This is the fourth edition ofthis textbook.Itincludes material on thelat-est standards and criteria of Manual on Uniform Traffic Control Devices 2003 Edition and forthcoming 2010 Edition
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ISBN-13: 978-0-13-613573-9 (alk paper)
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I Traffic engineering-United States I Prassas, Elena S II McShane William R JII Title.
Part CI
2
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www.pearsonhighered.com ISBN-IO:ISBN-I3: 978-0-13-613573-90-13-613573-0
Trang 4The transportation system is often referred to as the nation's
"lifeblood circulation system." Our complex system of roads
and highways, railroads, airports and airlines, waterways, and
urban transit systems provides for the movement ofpeople and
goods within and between ourdensest urban cities and the most
remote outposts ofthe nation Without theability to travel and
transport goods society must be structured around small
self-sufficient communities, each of which produces food and
material forall ofits needs locally and disposes ofits wastes in
manner The benefits ofeconomic specialization and
mass production possible only where transportation exists
to move needed materials of production centralized
loca-tions andfinished products towidely dispersed consumers
Traffic engineering deals with one critical element of
the transportation system: streets and highways, and their use
byvehicles This vast national system provides mobility and
access for individuals in private autos and forgoods in trucks
ofvarious sizes and forms, and facilitates public transport by
supporting buses, bicycles, and pedestrians
Because the transportation system issuch acritical part of
our public infrastructure, the traffic engineer is involved in a
wide range of issues, often in a very public setting, and must
bring wide range of skills to the table to be effective Traffic
engineers must have an appreciation for and understanding of
design, management, construction, operation, control,
and system optimization AIl of these functions involve traffic
engineers atsome level
This text focuses on thekey engineering skills required
to practice traffic engineering in a modern setting This is the
fourth edition ofthis textbook.Itincludes material on
thelat-est standards and criteria of Manual on Uniform Traffic
Control Devices (2003 Edition and forthcoming 2010
Edition), thePolicy on Geometric Design of Highways and
Streets (2004 Edition), the Capacity Manual (2000
Edition and forthcoming 201 0 Edition), and other critical
references It also presents both fundamental theory and abroad range ofapplications to modern problems
Thetext isorganized infive major functional parts:
• Part 1-Traffic Components and Characteristics
• Part 2-Traffic Studies and Programs
• Part 3-Freeways and Rural Highways
• Part4-The Intersection
• Part 5-Arterials Networks and Systems
for more detailed graduate courses At Polytechnic Institute
of New York University it is used for two undergraduatecourses and a series ofthree graduate courses
As in previous editions, the text contains many sampleproblems and illustrations that can be used in conjunction withcourse material A solutions manual is available The authorshope that practicing professionals and students find this textuseful and informative, and they invite comments and/or criti-cisms that will help them continue to improve the material
What's New in This Edition
This edition of the textbook adds a significant amount ofmaterial including, but notlimited to:
I New homework problems for most chapters
2 New chapters on Traffic Flow Theory, Analysis ofArterials in aMultimodal.Setting Critical MovementAnalysis of Signalized Intersections, and TrafficImpact Studies
3 Material from the latest editions of key traffic neering references, including the Traffic Engineering Handbook the Manual of Uniform Traffic Control
Trang 5Devices the Signal Timing Handbook, and the
Policy on Geometric Design ofHighways and Streets.
4 Substantial material from forthcoming new editions
Manila! Uniform Traffic Control Devices 1
which were obtained from research documents draft
included Since some of this material has not yet
been officially adopted, itprovides a preview but not
final information on these standard documents
PREFACE
5 New material on actuated signal systems and timing
6 New material on coordination of signal systems
7 Reference Jinks to important Web sites as well asdemonstration solutions using current softwarepackages
Trang 6Contents Preface ix
Design Elements 393.3 Horizontal Alignment ofHighways 403.4 Vertical Alignment of Highways 53Cross-Section Elements of Highways 603.6 Closing Comments 62
References 63Problems 64
4 Introduction to Traffic Control Devices 65
4.1 The Manual on Uniform Traffic Control
Devices 654.2 Traffic Markings 694.3 Traffic Signs 744.4 Traffic Signals 874.5 Special Types ofControl 934.6 Summary and Conclusion 94References 94
Problems 94
5.1 Types ofFacilities 955.2 Traffic Stream Parameters 96
5.3 Relationships among Flow Rate,
Speed, and Density 1035.4 Closing Comments 105References 105
Problems 105
Trang 76.3 Shock-Wave Theory and Applications JJ7
6.4 Characteristics ofInterrupted Flow 119
6.5 Closing Comments JJ9
References J20
Problems 120
Part 2 Traffic Studies and Programs 121
7 Statistical Applications in Traffic
7.4 Sample Size Computations J29
7.5 Addition ofRandom Variables 129
7.6 The Binomial Distribution Related to the
Bernoulli and Normal Distributions 131
7.7 The Poisson Distribution 133
9.4 Intersection Volume Studies 174
9.5 Limited Network Volume Studies 176
CONTENTS
9.6 Statewide Counting Programs 1849.7 Specialized Counting Studies 1899.8 Closing Comments 195
References 195Problems 195
10 Speed, Travel Time, and Delay
Studies 198
10.1 Introduction 19810.2 Spot Speed Studies J9910.3 Travel-Time Studies 21110.4 Intersection Delay Studies 21810.5 Closing Comments 222References 222
Problems 223
11 Highway Traffic Safety: Studies,
Statistics, and Programs 225
11.1 Introduction 22511.2 Approaches to Highway Safety 22711.3 Accident Data Collection and Record
Systems 23011.4 Accident Statistics 23411.5
11.6 Development ofCountermeasures11.7 Closing Comments 244References
Problems 248
12 Parking 250
12.1 Introduction 25012.2 Parking Generation and Supply Needs 25012.3 Parking Studies and Characteristics 25412.4 Design Aspects ofParking Facilities12.5 Parking Programs 270
12.6 Closing Comments 271References 272
Problems 272
Part 3 Freeways and Rural Highways 274
13 Fundamental Concepts for Uninterrupted
Flow Facilities 275
13.1 Types ofUninterrupted Flow Facilities 27513.2 The Highway Capacity Manual 27613.3 The Capacity Concept 27713.4 The Level ofService Concept 278J3.5 Service Flow Rates and Service Volumes 28113.6 Thevic Ratio and Its Use inCapacity
Trang 814.5 Calibration Speed-Flaw-Density Curves 309
14.6 Calibrating Passenger-Car Equivalents 309
14.7 Calibrating the Driver Population Factor 312
14.8 Adjustment Factorsto Free-Flow Speed 313
14.9 Software 313
14.10 Source Documents 3I3
References 313
Problems 314
15 Weaving, Merging, and Diverging
Movements on Freeways and
15.1 Turbulence Areas on Freeways and Multilane
Highways15.2 Level-of-Service Criteria 317
15.3 ACommon Point: Converting Demand
Volumes 31915.4 Weaving Segments: Basic Characteristics and
Variables 31915.5 Computational Procedures for Weaving Area
Analysis 325l5.6 Basic Characteristics of Merge and Diverge
Segment Analysis 33315.7 Computational Procedures for Merge and
Diverge Segments 33515.8 Sample Problems inWeaving, Merging, and
Diverging Analysis 34215.9 Analysis ofFreeway Facilities 352
16.4 Capacity and Level-of-Service Analysis of
Two-Lane Rural Highways 36616.5 Sample Problems in Analysis ofRuralTwo-Lane Highways 37716.6 The Impact ofPassing and Truck Climbing
Lanes 38416.7 Summary 387References 387Problems 387
17 Signing and Marking for Freeways and
17.1 Traffic Markings on Freeways and Rural
Highways 38917.2 Establishing and Posting ofSpeed Limits 39417.3 Guide Signing ofFreeways and Rural
Highways 39617.4 Other Signs on Freeways and RuralHighways 404
References 405Problems 407
Part 4 The Intersection 409
18 The Hierarchy ofIntersection
Control 410
18.I Levell Control: Basic Rules
of the Road 41118.2 Level IIControl: YIELD and STOPControl
18.3 Level III Control: Traffic
Control Signals 41718.4 Closing Comments 432References 432
Intersections 45419.6 Closing Comments 459References 459
Problems 459
Trang 920 Basic Principles ofIntersection
Signalization 461
20.1 Terms and Definitions
20.2 Discharge Headways, Saturation Flow Lost
Times, and Capacity 465
20.3 The Critical-Lane and Time-Budget
21 Fundamentals ofSignal Timing and
Design: Pretimed Signals 489
21.1 Development ofSignal Phase Plans 490
·21.2 Determining Vehicular Signal
Requirements 503
21 3 Determining Pedestrian Signal
Requirements 508
21.4 Compound Signal Timing 5] ]
21.5 Sample Signal Timing Applications II
22.1 Types ofActuated Control 527
22.2 Detectors and Detection 528
22.3 Actuated Control Features and
Operation 529
22.4 Actuated Signal Timing and Design 531
22.5 Examples inActuated Signal
Design and Timing 537
References 542
Problems 542
23 Critical Movement Analysis ofSignalized
Intersections 545
23.1 The TRB Circular212Methodology 546
23.2 ACritical Movement Approach toSignalized
Methodology24.3 The Basic Model 579
24.5 Complexities 60524.6 Calibration Issues 6J024.7 Summary 615References
Problems 615
25 Intelligent Transportation Systems
in Support ofTraffic Management and Control 620
25 I ITS Standards 621
25.2 National ITS Architecture 62225.3 ITS Organizations and Sources
ofInformation 62225.4 ITS-Related Commercial Routing
25.5 Traffic by Virtual and Other
Detectors 62325.6 Traffic Control in an ITS
Environment 624
25.9 Summary 63IProblems J
26 Signal Coordination fot Arterials and Networks: Undersaturated Conditions 632
26.1 Basic Principles ofSignal
Coordination 63226.2 Signal Progression on One-Way
Streets 63426.3 Bandwidth Concepts 63626.4 The Effect ofQueued Vehicles
at Signals 63826.5 Signal Progression for Two-Way Streets
and Networks26.6 Common Types ofProgression 64626.7 Software for Doing Signal
Progression 65026.8 Closing Comments 656References 657
Problems 658
Trang 1027 Signal Coordination for Arterials
and Networks: Oversaturated
Balanced Streets and Complete Streets 69129.5 Traffic Calming 694
29.6 Roundabouts 69829.7 Network Issues 69829.8 Special Cases 70129.9 Summary 706References 706Problems 707
30 Traffic Impact Analysis 708
30.1 Scope ofThis Chapter 70930.2 An Overview oftheProcess 70930.3 Tools, Methods, and Metrics 71230.4 Case Study I: Driveway Location 7I430.5 Case Study 2: Most Segments ofaTraffic
Impact Analysis 71530.6 Sumrnarv 726
References 726Problems 726
Index 728
Trang 11Introduction to Traffic
Engineering
1.1 Traffic Engineering
as aProfession
The Institute of Transportation Engineers defines traffic
engineering subset of transportation engineering as
follows [1]:
Transportation engineering is the application of
tech-nology and scientific principles to the planning,
func-tional design operation, and management offacilities
for any mode of transportation in order toprovide for
the safe, rapid comfortable economical,
and environmentally compatible movement of people
and goods
and:
Traffic engineering is that phase of transportation
engi-neering which deals with the planning, geometric
design and traffic operations ofroads, streets, and
high-ways, their networks, terminals, abutting lands, and
relationships with other modes oftransportation
These definitions represent a broadening of the profession to
include multimodal transportation systems and options, and
toinclude a variety of objectives in addition to the traditional
goals of safety and efficiency
1.1.1 Safety: The Primary Objective
The principal goal ofthe traffic engineer remains the provision
of safe system for highway traffic This is no small concern Inrecent fatalities on U.S highways have ranged betweenand per year Although this is a reduction fromthe highs experienced in the 1970s when highway fatalitiesreached more than 55,000 per year it continues to represent astaggering number Rising fuel prices in2008 and 2009 havehad an impact on both fatalities and vehicle-miles travelled In
2008, fatalities were reduced to 37.261, the first time the numberdipped below 40,000 in many years Some of this was due to
a reduction in vehicle-miles travelled, which dipped under3.0 trillion miles after two years over this level.Itremains tobeseen whether this reduction issustainable or whether fatalitieswill rise once again when (and if) the fuel cost issues areresolved One however, remains: More Americans havebeen killed on U.S highways than in all of the wars in whichthe nation has participated, including the Civil War
Although total highway fatalities per year haveremained relatively stable over the past two accidentrates based on vehicle-miles traveled have consistentlydeclined That is because U.S motorists continue to drivemore miles each year With astable total number offatalities,the increasing number of annual vehicle-miles traveled pro-duces a declining fatality rate This trend will also beaffected
by the decrease invehicle use in 2008 and 2009
Trang 121 CHAPTER 1 INTRODUCTION TOTRAFFIC ENGINEERING
• Convenience
The definitions oftransportation and traffic engineering
light additional objectives:
tc
leth
to
The traffic engineer has a very special relationship with
the traffic engineer deals with the daily safety of ment ofthepublic Although itcan be argued that
seg-who designs a product has this responsibility
have so many people using their product so routinely andfrequently and depending on it so totally Therefore.the trafficengineer also has a special obligation toemploy theavailableknowledge and state of the art within existing resources toenhance public safety
The traffic engineer also functions in a world in which anumber of key participants do not understand the traffic andtransportation issues orhow theytruly affect a particular project.These include elected and appointed officials with decision-making power, the general public and other professionals withwhom traffic engineers work on an overall project teameffort.
Because all of us interface regularly with the transportationsystem many overestimate theirunderstanding oftransportationand traffic issues The traffic engineer must deal productivelywith problems associated with naive assumptions plans anddesigns that are oblivious to transportation and traffic needs,oversimplified analyses, andunderstated impacts
Like all engineers, traffic engineers must understand andcomply with professional ethics codes Primary codes ofethicsfor traffic engineers are those of the National Society ofProfessional Engineers and the American Society of CivilEngineers The most up-to-date versions of each are available
1.1.3 Responsibility, Ethics, and Liability
in Traffic Engineering
construction, maintenance, andoperating expenditures most ofwhich are provided through general and user taxes and fees.Nevertheless every engineer, regardless of discipline is called
on toprovide the best possible systems for themoney
Harmony the environment isacomplex issue thathas become more important over time All transportationsystems have some negative impacts on the environment Allproduce air and noise pollution in someforms, all utilizevaluable land resources Inmany modem cities transportationsystems use asmuch as of thetotal land area "Harmony"
architecture provides for aesthetically pleasing facilities thatwith their surroundings
Thetraffic engineer is tasked with all of these goals andobjectives and with making the appropriatetrade-offs toopti-mize both the transportation systems and the of publicfunds to build, maintain and operate them
-,
1.1.2 Other Objectives
Most of these are self-evident desires of the traveler Most of
us want our trips to be fast comfortable convenient
and in harmony with theenvironment All of these objectives
are also relative and must be balanced against each other and
against the primary objective ofsafety
Although speed of travel is much to be desired, it is
limited by transportation technology, human characteristics
the need toprovide safety Comfort and convenience are generic
terms and mean different things todifferent people Comfort
involves the physical characteristics ofvehicles and roadways,
and it is influenced by our perception of safety Convenience
relates more tothe ease with which trips are made and the ability
oftransport systems to accommodate all of our travel needs at
appropriate times Economy is also relative There is little in
modern transportation systems that can be termed
Highway and other transportation systems involve massive
Improvements in[atality rates reflect a number oftrends
implementing Stronger efforts toremove dangerous drivers
from the road have yielded significant dividends in safety
under the influence(Dl.ll)and driving
(DWI) offenses are more strictly enforced, and licenses are
suspended or revoked more easily as a result of DUI/DWI
convictions, poor accident record and/or poor violations record
Vehicle design has greatly improved (encouraged byseveral acts
ofCongress requiring certain improvements).Today'svehicles
feature padded dashboards collapsible steering columns, seat
belts with shoulder harnesses, airbags (some vehicles now have
as many as eight), and antilock braking systems Highway
design has improved through the development and use of
advanced barrier systems for medians and roadside areas
surveillance systems can alert authorities toaccidents and
in thesystem
year still die in traffic accidents The of travel is
number one and isnever finished for thetraffic engineer
Trang 131.2 TRANSPORTATION SYSTEMS AND THEIR FUNCTION
online In general good professional ethics requires thattraffic
engineers work only intheir areas of expertise: do all work
completely and thoroughly: be completely honest with the
general public employers, and clients: comply with all
appli-cable codes and standards: and work tothe best oftheirability
In traffic engineering, the pressure to understate negative
impacts ofprojects, sometimes brought tobear by clients who
wish a project to proceed and employers who wish to keep
clients happy is a particular concern As in all engineering
professions, the pressure to minimize costs must give way to
basic needs for safety and reliability
Experiencehas shown that thegreatest risk to a project
because an impact was overlooked or analysis oversimplified
Sophisticated developers and experienced professionals know
that the environmental impact process calls for a fair and
reviewerson accepting the impacts, given anoverall good
analysis report The process does not require zero impacts: it
impacts so that policy makers can make informed decisions
Successful challenges to major projects are almost always
Indeed such disagreements are not avalid basis for a
to project In the case of the WestwayProject proposed in theI970s for the west side of Manhattan,
one of the bases for legal challenge that the impact of
project construction on striped bass in the Hudson River had
not been properly identified or disclosed In particular, the
project died due to overlooking the impact on the
reproduc-tive cycle ofstriped bass in the Hudson River Although this
topic was not the primary concern of the litigants it was the
The traffic engineer also has a responsibility to protect
the community from liability by good practice Agencies
charged with traffic and transportation responsibilities canbe
held liable in many areas These include (but are not limited
to)thefollowing:
• Placing control devices that donot conform to
applica-ble standards for their physical design and placement
• Failure to maintain devices in a manner that ensures
their effectiveness: the worst case ofthis is a
traffic signal in which no indication is given due to
bulb orotherdevice failure
• Failure to apply the most current standards and
guide-lines inmaking decisions ontraffic controL developing
afacility plan ordesign, orconducting an investigation
• Implementing traffic regulations (and placing
appropri-ate devices) without proper legal authority todoso
3
Ahistoric standard has been that "due care" be exercised
in the preparation of plans and that determinations made in theprocess be reasonable and "not arbitrary." It isgenerally recog-nized that professionals must make value judgments and thetermsdue careandnot arbitraryare continually under legal test.The fundamental ethical issue for traffic engineers is toprovide for the public safety through positive programs goodpractice, knowledge? and proper procedure The negative (albeitimportant) side ofthis isthe avoidance ofliability problems
1.2 Transportation Systems and Their Function
Transportation systems are a major component of the U.S.economy and have an enormous impact on the shape of thesociety and the efficiency ofthe economy ingeneral Table 1.1ilJustrates some key statistics for the U.S highway system forthebase year 2007 and two preliminary statistics for 2008.America moves on its highways Although public trans-portation systems are ofmajor importance in large urban areassuch as New York, Boston Chicago, and San Francisco, it isclear that the vast majority of person-travel as well as a largeproportion offreight traffic is entirely dependent on theway system The system is a major economic force in ownright: Over $90 billion per year iscollected by state and federalgovernments directly from road users in theform of focuseduser taxes and fees Such taxes and fees include excise taxes ongasoline and other fuels, registration fees, commercial vehiclesfees, and others Other funds areallocated from federal andstate general funds for highway use Asindicated in Table 1.1,
by 2007, $161 billion was being colJeeted and spent on way and traffic improvements byalJ units ofgovernment
high-Table 1.1: Important Statistics on U.S Highways
United States
Total receipts: Taxes, fees,
allocations
Federal, state, local
Trang 144 CHAPTER 1 INTRODUCTION TO TRAFFIC ENGINEERING
The American love affair with the automobile has
grown consistently since the 1920s when Henry Ford's
Model T made the car accessible to the average wage
earner This growth hassurvived wars, gasoline embargoes
depressions recessions, and almost everything else that has
happened in society As seen in Figure 1.1, annual
vehicle-miles traveled reached the I trillion mark in 1968 the trillion
mark in 1987 and the 3 trillion mark in 2005
This growth pattern is one ofthefundamental problems
to befacedbytraffic engineers Given the maturity of
our highway systems and the difficulty in trying to
add system capacity, particularly inurban areas the continued
growth in vehicle-miles traveled leads directly to increased
congestion on our highways The inability tosimply build
addi-tional capacity to meet the growing demand creates the need
to address alternative modes, fundamental alterations in
demand patterns, and management of the system to produceoptimal results
1.2.1 The Nature of Transportation
Demand
Transportation demand is directly related to land-usepatterns and to available transportation systems and facili-ties Figure 1.2 illustrates the fundamental relationship
is circular and ongoing Transportation demand
is generated by the types amounts, and intensity of landuse as well as its location The daily journey to work, forexample, is dictated by the locations of the worker'sresidence and employer and the times that the worker is
Figure 1.1: Public Highway Mileage and Annual Vehicle-Miles Traveled in the United States 1920-2007
(Source: Highway Statistics 2007 Federal Highway Administration U.S Department ofTransportation, Washington DC 2008, Table
VMT42J.)
Trang 151.2 TRANSPORTATION SYSTEMS AND THEIR FUNCTION 5
It isimportant that the traffic engineer understand thisprocess.Itis complex and cannot be stopped any moment
in time Demand-prediction techniques (not covered inthis text) must start and stop at arbitrary points in time.Theprocess is ongoing, and new or improved facilities
and proposals must recognize both this reality and theprofessional's inability to precisely predict its impacts
A demand forecast that comes
±20o/c of the actual vaiue is considered a significant success The essential truth however, is that
traffic engineers cannot simply build their way out ofcongestion
If anything, westill tend tounderestimate the impact oftransportation facilities on land-use development Often, theincrease in demand is hastened by development OCCUlTingsimply as aresult of the planning of a new facility
One of classic cases occurred on Long Island in
the development of suburban residential communities lurchedforward in anticipation While the expressway's link toExit 7was being constructed, new homes were being built at theanticipated Exit even though the facility would not heopen tothat point for several years The result that theexpressway was completed section by section the
even months This process has been repeated in manythroughout the nation
1.2.2 Concepts of Mobility
and Accessibility
Transportation systems provide the nation's population withbothmobility and accessibility Thetwoconcepts arestronglyinterrelated but have distinctly different elements
refers to the ability to travel to many different destinations
withrelative ease whereas accessibility refers tothe ability to
gain entry to aparticular site or area
Mobility gives travelers a wide range of choices as towhere to go to satisfy particular needs, and it provides forefficient trips to get to them Mobility allows shoppers tochoose from among many competing shopping centers andstores Similarly mobility provides the traveler with manychoices for all kinds of trip purposes, including recreationaltrips, medical trips, educational trips, and even the commute
to work The ranee of available choices isenabled bv havinz
an effective transportation network that connects to manyalternative trip destinations within a reasonable time withrelative ease and at reasonable cost Thus mobility provides
Land
Use
Transp.
Facilities
Figure 1.2: TheNature ofTransportation Demand
nearby lands more accessible and therefore more attractive
leads to further increases in land-use development which
(in turn') results in even higher transportation demands This
circular self-reinforcing characteristic'of traffic demand
creates central dilemma: Building additional
transporta-tion capacity invariably leads to incrementally increased
travel demands
more efficient transportation systems such as public transit
and car-pooling programs In some of the largest cities,
providing additional system capacity on highways is no
longer an objective because such systems are already
substantially choking in congestion In these places the
emphasis shifts to improvements within existing highway
rights-of-way and to the elimination of bottleneck locations
(without adding to overall capacity) Other approaches
include staggered work hours and workdays to reduce peak
hour demands, and even more radical approaches involve
development of satellite centers outside of the central
busi-ness district (CBD) to spatially disperse highly directional
demands into and out ofcity centers
Demand, however, is constrained by capacity inall
cities, and the normal process of attempting to accommodate
demand as it increases isfeasible in these areas At thesame
time, the circular nature ofthe travel/demand relationship will
lead tocongestion if care isnot taken tomanage both capacity
and demand tokeep them within tolerable limits
Trang 16access to many travel opportunities, and it provides relative
speed and convenience-for the required trips
Accessibility is a major factor in the value of land
When land can be accessed by many travelers from many
potential origins, it is more desirable for development and
therefore, more valuable Thus proximity ofland to major
highways and public transportation facilities is a major factor
determining its value
Mobility and accessibility may also refer to different
portions of a typical trip Mobility focuses on the through
portion oftrips and is most affected by the effectiveness of
through facilities that take a traveler from one general area to
another Accessibility requires the ability to make a transfer
from the transportation system to theparticular land parcel on
which the desired activity is taking place Accessibility,
therefore relies heavily on transfer facilities, which include
parking for vehicles, public transit stops, and loading zones
sys-tems are structured to separate mobility and access functions
because the two functions often compete and are not
neces-sarily compatible In highway systems, mobility is provided
by high-type facilities, such as freeways, expressways, and
primary and secondary arterials Accessibility is generally
provided by local street networks Except for limited-access
facilities serve only through vehicles (mobility) most
other classes of highway serve both functions to some
degree Access maneuvers (e.g parking and unparking a
vehicle, vehicles entering and leaving off-street parking via
driveways buses stopping to pick up or discharge
passen-gers, trucks stopped to load and/or unload goods) however
retard the progress of through traffic High-speed through
traffic in contrast tends to make such access functions more
dangerous
A good transportation system must provide for both
mobility accessibility and should bedesigned to separate
the functions to theextent possible to ensure both safety and
efficiency
1.2.3 People, Goods, and Vehicles
The most common unit used by the traffic engineer is
"vehicles:' Highway systems are planned, designed and
oper-ated tomove vehicles safely and efficiently from place toplace
Yet the movement of vehicles is not the objective; the goal is
themovement ofthepeople goods that occupy vehicles
Modem traffic engineering now focuses more onpeople
and goods Although lanes must be added to a freeway to
increase its capacity tocarry vehicles, itsperson-capacity can
be increased by increasing the average vehicle occupancy
Consider afreeway lane with a capacity of2,000 vehicles per
CHAPTER I INTRODUCTION TOTRAFFIC ENGINEERING
hour(veh/h).Ifeach vehicle carries one person thelane has acapacity of 2.000 persons/hour as well If the average caroccupancy is increased to 2.0 persons/vehicle thecapacity interms ofpeople is doubled to persons/hour If the lanewere established anexclusive bus lane the vehicle-capacitymight be reduced to 1.000 veh/h due to the larger size andpoorer operating characteristics of buses as comparedwith automobiles However ifeach bus carries 50 passengers,the people-capacity of the lane is increased to 50.000persons/hour
The efficient movement of goods is also vital to thegeneral economy of the nation The benefits of centralizedand specialized production of various products are possibleonly if raw materials can be efficiently shipped to manu-facturing sites and finished products can be efficientlydistributed throughout the nation and the world forconsumption Although long-distance shipment of goodsand raw materials is often accomplished water,rail orair transportation the final leg of the trip todelivera good
to the local store or the home of an individual consumergenerally takes place on truck using the highway system
heavily on the characteristics the vehicle and of the driver
In the final analysis however theobjective isto move peopleand goods not vehicles,
1.2.4 Transportation Modes
Although the traffic engineer deals primarily with highwaysand highway vehicles there are other important transportationsystems that must be integrated into a cohesive national.regional and local transportation network Table J.2provides
a comprehensive listing of various transportation modes andtheirprincipal uses
The traffic engineer deals with all of these modes in anumber ofways All over-the-road modes-automobile bustransit, trucking-are principal users of highway systems.Highway access to rail and air terminals is critical to theireffectiveness, as is the design of specific transfer facilities forboth people and freight General access, internal circulation,parking, pedestrian areas, and terminals for both people andfreight are all projects requiring the expertise of the trafficengineer Moreover, the effective integration of multimodaltransportation systems isa major goal inmaximizing efficiencyand minimizing costs associated with all forms oftravel.'
Trang 171.2 TRANSPORTATION SYSTEMS AND THEIR FUNCTION
Table1.2: Transportation Modes
Rail service using one- totwo-car unitsalong fixed
Heavy rail vehicles inmulticar trains along fixedroutes with schedules on fully separated
Public transportation along a fixed intercity
Provides service to a central terminal
in each city
Approximate Range of Capacities*
1-6persons/vehicle: approx
2,000veh/h perfreeway lane:
400-700veh/h per arterial lane
1-6persons/vehicle; total capacitylimited byavailability
40-70persons/bus: capacitylimited byschedule: usually
100-5,000persons/h/route
40-50persons/bus (no standees);
capacity limited by schedule
Variable seating capacity depends onvehicle design: total capacity depends
on number of available vehicles
80-120persons/car: up to15,000
persons/h/route
150-300persons/car depending onseating configuration andstandees:
up to60.000persons per track
Highly variable with ferry andterminal design and schedule
Same as urban automobile
40-50passengers per bus: scheduleshighly variable
A variety ofair-passenger services from smallcommuter planes tojumbo jets on fixed routes andfixed schedules
Passenger ship service often associated withonboard vacation packages on fixed routes andschedules
500-1,000passengers pertrain,depending on configuration:
schedules highly variable
per aircraft depending on size andconfiguration Schedules depend ondestination and are highly variable
Ship capacity highly variable from
schedules often extremely limited
( COiltinned)
Trang 188 CHAPTER 1 INTRODUCTION TOTRAFFIC ENGINEERING
Intercity haulage of bulk commodities withsome local distribution tolocations with rail sidings
International and intercity haulage ofbulk ties on a variety of container ships and barges
commodi-International and intercity haulage of small andmoderately sized parcels and/or time-sensitiveand/or high-value commodities where high
is not adisincentive
Approximate RangeofCapacities"
capacity
vehicle (orpipeline) and limitations
intercity and local distribution networks possible
represent not the full range of possibilities.
1.3 Highway Legislation and
History in the United States
Thedevelopment of highway systems in the United States is
strongly tied to federal legislation that supports and regulates
discussed in the sections that follow
1.3.1 The National Pike and the States'
Rights Issue
Before the 1800s roads little more than trails cleared
through the wilderness by adventurous travelers and
explor-ers.Private roadways began to appear in the latter part ofthe
1700s These roadways ranged in quality and length from
cleared trails to plank roadways They were built by private
owners, and fees were charged for their use At points where
fees were to be collected, a barrier usually consisting of a
single crossbar was mounted on a swiveling stake, referred to
as a "pike," When the fee was collected, the pike would be
swiveled or turned, allowing the traveler to proceed This
early process gave birth to the term turnpike often used to
describe toll roadways in modern times
The National Pike
In 1811 the construction the first national roadway
govern-ment Known as the "national pike" or the "Cumberlandthis facility stretched for 800miles Cumberland.Maryland in the east to Vandalia Illinois in the
Highways as a States' Right
The course of highway development in the United States.however was forever changed as a result of an 1832Supreme Court case brought by the administration ,ofPresident Andrew Jackson A major proponent of states'rights, the Jackson Administration petitioned the court
define transportation and roadways as federal functions;they were, therefore, the responsibility of the individualstates The Supreme Court upheld this position andthe principal administrative responsibility for transporta-tion and highways was forevermore assigned to stategovernments
f
o
ati
Trang 191.3 HIGHWAY LEGISLATION AND HISTORY IN THE UNITED STATES 9
The Governmental Context
Ifthe planning, design, construction, maintenance, and
opera-tion of highway systems is a state responsibility, what is the
role of federal agencies-for example, the U.S Department
ofTransportation and its components such as the Federal
Highway Administration the National Highway Safety
Administration, and others in these processes?
The federal government asserts its overall control of
highway systems through thepower ofthepurse strings The
federal government provides massive funding for the
struction, maintenance, and operation of highway and other
transportation systems States are not required tofollow
fed-eral mandates and standards but must do so to qualify for
federal funding of projects Thus the federal government
does notforce a state toparticipate infederal-aid
transporta-tion programs If it chooses to participate, however it must
follow federal guidelines and standards Because no state
can afford to give up this massive funding source, the federal
government imposes strong control of policy issues
standards
The federal role in highway systems has four major
components:
I Direct responsibility for highway systems on
Native American reservations
Provision offunding assistance in accord with
cur-rent federal-aid transportation legislation
3 Development ofplanning, design andother relevant
standards and guidelines that must be followed to
qualify for receipt of federal-aid transportation
funds
4 Monitoring and enforcing compliance with federal
standards and criteria, and the use offederal-aid funds
State governments have the primary responsibility
for the planning, design, construction, maintenance, and
operation of highway systems These functions are
gener-ally carried out through a state department of
transporta-tion or similar agency States have:
1 FuJ] responsibility for administration of highway
systems
2 Full responsibility fortheplanning, design,
construe-tion, maintenance, and operation ofhighway systems
in conformance with applicable federal standards
andguidelines
3 The right to delegate responsibilities forlocal
road-way systems tolocal jurisdictions or agencies
Local governments have general responsibility forlocal roadway systems as delegated in state law In general,local governments are responsible for the planning, design,construction, maintenance, and control of local roadwaysystems Often, assistance from state programs and agenciesisavailable to local governments infulfilling these functions
At intersections of state highways with local roadways it isgenerally the state that has the responsibility to control theintersection
Local organizations for highway functions range fromfull highway or transportation department to local police to
a single professional traffic or city engineer
There are also anumber of special situations across theUnited States In New York State, for example, the stateconstitution grants "home rule" powers to any municipalitywith a population in excess of 1 million people Under thisprovision, New York City has full jurisdiction over all high-ways within its borders includins those on the state hishwavsvstern
1.3.2 Key Legislative Milestones
Federal-Aid Highway Act of1916
alloca-tion offederal-aid highway funds for highway construcalloca-tion
secondary and tertiary federal-aid highways, and provided
of the funding for construction of highways in thissystem Revenues for federal aid were taken from thefederal general fund and the act was renewed every two tofive years (with increasins amounts dedicated) No major. .
changes in funding formulas were forthcoming for a period
of years
Federal-Aid Highway Act of1934
In addition to renewing funding for the A-B-C System,this act authorized states to use up to1.5% of federal-aidfunds for planning studies and other investigations It rep-resented the entry ofthe federal government into highwayplanning
Federal·Aid Highway Act of1944
This act contained the initial authorization of what becamethe National System of Interstate and Defense Highways
No appropriation offunds occurred, however, and the systemwas not initiated for another 12years
Trang 20Federal-Aid Highway Act of 1956
The authorization and appropriation offunds for the
imple-mentation of the National System of Interstate andDefense
Highways occurred in 1956 The act also set the federal
share of the cost of the Interstate System at the first
major change in funding formulas since 1916 Because
of the major impact on the amounts of federal funds to
be spent the act also created theHighwa» Trust Fund
and enacted a series of road-user taxes to provide it with
revenues These taxes included excise taxes on motor
fuels vehicle purchases motor oil and replacement parts
Most of these taxes, except for the federal fuel tax, were
dropped during the Nixon Administration The monies
housed in the Highway Trust Fund may be disbursed
only for purposes authorized by the current federal-aid
highway act
Federal-Aid Highway Act of 1970
Also known as the Safety Act of1970 this
tion increased the federal subsidy of non-Interstate highway
projects to and required all states to implement highway
safety agencies and programs
Federal-Aid Highway Act of 1983
This act contained the "Interstate trade-in" provision that
allows states to "trade in" federal-aid funds designated for
urban interstate projects for alternative transit systems This
historic provision was the first to allow road-user taxes to
be used topayfor public transit improvements
ISTEA and TEA-21
The single largest overhaul of federal-aid highway programs
occurred with the passage of the Intermodal Surface
Transportation Efficiency Act OSTEA) in 1991
anditssuc-cessor, the Transportation Equity Act for the 21 st Century
(TEA-21) in 1998
Most importantly these acts combined federal-aid
programs for all modes of transportation and greatly
liber-alized the ability of state and local governments to make
decisions on modal allocations These are the key
provi-sions ofISTEA:
I Greatly increased local options in the
useoffederal-aid transportation funds
2 Increased the importance and funding toMetropolitan
Planning Organizations (MPOs) and required that
CHAPTER I INTRODUCTION TOTRAFFlC ENGINEERING
each state maintain astate transportation improvementplan (STIP)
Tied federal-aid transportation funding to ance with the Clean AirAct and itsamendments.Authorized $38 billion for ISS.OOO-mile NationalHighway System
compli-5. Authorized an additional $7.2 billion to completethe Interstate System and $17 billion to maintain itaspartofthe National Highway System
6 Extended federal funding of Interstate-eligibleprojects
7 Combined all other federal-aid systems into asinglesurface transportation system with 80% federalfunding
8 Allowed (for the first time) the use of federal-aidfunds in the construction oftoll roads
TEA-21 followed in increasing funding: levels.further liberalizing local options for allocation of funds.further encouraging intermodality and integration oftransportation systems, and continuing the link betweencompliance with clean-air standards and federal transporta-tion funding
The creation of the National Highway System (NHSJanswered a key question that had been debated for years:What comes after the lnterstate System? The new expandedNHS is not limited to freeway facilities and is over threetimes the size of the Interstate System, which becomes partofthe NHS
SAFETY·LU
President Bush signed the most expensive transportationfundins act into law on Aucust 10 2005 The act was a milewide, and more than four years with intervening highwayfunding being accomplished through annual continuationlegislation thatkept TEA-21 ineffect
The Accountable Flexible and Efficient portation Equity Act-A Legacy for Users (SAFETY-LU)has been both praised and criticized Although it retains most
Trans-of the programs Trans-of ISTEA and TEA-21 and expands thefunding for most of them the act also adds many newprograms and provisions, leading some lawmakers and politi-cians to label it most pork-filled legislation in U.S.history." Table 1.3 provides a simple listing of the programscovered under thislegislation Theprogram, which authorizesover $248 billion in expenditures, includes many programsthat represent items of special interest inserted by membersofCongress
Trang 21HIGHWAY LEGISLATION AND HISTORY IN THE UNITED STATES
Table 1.3: Programs Covered by SAFETY-LU*
Interstate Maintenance ProgramNational Highway SystemSurface Transportation SystemCongestion Mitigation/Air Quality Improvement ProgramHighway Safety Improvement Program
Appalachian Development/Highway System ProgramRecreational Trails Program
Federal Lands Highway Program,National Corridor Infrastructure Improvement ProgramCoordinated Border Infrastructure Program
National Scenic Byways ProgramConstruction ofFerry BoatslTerminalsPuerto Rico Highway ProgramProjects of National andRegional Significance ProgramHigh-Priority Projects Program
Safe Routes to School ProgramDeployment ofMagLev Transportation Projects
Highways for Life Program
amounts are inbillions ofdollars.
$5.1
50.850.2
The legislation cloes recognize the need for massive
funding of Interstate highway maintenance, as the system
continues to age with many structural components well
past their anticipated service life.It also provides massive
funding for the new NHS, which is the successor to the
Interstate System in terms of new highways Italso retains
the flexibility for local governments to push more funding
into publictransportation modes
Although discussions in Congress on a successor act
have begun it is not clear, at this writing, when the next
major funding legislation will be passed, or what is will and
will notcontain,
1.3.3 The National System of Interstate
and Defense Highways
The Interstate System has been described as the largest
public works project in the history of humankind In 1919,
a young army officer, Dwight Eisenhower, was tasked with
moving a complete battalion of troops and military
equip-ment from coast to coast on the nation's highways to
determine their utility for such movements in a time of
potential war The trip took months and left the young
officer with a keen appreciation for the need to develop a
Interstate System was initiated in the administration of
bears his name
After the end of World War II, the nation entered a
period of sustained prosperity One of the principal of
with theexpanding desire ofowners touse their cars for dailycommuting and for recreational travel Motorists groups such
as the American Automobile Association (AAA l wereformed and began substantial lobbying efforts to expand the
previous rail monopoly on intercity freight haulage Tflickersalso lobbied strongly for improved highway systems Thesesubstantial pressures led to the inauguration ofthe InterstateSystem in 1956
The System Concept
Authorized in 1944 and implemented in 1956, the NationalSystem of Interstate and Defense Highways isa42.500-milenational system of multilane limited-access facilities Thesystem was designed to connect all standard metropolitan sta-
Trang 22a continuous system of limited-access facilities The
alloca-tion of of thecost of the system to the federal
govern-ment justified on the basis of the potential military use of
System Characteristics
Key characteristics of the Interstate System include the
following:
use oftraffic ineach direction
All highways have full control ofaccess
3 The system must form aclosed loop: All1nterstate
highways must begin and end at a junction with
another Interstate highway
North-south routes have odd two-digit numbers
1-95)
East-west routes have even two-digit numbers (e.g
Interstate routes serving as bypass loops oracting as
a connector to a primary Interstate facility have
three-digit route numbers with the last two digits
indicating primary route
Figure 1.3 shows a map of the Interstate System
Status and Costs
unfinished sections were not expected to ever be completed
for a variety of reasons The total cost of the system was
approximately $125 billion
Figure 1.3: The Interstate System
CHAPTER I INTRODUCTION TOTRAFFIC ENGINEERING
The impact of the Interstate System on the nation ,cannot be understated The system facilitated and enabledthe rapid suburbanization of the United States by providing
a means for workers to commute from suburban homes tourban jobs.The economy of urban centers suffered as shop-pers moved in droves from traditional CBDs to suburbanmalls
The system also had serious negative impacts on some ofthe environs through which it was built Following thetraditional theoryofbenefit-cost urban sections were often builtthrough the low-income parts of communities where land wasthe cheapest The massive Interstate highway facilities createdphysical barriers, partitioning many communities, displacingresidents, and separating othersfrom their schools, churches,and local shops Social unrest resulted in several parts of thecountry, which eventually resulted in important modifications tothe public processand in the ability of local opponents
to legally stop many urban highway projects
Between 1944and 1956, a national debateover whether theInterstate System should be built into andout of urban areas orwhether all Interstate facilities shouldterminate in ring roads built around urban areas Proponentsofthe ring-road option (including ironically Robert Moseswho built many highways into and out of urban cities)
would lead to massive urban congestion On the other side.the argument was that most of the road users who
urban areas and should be served The latterbut the predicted rapid growth of urban congestionbecame a reality
1.4 Elements of Traffic Engineering
There are a number ofkey elements oftraffic engineering:
I Traffic studies and characteristicsPerformance evaluation
3 Facility design
4 Traffic control
5 Traffic operations
6 Transportation systems management
7 Integration of intelligent transportation systemtechnologies
Traffic studies Gild characteristics involve measuringand quantifying various aspect of highway traffic Studiesfocus ondatacollection and analysis that is used tocharacterize
Trang 2311
1.5 MODERN PROBLEMS FOR THE TRAFFIC ENGINEER
including (but not limited to) traffic volumes and
speed and time, delay, accidents, origins and
modal use andother variables
Performance evaluation is a means by which traffic
can rate the operating characteristics of ual sections offacilities andfacilities as a whole inrelative
individ-terms Such evaluation relies on measures of performance
qual ity and is often stated in terms of "levels of service."
Levels of service are letter grades from A to F describinz
how well a facility is operating using specified
whereas F connotes failure (on some level) As part of
per-formance evaluation the capacity of highway facilities
must be determined
Facility design involves traffic engineers in the
facilities Traffic engineers, per se, are not involved in the
structural design ofhighway facilities hut should have some
appreciation for structural characteristics of their facilities
contro! is a central function of traffic engineers
and involves the establishment oftraffic regulations and their
communication to the driver through the useoftraffic control
devices such as markings and signals
Traffic operations involves measures that influence
erall operation of traffic facilities such as one-way street
transit operations curb management
surveil-lance and network control systems
Tronsponation systems management (TSM) involves
virtually all aspects oftraffic engineering inafocus on
optimiz-ing system capacity and operations Specific aspects of TSM
include high-occupancy vehicle priority systems, car-pooling
programs pricing strategies to manage demand, and similar
functions
Intelligent transportation .I (ITS) refers to the
application of modern telecommunications technology to
the operation and control of transportation systems Such
systems include automated highways automated
toll-collection systems vehicle-tracking systems, in-vehicle
global positioning systems (GPS) and mapping systems,
automated enforcement of traffic lights and speed laws,
smart control devices, and others This is a rapidly
emerg-family of technologies with the potential to radically
alter the way we travel as well as the way in which
trans-portation professionals gather information and control
facilities While the technology continues to expand,
soci-ety will grapple with the substantial "big brother" issues
that such systems invariably create
This text contains material related to all of these
components of the broad and complex profession of traffic
ensineerins
13
1.5 Modern Problems for the Traffic Engineer
We live in a complex and rapidly developing world.Consequently, the problems that traffic engineers are involved
in evolve rapidly
Urban congestion has been a major issue for manyyears Given the transportation demand cycle, itisnot alwayspossible to solve congestion problems through expansion ofcapacity Traffic engineers therefore are involved in the
in both time and space and to discourage growth wherenecessary A real question is not "how much capacity isneeded to handle demand?" but rather "howmany vehiclesand/or people can beallowed toenter congested areas withindesignated time periods?"
Growth management isamajor current issue Anumber
of states have legislation that ties development permits to
system Where development will cause substantial tion in the quality oftraffic service, either such developmentwill be disallowed or the developer will be responsible for
negative impacts Such policies are more easily dealt with insood economic times When the economv is slusaish thep'
issue will often be a clash between the desire to reduce gestion and the desire to encourage development a meansofincreasing the tax base
con-Reconstruction ofexisting highway facilities also causesunique problems The entire Interstate System has been aging,and many of its facilities have required major reconstructionefforts Part ofthe problem is that reconstruction of Interstatefacilities receives the 90% federal subsidy whereas routinemaintenance on the same facility been until recently,primarily the responsibility of state and local governments.Deferring routine maintenance on these facilities in favor ofmajor reconstruction efforts has resulted fromfederal fundingpolicies over the years Major reconstruction efforts have asubstantial major burden not involved in the initial construc-tion ofthese facilities: traffic It is easier tobuild anew facility inadedicated right-of-way than torebuild itcontinuing to serve 100,000 or more vehicles per day Thusissues of long-term and short-term construction detours aswell as the diversion of traffic to alternative routes requiremajor planning by traffic engineers
Since 200I, the issue of securi ty of transportationfacilities hascome to the fore The creation offacilities andprocesses for random and systematic inspection of trucksand other vehicles at critical locations is a major challenge,
Trang 24as is securing major public transportation systems such as
railroads airports rapid transit systems
decline for the first time in Transportation planners
and engineers must be careful in determining whether thisis
reliable trend with long-term implications or simply a
short-term market perturbation The economic crisis of2008
bankruptcy with major industry reductions and changes
anticipated Government loans to both banks and industries
brought with it more governmental control of private
indus-tries A major shift of U.S automakers to smaller more
fuel-efficient and "green" vehicles begun, with no clear
shift For perhaps the first time in decades
improved mobility and accessibility Will people learn new
da: commute? Itis an unsett lime that will continue to
into challenges for traffic and transportation
With new challenges, however, comes the ability
for new and innovative approaches that might not been
feasible only a few years ago
engi-neer must be ready to face current problems and to play an
important role in any situation that involves transportation
and/or traffic systems
1.6 Standard References
for the Traffic Engineer
To remain uptodate and aware the traffic engineer must keep
up with modem developments through membership and
par-ticipation inprofessional organizations regular review of key
periodicals, and an awareness ofthe latest standards and
crite-ria for professional practice
Key professional organizations for the traffic engineer
include the Institute of Transportation Engineers (ITE) the
CHAPTER I INTRODliCTION TO TRAFFIC ENGINEERING
Group of the American Society ofCivil Engineers (ASCE),ITS America and others All of these provide literature and
meetings, TRB is branch of the National Academy ofEngineering and is major source of research papers andreports
Like many engineering fields, the traffic engineering
ofwhich will hereferred to in the chapters of this text Majorreferences include following:
Traffic Engineering Handbook 6th edition[1]
• Uniform Code and Model Traffic Ordinance [2]
• Manual on Uniform Traffic Control Devices 2003 edition anticipated in 2009-2010)[3J
• Capacit: Manna! edition 15th edition
in 2(10)[4J
• A Polie: on Geometric Highways and
Traffic Timing Manual lst edition[6J
• Transporunum Planning Handbook 3rd [7]
• Trip Generation Sth edition
• Parking Generation, 3rdeduion
All of these documents are updated periodically.the traffic engineering professional should be aware ofupdates arepublished and where they be accessed
of traffic engineering These references documentthe current state of art in traffic engineering and thosemost frequently used should be part of the professional'spersonal library
There are also awide variety ofinternet sites that are of
here because they change rapidly All of the professionalorganizations, as well as equipment manufacturers maintainWeb sites The federal Department ofTransportationFederal Highway Administration (FHWA) NationalHighway Traffic Safety Administration (NHTSA) andpri-vate highway-related organizations maintain Web sites The
entire Manual onUnijonn Traffic Control Devices isavailable online through the FHWA Website as is.the Manual of
Traffic Signal Timing.
Because traffic engineering is a rapidly changing field,you cannot assume that every standard and analysis processincluded in this text is current, particularly as the time sincepublication increases Although we will continue toproduce
Trang 25latest developments as professional responsibility.
1.7 Metric versus U.S Units
This text is published in English (or standard U.S.) units
Despite several attempts to switch to metric units in the
United States most states use English units indesign
control
lane converts to a standard 3.6-01 lane, which is
narrower than J feet Standards for a 70-milh design speed
convert to standards for 120-km/h design speed, which are
not numerically equivalent This is because even units are
used in both systems rather than the awkward fractional
values that result from numerically equivalent conversions
That is why a metric setofwrenches for use on a foreign car
is different from standard U.S wrench set
Because more states are on the U.S system than on the
because the sizeof the text would be unwieldy if dual units
U.S units
1.8 Closing Comments
The profession of traffic engineering is a broad complex
one Nevertheless it relies on key concepts and analyses and
emphasizes both the basic principles and current (in 2009)
standards and practices You must keep abreast of changes
that influence the latter
At this writing drafts oftheManual all Uniiorm
Devices (MUTeD) expected tobeofficially released
inlate2009 or IO.are available online Also a great dealof
Capacitv Nanna! is available as well Because of this they
15
have been incorporated into this text for completeness
It should be remembered however, that until they areofficially released some ofthis material issubject to change.even if major changes are not expected Consult these docu-ments directly to ensure that you are using the officialversions ofthe methodologies standards included intheseimportant source documents
References
6th Edition, Institute of Transportation Engineers
2. Uniform Vehicle Code and Model Traffic
National Committee on Uniform Traffic Laws and
3 Manual on Uniform Control Devices,
Highway Administration, Washington DC
available
4. Munua]. Edition TransportationResearch Board Washington DC 2000
5. A Oil Geometric andSirens.
5th Edition American Association of State Highway
6 Koonce P et.11 Signal Timing Manual Federal
Highway Administration, Washington DC 2009
7 Edwards, J.D Jr., Editor, Transportation Planning
Handbook, Edition, Institute of Transportation
8. Trip Generation 8th Edition.lTEInformational Report.Institute of Transportation Engineers, Washington DCJanuary 2008
9. Parking Generation 3rd Edition lTE Informational
Report Institute ofTransportation Engineers Washington
DC January
Trang 27Road User and Vehicle
Characteristics
2.1 Overview of Traffic Stream
Components
To begin to understand the functional and operational aspects
under-stand how the various elements of a traffic system interact
Further the characteristics of traffic streams are heavily
influenced by the characteristics and limitations of each of
these elements Five critical components interact in a traffic
• Traffic control devices
• The general environment
This chapter provides an overview ofcritical road user
and vehicle characteristics Chapter 3 focuses on the
charac-teristics ofstreets and highways, and Chapter provides an
overview oftraffic control devices and their use
The general environment also has an impact on traffic
operations, but this is difficult to assess in any given situation
Such factors as weather, lighting, density ofdevelopment, and
local enforcement policies all playa role in affecting traffic
operations These factors are most often considered qualitatively
with occasional supplemental quantitative information available
2.1.1 Dealing with Diversity
Traffic engineering would be a great deal simpler ifthevariouscomponents of the traffic system had uniform characteristics.Traffic controls could be easily designed if all drivers reacted
to them in exactly the same way Safety could be more easilyachieved ifall vehicles had uniform dimensions weights, andoperating characteristics
Drivers and other road users, however, have widelyvarying characteristics The traffic engineer must deal withelderly drivers as well as 18-year-olds aggressive drivers andtimid drivers and drivers subject to myriad distractions bothinside and outside their vehicles Simple subjects likereactiontime, vision characteristics, and walking speed become com-plex because notwo road users are thesame
Most human characteristics follow the normal bution (see Chapter7).The normal distribution is character-izedbya strong central tendency (i.e., most people havecharacteristics faJling into a definable range) For example.most pedestrians crossing a street walk at speeds between3.0 and 5.0 ft/s However, a few pedestrians walk either
the proportions of the population expected to fall into theseranges Because of variation, it is not practical to design a
Trang 2818 CHAPTER ROAD USER AND VEHICLE CHARACTERISTICS
system for "average characteristics If a signal is timed for
example to accommodate the speed of crossing
pedestrians about half of pedestrians would walk at a
slower rate and beexposed to unacceptable
Thus most standards are geared tothe percentile"
(or percentile") characteristic In general terms,
percentile is in a distribution for which the stated
percentage of the population has characteristic that is less
than or equal to the specified value In terms of walking
speed, for safety demands that we accommodate
slower walkers The lSth walking is used
because only of the population walks slower than this,
Where driver reaction time isconcerned, the percentile
value is used because 85% of the population has a reaction
time that is numerically equal to or less than this value This
approach leads to design practices and procedures that safely
accommodate of the population What about the
remain-ing I One ofthecharacteristics of normal is
that the extreme ends of the distribution highest and
lowest extend to plus or minus In practical
terms the highest and lowest of the distribution
rep-resent very extreme that could not be
accommodated into design practices the
exis-tence of road users who may possess characteristics not
the (or 15th) percentile is considered but most
standard practices and criteria do not directly accommodate
them Where feasible higher percentile characteristics can be
employed
Just as road-user characteristics vary the characteristics
of vehicles widely as well Highways must he designed
to accommodate motorcycles, the full range of automobiles
and a wide range of commercial vehicles including
double-and triple-back tractor-trailer combinations Thus widths,
forexample, must accommodate the largest vehicles expected
to use the facility
The economic crises of2008-2009 and the poor
condi-tion of the U.S automobile industry may very well lead to
drastic changes in the vehicle fleet With the emphasis on
cleaner and more efficient vehicles cars may be getting
smaller and lighter Their relative safety within a mixed traffic
stream still containing large trucks and buses may become an
important issue requiring new planning and design
approaches The traffic professional must be prepared to deal
with this and other new issues when they arise
Some control over the range of road-user and vehicle
characteristics is maintained·through licensing criteria and
federal and state standards on vehicle design and operating
characteristics Although these are important measures the
traffic engineer must still deal with a wide ofroad-user
and vehicle characteristics
2.1.2 Addressing Diversity through
Uniformity
Although traffic engineers little control mer drivervehicle characteristics design roadway trafficcontrols is in the core of their professional practice In bothcases a strong degree of uniformity of approach is desirable.Roadways ofasimilar type and function should familiar
"look" to drivers traffic control devices should as uniform
as possible Traffic engineers to information todrivers inuniform Although this does not uniformreactions from drivers it least naITOWS the range ofbehavior
as become accustomed to and familiar with the cuestraffic engineers design into the system Chapters and 4 dealwith roadways and controls, respectively, and treat the issue ofuniformity in detail
2.2 Road Users
ofcharac-teristics that and doinfluence the driving task In systemwhere the driver is in complete control of operations
driver characteristics Much the task of traffic is
to find to drivers with information in a clear.effective manner induces proper
The characteristics ofutmost importance arevisual acuity factors and the reaction process The two overlap,inthat reaction requires the use ofvision for most cues.Understanding how information is received and processed iskey element in the design of and controls
There are other important characteristics as well
an important element in the (i.e horns.emergency vehicle sirens, brakes squealing erc.) Althoughnoting this is important, however, no traffic element candesigned around audio cues because hearing-impaired andeven deafdrivers are licensed Physical strength maybeen important in the past, but the evolution ofpower-steeringand power-braking systems has eliminated this as a majorissue with possible exception of professional drivers oftrucks, buses and other heavy vehicles
Ofcourse one ofthe most important human factors thatinfluences driving is the personality and psychology of thedriver This however, is not easily quantified and is difficult toconsider in design.Itis dealt with primarily through enforce-ment and licensing procedures that attempt toremove orrestrictdrivers who periodically display inappropriate tendencies, asindicated by accident and violation experience
Trang 292.2 ROAD USERS· 19
When drivers initially apply for, or their licenses,
are asked to take an eye test, administered either by the state
motor vehicle agency orby anoptometrist orophthalmologist
who fills out an appropriate form for the motor vehicle
agency The test administered is a standard chart-reading
exercise that measuresstatic visual is, the ability
tosee small stationary details clearly
Many of theother factors listed in Table 2.1 reflect thedynamic nature of the driving task and the fact that mostobjects to be viewed by drivers are in relative motion withrespect to the driver's eyes
Because static visual acuity is the only one of thesemany visual factors examined as a prerequisite to issuing adriver's license, traffic engineers must expect and deal withsignificant variation in many of the other visual characteris-tics ofdrivers
Table2.1: Visual Factors in the Task
Although static visual acuity is certainly an important
charac-teristic, it is hardly the only visual factor involved in the
driv-ing task TheTraffic Engineering Handbook [1)provides an
excellent summary of visual factors involved in driving, as
shown in Table 2.1
Discrimination between different colors
Detecting dark-clothed pedestrians
at night
Judging speed of an approaching vehicle.Identifying the colorofsignals
• Acute orclear vision cone-3 Dto 10° around the line
of sight; legend can be read only within this narrowfield of vision
Reading distant traffic signs
Figure2.1 illustrates three distinct fields of vision, each ofwhich isimportant to the driving task [2]:
Judging the speed of cars crossingourpaths
Adjusttochanges in light uponentering a tunnel
Changing focus from dashboarddisplays to roadway
Sample Related Driving Task(s)Definition
Ability to see small details clearly
Detecting changes in visual image size
Change in the shape ofthe lenstobring images intofocus
Change in sensitivity to differentlevels oflight
Seeing objects that are similar inbrightness to their background
Seeing objects moving acrossthe field ofview
Static Visual Acuity
Depth Perception Judgment ofthedistance ofobjects Passing on two-lane roads with
oncoming traffic
Dynamic Visual Acuity Ability to see objects that are in motion
relative to the eye Reading traffic signs while moving.
Glare Sensitivity
Changing the direction ofgaze
Ability toresist andrecover fromthe effects ofglare
Scanning the road environment forReduction in visual performance due
to headlight glare
Peripheral Vision Detection ofobjects atthe side of
Vergence Angle between the eyes' line ofsight Change from looking at the dashboard
to the road
(Source: Used with permission ofInstitute ofTransportation Engineers, Dewar, Users," Traffic Engineering Handbook,
Edition, Chapter 2 Table 2-2, p.8,1999.)
Trang 3020 CHAPTER 2 ROAD USER AND VEHICLE CHARACTERISTICS
Acute vision cone
Figure 2.1: Illustration ofFields ofVision
• Fairly clear vision cone-lO° to 12° around the line
ofsight; color and shape can beidentified inthis field
• Peripheral vision-This field may extend up to90° to
the right and left ofthe centerline of the pupil, and
up to 60° above 70° below the line of sight
Stationary objects are generally not seen in the
peripheral vision field but the movement of objects
through this field is defected
Illustration of FieldsofVision
These fields ofvision, however are defined for astationary
per-son In particular, the peripheral vision field narrows, as speed
increases, to as little as 1000 at 20 milh and to 40° at 60
The driver's visual landscape is both complex and
rap-idly changing Approaching objects appear to expand in size
while other vehicles and stationary objects are in relative
motion both tothe driver and toeach other The typical driver
essentially samples the available visual information and
selects appropriate cues to make driving decisions
The fields of vision affect a number oftraffic
engineer-ing practices and functions Traffic signs, for example, are
placed so that they can be read within the acute vision field
without requiring drivers to change their line of sight Thus
they are generally placed within a 10° range of the driver's
expected line of sight, which isassumed to beinline with the
highway alignment This leads tosigns that are intended tobe
read when they are a significant distance from the driver; in
tum, this implies how large the sign and its lettering must be
tobecomprehended atthat distance Objects orother vehicles
located in the fairly clear and peripheral vision fields may
draw the driver's attention to an important event occurring in
that field, such as theapproach ofa vehicle on an intersection
street ordriveway or achild running into the street after aball
Once noticed the driver mav tum hisorher head to examine. .
the details ofthesituation
Peripheral vision is the single most important factorwhen drivers estimate their speed The movement of objectsthrough the peripheral vision field is the driver's single mostimportant indicator of speed Old studies have demonstratedtime and again that drivers deprived of peripheral vision(using blinders in experimental cases) and deprived of aworking speedometer have little idea of how fast they aretraveling
2.2.2 Important Visual Deficits
Anumber ofvisual problems can affect driver performance andbehavior Unless the condition causes asevere visual disability,drivers affected by various visual deficits often continue todrive Reference 3 contains an excellent overview and discus-sion ofthese
Some of the more common problems involve cataracts,glaucoma peripheral vision deficits ocular muscle imbal-ance, depth perception deficits and color blindness Driverswho have surgery to correct problem experiencetemporary orpermanent impairments Other diseases, such asdiabetes, can have a significant negative impact on vision ifnot controlled Some conditions like cataracts glaucoma
jf untreated lead to blindness
Although color blindness is not the worst of these ditions, it generally causes some difficulties for the affecteddriver because color is one of the principal means to impartinformation Unfortunately, one ofthe most common forms ofcolor blindness involves the inability to discern thedifferencebetween red and green In the case oftraffic signals this could.
con-have a devastating impact on the safety of such drivers Toameliorate this difficulty tosome degree, some blue pigmenthas been added to green lights and some yellow pigment hasbeen added to red lights, making them easier to discern bycolorblind drivers Also, the location ofcolors on signal headshas long been standardized, with redon the top and green onthe bottom ofvertical signal heads On horizontal heads, red
is on the left and green on the right Arrow indications areeither located ona separate signal head orplaced below ortothe right ofball indications on a mixed signal head
2.2.3 Perception-Reaction Time
The second critical driver characteristic isperception-reactiontime (PRT) During perception and reaction the driver mustperform four distinct processes [4]:
• Detection In this phase, an object or condition of
concern enters the driver's field of vision, and the
Trang 31I
2.2 ROAD USERS
driver becomes consciously aware that something
requiring a response is present
• Identification. In this phase, the acquires
suffi-cient information concerning the object or condition
to allow the consideration ofan appropriate response
• Decision.Once identification ofthe object orcondition
is sufficiently completed, the driver must analyze the
information and make adecision about how torespond
• Response. After a decision has been reached, the
response is now physically implemented by the driver
Insome ofthe literature, the four phases ofPRT are referred
toas perception, identification, emotion, and volition, leading
to the term "PIEV This text uses but you should
understand that it is equivalent to PIEV time
Design Values
Like all human characteristics, perception-reaction times
widely among drivers, as do a variety ofother factors
includ-ing the type and complexity of the event perceived and the
environmental conditions at the time ofthe response
Nevertheless design values forvarious applications must
be selected The American Association ofState Highway and
Transportation Officials (AASHTO) mandates the use of
2.5 seconds for most computations involving braking reactions
based on a number of research [6-9] This value
is believed to be approximately a 90th percentile criterion
(i.e., ofall drivers have a PRT as fast orfaster than 2.5 s)
For signal timing purposes, the Institute ofTransportation
Engineers[J0]recommends aPRT time of1.0 s.Because ofthe
simplicity ofthe response and the preconditioning of drivers to
respond to signals, the PRT time is significantly less than that
for a braking response on an open highway Although this is a
lower value it still represents an approximately 85th percentile
for the particular situation ofresponding to atraffic signal
AASHTO criteria recognize that in certain
more complex situations, drivers may need considerably more
time to react than 1.0 or2.5 s Situations where drivers must
detect and react tounexpected events, or adifficult-to-perceive
information source in a cluttered highway environment ora
situation in which there is a likelihood of error involving
either information reception, decisions, or actions all would
result in increased PRT times Some ofthe examples cited by
AASHTO of locations such situations might exist
include complex interchanges and intersections where
unusual movements are encountered and changes in highway
cross sections such astoll plazas, lane drops, and areas where
the roadway environment iscluttered with visual distractions
Where acollision avoidance maneuver is required, AASHTO
21
criteria call foraPRT of3.0 sforstops on rural roads and 9.1 sfor stops onurban roads Where collision avoidance requirespath, and/or direction changes AASHTO recommends
a PRT of between 10.2 and 1].2 s on rural roads, 12.1 and12.9 son suburban roads and 14.0 and 14.5 s onurban roads
Expectancy
The concept ofexpectancy isimportant to the driving task andhasasignificant impact on the perception-reaction process andPRT Simply drivers react more quickly to situations they
expectto encounter as opposed to those that theydonot expect
toencounter There are three different types of expectancies:
• Continuity Experiences of the immediate past are
generally expected to continue Drivers do forexample, expect the vehicle they are following tosuddenly slow down
• Event.Things that have not happened previously willnot happen Ifno vehicles have been observed enter-ing theroadway from a small driveway over a reason-able period oftime, then the driver will assume thatnone will enter now
• Temporal. When events are cyclic, such as a trafficsignal the longer a given state is observed, driverswill assume that it more likely achange will occur.Figure 2.2 illustrates the impact ofexpectancy on PRT.This study byOlsen et al.lI l]in 1984 was controlled obser-vation of student reacting to a similar hazard whenthey were unaware it would appear, and again where theywere told to look forit In a third experiment, a red light wasadded to the dash to initiate the braking reaction The PRTunder the "expected" situation was consistently about 0.5 sfaster than under the "unexpected" situation
Given the obvious importance ofexpectancy on PRT.traffic engineers must strive to avoid designing
events into roadway systems and traffic controls If there areall right-hand ramps on given freeway, for example, left-hand ramps should be avoided if at all possible If absolutelyrequired, guide signs must be very carefully designed to alertdrivers tothe existence and location ofthe left-hand ramp; sothat when they reach itis no longer "unexpected."
Other Factors Affecting PRT
In general, PRTs increase with a number offactors, including
(1)age, (2) fatigue, (3) complexity ofreaction, and (4) ence ofalcohol and/or drugs in the driver's system Althoughthese trends are well documented, they are generallyaccounted for in recommended design values, with the
pres-,
Trang 3222 CHAPTER ROAD USER AND VEHICLE CHARACTERISTICS
Thus the reaction distance may computed
The reaction distance is simply the PRT multiplied bythe initial speed ofthe vehicle Because speed is generally inunits ofmi/h and PRY isin units ofseconds itis convenient toconvert speeds toftls for use:
2.2.4 Pedestrian Characteristics
The vehicle will travel 220.5ft (approximately II to 12 i
car lengths) before the driver even engages the brake The cation of this is frightening If the overturned truck is tothe vehicle than 220.5 ft when noticed by the not onlywill the driver hit the he orshe will do soat full speed- r
impli-r.
60 milh Deceleration begins only when the brake is
after the perception-reaction process has been completed.
d,= 60 = ft
One of the most critical safety problems in any andstreet system involves the interactions of vehicles and pedes-trians A substantial number of traffic accidents and fatalitiesinvolve pedestrians This is not surprising because in anycontact between a pedestrian and a vehicle, the pedestrian isatasignificant disadvantage
Virtually all ofthe interactions between pedestrians andvehicles occur as pedestrians the street at intersectionsand at mid-block locations At signalized intersections safe accommodation ofpedestrian crossings is as critical as vehi-cle requirements in establishing an appropriate timing pattern.Pedestrian walking speed in crosswalks is the most importantfactor inthe consideration ofpedestrians in signal timing
x
., '.:
I I
Figure 2.2: Comparison of Perception-Reaction Times
between Expected and Unexpected Events
iSonrce: with permission of Transportation Research
Board National Research Council, Olson P et al "Parameters
Affecting Stopping Sight Distance." NCHRP Report 270.
Washington DC.
exception of the impact of alcohol and drugs The latter are
addressed primarily through enforcement of ever-stricter
driving while intoxicated/driving under the influence
(OWl/OUT) Jaws in the various states, with the intent of
removing such drivers from the system, especially where
repeated violations make them a significant safety risk Some
ofthe more general affects of alcohol and drugs, as well as
aging ondriver characteristics are discussed inalater section
The most critical impact of perception-reaction time is the
distance the vehicle travels while the driver goes through the
process In the example ofa simple braking reaction, the PRT
begins when the driver first becomes aware of an event or
object in his or her field of vision and ends when his or her
foot is applied to the brake During this time, the vehicle
con-tinues along itsoriginal course atits initial speed Only after
the foot is applied to the brake pedal does the vehicle begin to
slow down in response tothe stimulus
Trang 33the pedestrian population In 2008 and serious sion oflowering the general standard speed to3.5 ftls
thought tobelikely at this writing
One problem with standard walking speeds involvesphysically impaired pedestrians A study of pedestrians withvarious impairments and assistive devices concluded thataverage walking speeds forvirtually all categories were lowerthan the standard ftls usedin signal timing [13] Table 2.3includes some ofthe results of this study These and similarresults of other studies suggest that more consideration needstobe given to the needs of handicapped pedestrians
Cane/CrutchWalkerWheelchairImmobilized KneeBelow-Knee AmputeeAbove-Knee AmputeeHip Arthritis
Rheumatoid Arthritis (Knee)
When a pedestrian crosses at an uncontrolled (either by nals or YfELD signs) location, either at an intersec-tion orat a mid-block location, the pedestrian must select anappropriate in the traffic stream through which to cross
vehicles in any encroaching on the pedestrian's crossingpath As the pedestrian waits to cross he or she views gapsand decides whether to"accept"or the gapfor asafecrossing Some studies have used a gap defined as the dis-tance between the pedestrian and the approaching vehicle atthe time the pedestrian begins his or her crossing An earlystudy [14] using the latter approach resulted in an 85th per-centile gap of approximately 125 ft
Gap acceptance behavior, however is quite complexand varies with anumber of other factors, including the speedofapproaching vehicles the width ofthe street the frequencydistribution of gaps in the traffic stream waiting time, andothers Nevertheless,this is animportant characteristic that
50thPercentile WalkingSpeed(ftls)
At unsignalized crossing locations, gap-acceptance
behavior of pedestrians another important consideration
"Gap acceptance" refers to the clear time intervals between
vehicles encroaching on thecrossing path and the behavior of
pedestrians in "accepting" them tocross through
Table 2.2 shows 50th percentile walking speeds
forpedestri-ans ofvarious ages Note thatthese speeds were measured as
part of a controlled experiment [12] and not specifically at
intersection or mid-block crosswalks Nevertheless the
results are interesting The standard walking speed used in
timing signals is 4.0 ft/s, with 3.5 ftls recommended where
older pedestrians are predominant Most studies indicate that
these standards are reasonable and will accommodate 85% of
Walking Speeds
(Source: Compiled from Eubanks, 1 and Hill, P., Pedestrian
Accident Reconstruction and Litigation, 2nd Edition, Lawyers &
Judges Publishing Tucson J 999.)
Trang 3424 CHAPTER 2 ROAD USER AND VEHlCLE CHARACTERISTICS
must be considered due to its obvious safety implications
Chapter 18, for presents warrants for (conditions
justifying) the imposition of traffic signals One of these is
devoted entirely tothe safety ofpedestrian crossings
Pedestrian Comprehension ofControls
One of the problems in designing controls for pedestrians is
generally poor understanding of and poor adherence to such
devices One questionnaire survey of 4,700 pedestrians [J5]
detailed many problems of misunderstanding' The proper
response to a flashing "DON'T WALK" signal, forexample,
was not understood by of road users who thought it
meant they should return to the curb from which they started
The meaning ofthis signal is not to start crossing while it is
flashing; itissafe tocomplete a crossing if the pedestrian has
already started todo so Another study[J6]found that
viola-tion rates for the solid "DON'T signal were higher
than in most cities, that the use of the flashing "DON'T
for pedestrian clearance was not well understood,
and that pedestrians tend not to use pedestrian-actuated
sig-nals Chapter (on signal timing) discusses some of the
problems associated with pedestrian-actuation buttons and
theiruse that compromise both pedestrian'comprehension and
the efficiency ofthe signalization Since this study was
com-pleted the flashing and solid "DON'T WALK" signals have
been replaced by the Portland orange "raised hand" symbol,
Thus the of providing forasafe environment for
pedestrians is not an easy one The management and control of
conflicts between vehicles and pedestrians remains difficult
2.2.5 Impacts ofDrugs and Alcohol
on Road Users
The effect ofdrugs and alcohol on drivers has received
well-deserved national attention for many years, leading to
substan-tial strengthening of DWI/DUI laws and enforcement These
factors remain, however, a significant contributor to traffic
fatalities and accidents And drivers are not the only road users
who contribute tothe nation's accident and fatality statistics
Consider that in 1996, 47.3% of fatal pedestrian accidents
involved either adriver orapedestrian with detectable levels of
alcohol in their systems For this group, 12.0% of the drivers
and32.3%ofthe pedestrians had blood-alcohol levels above
0.10%, the legal definition of"drunk" in many states More
telling is that 7% of the drivers and 6%ofthe pedestrians had
detectable alcohol levels below this limit
The importance of these isolated statistics is to make
the following point: Legal limits for DWUDUI do not define
the point at which alcohol andlor drugs influence the road
user Recognizing this is important for individuals to ensuresafe driving itisnow causing many statestoreduce their legallimits on alcohol to 0.08% and for some to considertolerance" criteria (0.01%)for drivers for thefirstyearortwo they are licensed
Figure 2.3summarizes some studies on the effects ofdrugs and alcohol on various factors Note that formany factors, impairment of driver function begins at levelswell below the legal limits-for some factors at blood-alcohollevels aslow as0.05%
What all ofthese factors add up to is an impaired driver.This combination ofimpairments leads to longer PRT times.poor judgments, and actions that can and do cause accidents .Because few ofthese factors can be ameliorated by design orcontrol (although good designs and well-designed controlshelp both impaired and unimpaired drivers), enforcement andeducation are critical elements in reducing the incidence ofDWIIDUI and the accidents and that result
The statistics cited in the opening paragraph ofthis tion also highlight thedanger caused bypedestrians who are
sec-Stecrina BeC'lll1t' andConfused Faster Abrupt andStarting Phvsical Uncoordination Tendcnc\'to Distracted Drive Closer
10
Decision Visual
Information I
-0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 DAD
BAC('Yo)
Figure 2.3: Effects of Blood-Alcohol Level on Driving
Tasks
(Source: Usedwith permission of Institute of Transportation Engineers, Blaschke,1.,Dennis M., and Creasy, E."Physical and Psychological Effects of Alcohol and Other Drugs on Drivers,"
ITE Journal 59, Washington DC 1987.)
Trang 352.3 VEHlCLES
impaired bydrugs or alcohol In the case of impaired
pedes-trians, the danger is primarily to themselves Nevertheless, if
crossing a street or highway is required, "walking while
impaired" is also quite dangerous Again, enforcement and
education are the major weapons in combating the problem
because not agreat deal can be done through design orcontrol
to address the issue
Both motorists and pedestrians should also be aware of
the impact ofcommon prescription and over-the-counter
med-ications on their performance capabilities Many legitimate
medications have effects that are similar to those ofalcohol
and/or illegal drugs Users ofmedications should always be
aware ofthe side effects ofwhat they use (amost frequent effect
ofmany drugs is drowsiness), and to exercise care and good
judgment when considering whether ornottodrive Some
legit-imate drugs can have adirect impact onblood-alcohol levels and
can render a motorist legally intoxicated without "drinking."
2.2.6 Impacts of Aging on Road Users
As life expectancy continues to rise the number ofolder
driv-ers has risen dramatically over the past several decades Thus
it becomes increasingly important to understand how aging
affects driver needs and limitations and how these should
impact design and control decisions Reference 17 isan
excel-lent compilation sponsored by the National Academy of
Sciences ona wide range oftopics involving aging drivers
Many visual acuity factors deteriorate with age,
incIud-ing both static and dynamic visual acuity, glare sensitivity and
recovery, night vision, and speed of eye movements Such
ailments as cataracts glaucoma, macular degeneration, and
conditions have negative impacts on vision
The increasing prevalence of older drivers presents a
number of problems for both traffic engineers and public
officials On one hand, at some pointdeterioration ofvarious
capabilities must lead to revocation of the right todrive On
the otherhand, driving is the principal means ofmobility and
accessibility in most parts of the nation, and the alternatives
for those who can no longer drive are either limited
orexpen-sive The response to the issue ofan aging driver population
must have many components, including appropriate licensing
standards, consideration of some license restrictions on older
drivers (e.g., a daytime-only license), provision of efficient
and affordable transportation alternatives, and increased
con-sideration oftheir needs, particularly inthe design and
imple-mentation of control devices and traffic regulations Older
drivers may be helped, for example, by such measures as
larger lettering on signs, better highway lighting, larger and
brighter signals, and other measures Better education can
25
serve tomake older drivers more aware ofthe types ofdeficitsthey face and how best to deal with them More frequenttesting ofkey characteristics such aseyesight may help ensurethat prescriptions for glasses and/or contact lenses arefrequently updated
2.2.7 Psychological, Personality,
and Related Factors
Over the past decade traffic engineers and thepublic
Commonly applied to drivers who lose control ofthemselvesand react toawide variety of situations violently, improperly,and almost always dangerously, the problem (which hasalways existed) is now getting well-deserved attention Road rage, however is a colloquial term, and is applied to every-thing from a direct physical assault by one road user onanother to a variety of aggressive driving behaviors
According to the testimony of Dr John Larsen to theHouse Surface Transportation Subcommittee on July 17,
1997 (as summarized in Chapter of Reference I), thefollowing attitudes characterize aggressive drivers:
• The desire to get to one's destination quickly aspossible leading to the expression of anger at otherdrivers/pedestrians who impede this desire
• The need tocompete with other fast cars
• The need to respond competitively to other sive drivers
aggres-• Contempt for other drivers who do not drive, look,and act as they do onthe road
• The belief that it is their right to "hit at otherdrivers whose driving behavior threatens them
psycho-logical and personal displeasure over the traffic situation heorshe has encountered It does, however, remind traffic engineersthat drivers display a wide range of behaviors in accordancewith their own personalities and psychological characteristics.Once again, most of these factors cannot be addresseddirectly through design orcontrol decisions and are best treatedthrough vigorous enforcement and educational programs
2.3 Vehicles
In 2007, approximately 240 million registered vehicles were
in the United States, a number that represents more than onevehicle perlicensed driver The characteristics ofthese vehi-cles vary aswidely asthose ofthe motorists who drive them
Trang 3626 CHAPTER 2 ROAD USER AND VEHICLE CHARACTERISTICS
In general, motor vehicles are classified by AASHTO
[5] into four main categories:
o Passenger cars-all passenger cars SUVs, minivans.
and pickup trucks
o Buses-intercity motor coaches transit buses school
buses and articulated buses
o Trucks-single-unit trucks, trailer and
tractor-semi-trailer combination vehicles
o Recreational \'ehicles-motor homes cars with
vari-ous types of trailers (boat campers motorcycles etc.)
Motorcycles and bicycles also use highway and street facilities
but are not isolated as aseparate category because their
charac-teristics do not usually limit ordefine design orcontrol needs
Anumber ofcritical vehicle properties mustbeaccounted
forin the design ofroadways and traffic controls These include:
o Braking and deceleration
• Acceleration
• Low-speed turning characteristics
• High-speed turning characteristics
In more general terms, the issues associated
of vastly differing size weight and operating characteristics
sharing roadways must also beaddressedbytraffic engineers
2.3.1 Concept of the Design Vehicle
Given the immense range ofvehicle types using street
high-way facilities, it is necessary to adopt standard vehicle
charac-teristics for design and control purposes For geometric design
AASHTO has defined 20 vehicles," each with specified
characteristics The 20 design vehicles are defined as follows:
SU = single-unit truck
BUS-40 = intercity bus with
wheelbaseBUS-45 = intercity bus with a45-ft
wheelbaseCITY-BUS = transit bus
S-BUS36 = conventional school bus for
65 passengersS-BUS40 = large school bus for 84passengers
A-BUS = articulated bus
WB-40 = intermediate semi-trailer
(wheelbase =40ft)
WE-50 = intermediate semi-trailer
(wheelbase =50ft)WB-62 = interstate semi-trailer
(wheelbase =62 ft)WB-65 = interstate semi-trailer
(wheelbase =65 ft)WB-67D = double trailer combination
(wheelbase =67ft)WB-IOOT = triple semi-trailer/trailers
(wheelbase =J00ft)WB-J09D = turnpike double semi-trailer/trailer
(wheelbase =109ft)
PIT = passenger car and camperP/B = passenger car and boat trailerMHIB = motor home and boat trailer
In considering the selection ofadesign vehicle, itmust
be remembered that all of the street and highway work must be accessible toemergency vehicles, including fireengines, ambulances emergency evacuation vehicles andemergency repair vehicles among others Therefore thesingle-unit truck is usually the minimum design vehicleselected for most local street applications The mobility ofhook-and-Iadder fire vehicles is enhancedbyhaving rear-axlesteering that allows these vehicles to negotiate sharper turnsthan would normally be possible forcombination vehicles, sothe use of a single-unit truck as a design vehicle forlocalstreets isnot considered tohinder emergency vehicles.The passenger car is used as a design vehicle only inparking lots, and even there, access to emergency vehiclesmust be considered For most otherclasses or types ofhigh-ways and intersections, the selection ofadesign vehicle mustconsider the expected vehicle mix In general, the designvehicle selected should easily accommodate 95% or more ofthe expected vehicle mix
Trang 37net-2.3 VEHICLES 27
Path of front
7.77 m [25.5 ft]
o5ft 10 ft
o 2.5 m scale
• Assumed steering angle is
tSovrce: Used with permission of American Association of
State Highway and Transportation Officials, APolicy all
Geometric Design ofHighways andStreets, 5thEdition,
Washington Exhibit 2-13.)
10.67 ftl 12.20 m ftlwheelbase I I
13.87 m [45.5 ftl Typical tire size and space between tires applies toall trailers.
I
Turning templates provide illustrations of the manydifferent dimensions involved in a low-speed tum.Indesign-ing for low-speed turns, the minimum design turning radius isthe minimum centerline radius plus halfof the width ofthefront ofthe vehicle
• Low-speed turns I0 mi/h )
High-speed turns(>1O mi/h)
2.3.2 Turning Characteristics of Vehicles
Low-speed turns are limited by thecharacteristics of
thevehi-cle because the minimum radius allowed by the vehithevehi-cle's
steering mechanism can be supported at such speeds
High-speed turns are limited by the dynamics of side friction
between the roadway and the tires, and by the superelevation
(cross-slope) of the roadway
The physical dimensions of design vehicles are also
4.25 ft for passenger car to 13.5 ft for the largest trucks
Overhead clearances of overpass and sign structures,
electri-cal wires and other overhead appurtenances should be
suffi-cient to allow the largest anticipated vehicles to proceed
Because all facilities must accommodate a wide variety of
potential emergency vehicles use of 14.0 ft for minimum
clearances isadvisable for most facilities
The width of design vehicles ranges from 7.0ft
forpas-senger cars to 8.5 ft for the largest trucks (excluding special
"wide vehicles such as a tractorpulling a prefabricated
or motor horne) This should influence the design of such
fea-tures as lane width and shoulders Formost facilities, it is
desirable touse the standard 12-ft lane width Narrower lanes
may be considered for some types of facilities when
neces-sary but given the width ofmodem vehicles 10ftis a
reason-able minimum for virtually all applications
Low-Speed Turns
There are conditions under which vehicles must make turns:
AASHTO specifies minimum design radii for each of the
design vehicles based onthe centerline turning radius and
min-imum inside turning radius ofeach vehicle Although theactual
turning radius of a vehicle is controlled bythe front wheels,
rear wheels do not foJlow the same path Rear wheels
"off-track" asthey are dragged through the turning movement
Reference 5 contains detailed low-speed turning
templates for all AASHTO design vehicles Figure 2.4 shows
an example (for a WB-40 combination vehicle) Note that the
minimum turning radius is defined by the track of the front
outside wheel The combination vehicle, however,
demon-strates considerable "off-tracking" of the rear inside wheel,
effectively widening thewidth of the "Jane" occupied by the
vehicle asitturns Thepath oftheinside rear wheel is not
cir-cular and has a variable radius
Trang 3828 CHAPTER 2 ROAD USER AND VEHICLE CHARACTERISTICS
High-Speed Turns
When involved in ahigh-speed tum onahighway curve,
cen-tripetal forces ofmomentum are exerted on the vehicle
con-tinue in a straight path To hold the curve these forces are
opposed by side friction and superelevation
Superelevation is the cross-slope of the roadway
always with the lower edge in the direction of the The
sloped roadway provides anelement ofhorizontal support for
the vehicle Side-friction forces represent the resistance to
sliding provided across the plane of the surface between the
physics, the relationship governing vehicle operation on a
curved roadway is:
Minimum design turning radii range from 24.0 ft for
passenger car to high of 60.0 ft for the WB-109D
double tractor-trailer combination vehicle Depending
on the specific design vehicle, the minimum inside curb
radius is generally considerably smaller than the minimum
design turning radius, reflecting the variable radius of the
off-tracking characteristics of the design vehicle should be
con-sidered when determining how far from travel lanes to
locate(orcut back) the curb In a good design, the outside
tonego-tiate its path without"spilling over" into adjacent lanes as
the turn is negotiated This requires that the curb setback
must accommodate the maximum off-tracking of the design
vehicle
where: e =superelevation rate,
f =coefficient of side friction
S=speed ofthe vehicle ftls
normal range of superelevation rates andside-friction factors.
It is also convenient to express vehicle speed in milh Thus:
where all terms are previously defined, except that isthe speed in milh rather thanft/sasin Equation 2-2
The normal range of superelevation rates is from aminimum of approximately0.5%tosupport side drainage to a
rates are used Where icing conditions are expected the mum superelevation rate is generally limited to toprevent astalledvehiclefrom sliding toward the inside ofthecurve
maxi-for design are based on wetconditions.They vary with speed and are shown inTheoretically a road can be banked to fully oppose cen-tripetal force without using side friction at all This is ofgenerally not done because vehicles travel at a range ofspeeds,and the superelevation rate required in many cases would beexcessive High-speed turns ona flat pavement may be fully sup-ported byside friction aswell, butthis generally limits the radiusofcurvature orspeed at which thecurve may besafely traversed.Chapter 3 treats the design ofhorizontal curves and therelationships among superelevation, side friction, curve radii,and design speed in greater detail
design a minimum radius of curvature is computed based onmaximum values ofeandf For example if a roadway has a
e= andf =O II.the minimum radius is computed as:
-The superelevation rate is the total rise in elevation across
the travel lanes ofthe crosssection(ft)divided bythe width
of the travel lanes (ft), expressed as a percentage (i.e.,
multiplied by 100) AASHTO [5] expresses supereJevation
as a percentage in its2004criteria, but many other
publica-tions still express thesuperelevation rate as a decimal
proportion
Equation 2-2 is simplified by noting that the term
"0.0 Ief' is extremely smaJl and may be ignored for the
Trang 39-whereF=coefficient offorward rolling or skidding friction.
When the effects ofgrade are considered, and where a ing cycle Jeading to a reduced speed other than are con-sidered, theequation becomes:
brak-where: G=grade.%
S, =initiaJ speed, mi/h
5 f =final speed, mi/hWhen there isanupgrade, a isused: a isused for down-grades This results in shorter braking distances on upgrades,where gravity helps deceleration, and longer braking distances
on downgrades, where gravity iscausing acceleration
In previous editions of Reference5, braking distanceswere based on coefficients of forward skidding friction thatvaried with speed lnthe latest standards, however, a standarddeceleration rateof11.2ftls2is adopted asa design rate Thisisviewed as a rate that can bedeveloped on wetpavements bymost vehicles.It is also expected that 90% of drivers
(1.0755
2 )32.2
where:db =braking distance, ft
5= initial ftls
a=deceleration rate, ftls2
where Sis the speed inmi/h. Note that the 1.075 factor isderived from the more exact conversion factor between mi/hand ft/s (1.4666 ).It is often also useful to express this
equation in terms ofthe coefficient offorward rolling ding friction, whereF=alg and gisthe acceleration due
Another criticaJ characteristic of vehicles is their ability to
stop (or decelerate) once the brakes have been engaged
Again, basic physics relationships are used The distance
Thus, for the curve as described, 49.1 mi/h is the maximum
safe speed at which itshould be negotiated
Note thatthis isbased on the design condition ofa wet
pavement andthat higher speeds would be possible under dry
For the example given the equation issolved for the given
values ofe(6%)andR(800ft)using various values ofjfrom
Table 2.4 Computations continue until there is closure
between the computed speed and the speed associated with
the coefficient ofside friction seJected Thus:
mi/h(70mi/hassumed)
Jt can also beused to solve for a maximum safe speed, given a
radius ofcurvature and values fore and!Jf
ahigh-way curve with radius of 800 ft has a superelevation rate of
the maximum safe speed can be estimated However,
doing sorequires that the relationship between thecoefficie11l
ofside friction, [, and speed, as indicated in Table be
taken into account Solving Equation 2-3 for 5 yieJds:
=49.0 mi/h (50mi/hassumed)
46.5 mi/h (60mi/h assumed)
5 \/]5*800*(0.06+0.J2)
The correct result isobviously between 49.0 and50.2 mi/h If
straight-line interpolation is used
"
Trang 4030 CHAPTER 2 ROAD USER AND VEHICLE CHARACTERISTICS
decelerate at higher rates This, then, suggests a standard
friction factor for braking distance computations of
db = 30(0.348 ±O.OJ G) (2-7)
It therefore, estimated that the speed of the vehicle
imme-diately before the pavement skid was 44.9 mi/h This, of
course can be compared with the speed limit to determinewhether excessive speed factor in the accident
2.3.4 Acceleration Characteristics
Acceleration Rate (ft/s2)for:
Table 2.5: Acceleration Characteristics of a Typical
(Source: Compiled from Traffic Engineering Handbook, 5th Edition,
Institute ofTransportation Engineers Washington 2000, Chapter Tables 3-9and 3-10.)
for a passenger car with a weight-to-horsepower ratio of
30 Ibs/hp and atractor-trailer with aratio of 200 lbs/hp.Acceleration is highest atJaw speeds and decreaseswith increasing speed The disparity between passenger carsand trucks is significant Consider the distance required forcar and a truck to accelerate to mi/h Converting speedfrom mi/h toft/s:
Once again note that the 1.075 factor is derived using
the more precise factor for converting milh to fils
Consider the following case: Once the brakes are
engaged, what distance iscovered bringing a vehicle traveling
(Sf=0 mi/h) Applying Equation 2-7:
S, =\/05*30*0.25) +202=
The braking distance formula is also a favorite tool of
speed of a vehicle using measured skid marks and an estimated
final speed based on damage assessments In such cases, actual
estimated values ofF are rather than thestandard design
value recommended by AASHTO Thus Equation 2-6 is used
Consider the following case: An accident investigator
estimates that a vehicle hit a bridge abutment at speed of
mi/h, based on or her assessment of damage Leading up
to the accident location heorshe observes marks of 100 ft
on the pavement(F =0.35) and 75 fton the grass shoulder
(F =0.25) There isnograde An estimation ofthe speed ofthe
vehicle at the beginning ofthe skid marks isdesired
In this case Equation 2-6 is used to find the initial speed
ofthe vehicle(S;)based on a known (orestimated) final speed
(Sf)' Each skid must be analyzed separately, starting with the
grass skid (for which a final speed has been estimated) Then:
This is the estimated speed of the vehicle at thestart of the
grass skid; it isalso the speed ofthe vehicle at theendof the
pavement skid Then: