Most messages provide information about a location (e.g., a stretch of road, an intersection, or a region) and they refer to it by using a location reference. This is an identifier that can be interpreted without ambiguity by the receiving sys- tem. In RDS-TMC, locations are predefined and precoded, and the codes are stored in location code tables. The maximum number of codes in one table is determined by the field length for location codes in RDS-TMC; that is, 16 bits, which corresponds to 65,536 possible codes. The disadvantage of these tables is that they need to be created and maintained as was explained above, and the receiving system must use exactly the same table as the one used for encoding of the message. Otherwise, the message is not receivable.
The rules for location reference coding are specified in CEN prestandard prENV 278/7/3/0005: May 1997. These rules apply to ALERT-C messages only. ALERT-Plus uses a different system. Because of the provisional nature of ALERT-Plus in 1997, no further reference is made to that particular coding method, and all details that follow are extracted from the prestandard and apply thus to ALERT-C messages only.
Predefined locations are referenced by their location code, which is the tabular address of a number of prestored location details. Each table of stored Intelligent Transport Systems and RDS-TMC 139
140 RDS: The Radio Data System Table 7.6
List of Event Categories (Update Class) Used to Structure the Event Database and Number of Event Codes Falling Into Each of These Classes
Update Class Events
1. Level of service 418
2. Expected level of service 58
3. Accidents 28
4. Incidents 14
5. Closures and lane restrictions 123 6. Carriageway restrictions 19
7. Exit restrictions 12
8. Entry restrictions 6
9. Traffic restrictions 56 10. Carpool information 11
11. Roadworks 30
12. Obstruction hazards 70 13. Dangerous situations 17
14. Road conditions 54
15. Temperatures 8
16. Precipitation and visibility 58 17. Wind and air quality 13
18. Activities 41
19. Security alerts 13
20. Delays 142
21. Cancellations 31
22. Travel time information 9
23. Dangerous vehicles 15
24. Exceptional loads / vehicles 20 25. Traffic equipment status 30 26. Size and weight limits 11 27. Parking restrictions 5
28. Parking 29
29. Reference to audio broadcasts 8
30. Service messages 22
31. Special messages 4
Number of events (max. 2048) 1375
locations must be given a unique location table number by one unique agency in each country or state. A country code (note: RDS-TMC uses the RDS coun- try codes given in Appendix G) identifies the agency responsible for location reference coding and which one defined the location table and its number.
Many location references extend through several adjacent areas or road sections. The concept of primary and secondary locations is then used to indi- cate the extremities of the affected sections without having to list all the inter- vening places. For example, if an accident occurs at km 14.2 on the E15 (A26 road in France) and the resulting queue extends back to km 10.9, the situation location can be defined as E15, “km 14.2–10.9," where km 14.2 is defined as the primary location and km 10.9 is the secondary location. The primary loca- tion is taken to be where the cause of the problem can be found, whenever a cause can be pinpointed geographically. However, both primary and secondary locations will lie on the same road.
For the primary location, the location reference is the nearest downstream location in the direction of travel. The secondary location is indicated in terms of extent; that is, the number of steps back along the road through other prede- fined locations. Alternatively, a distance marker may be used.
All location codes belong to a unique location table. Within any particu- lar location code table, each location has one unique number in the range 1–63,487. The other 2,048 numbers are reserved for EUROAD, an agreed upon concept used for coding messages to international travellers on the Trans European Road Network (TERN).
RDS-TMC uses a hierarchical structure of predefined locations. A system of pointers provides upward references to higher level locations containing the specified location. For example, Kent would have an upward area reference to southeast England, which will be upwards referenced to the United Kingdom, then the British Isles, then Europe.
Junction 25 on the M1 motorway in the United Kingdom would have a section of route referenced to a motorway segment (e.g., Leicester–Sheffield).
This segment will then be referenced upwards to the whole road (i.e., the motorway M1).
Also, Junction 25 on a motorway may be offset to Junction 26 in the positive direction, and to Junction 24 in the negative direction.
In many cases, events affecting road traffic cover a number of locations, such as where an accidents results in long tailbacks. The ALERT-C protocol defines such occurrences by addressing the location of the accident as the pri- mary location, then identifying the end of the tailback by using the direction and extent fields. These fields consist of four bits in total: one direction bit and three extent bits. The direction bit indicates the queue growth and not the direction of traffic flow. The extent bits identify the number of locations along Intelligent Transport Systems and RDS-TMC 141
the road that are affected by the problem, with a maximum of eight (primary location and seven related locations). An extent of 1 would identify the secon- dary location (the end of the event’s extent) as being the next location along the same road from the primary location. An extent of 3 would force the receiver to search the database for the third location along the same road from the primary location as is illustrated in the example given in Table 7.7.
Three additional optional columns exist. These are the motorway exit number and the deviation reference road number in positive and negative directions to give diversion advice.
142 RDS: The Radio Data System
Table 7.7
Coding Example for Location Referencing Location
Code Type
Road
Number Name 1 Name 2 Ref A Ref L
Neg.
Offset Pos.
Offset
949 L3 E1 X-town Y-town 2009 — 948 950
2009 A6.2 — Greater
neighbourhood
— 1 — — —
4420 P3.2 E1 Bridge — 2009 949 4456 4423
4423 P1.3 E1 Place A N207 2009 949 4420 4459
4459 P3.3 E1 Parking — 2009 949 4423 4460
4460 P1.3 E1 Place B — 2009 949 4459 4461
Location code: Identifier in the range 0 to 65,536 describing a stretch of road, an intersection or a region;
Type: Defines the kind of location in three main categories and a number of sub-categories: Areas (A), Linear locations (L), and Points (P). In the example given above, L3 means a road link, A6.2 a metropolitan area, P3.2 a bridge, P1.3 a motorway junction, and P3.3 a service area;
Road number: The road reference number;
Name 1: Name of the primary location that will be presented by the receiver;
Name 2: Name of a secondary location—required to describe a road segment;
Ref A: Pointer to the area to which the location belongs;
Ref L: Pointer to the stretch of road to which the location belongs;
Negative offset: Pointer to the previous area or location (e.g., on the same stretch of road);
Positive offset: Pointer to the next area or location (e.g., on the same stretch of road).