Discussion
RDS-TMC is an extremely low-bandwidth system, with each RDS-TMC message comprising only 37 data bits sent at most 1–3 times per second, using a very basic data channel primarily designed for FM radio tuning and station name identification. Compressing entire traffic incident descriptions in multiple languages into a mere 16 bits for a location, 11 bits for an event code, plus 5 bits for an extent and a few more bits for the duration and system management was necessary due to constraints already set in the pre-existing RDS standard (almost all the broadcast data bits were already taken).
Given so few bits to work with, one major design challenge of RDS-TMC was to find a way of using 16 bits (about 65000 locations) to describe an entire state or country. Such a system could not convey latitude-longitude data (so easily available 25 years later with great precision using GPS). Instead, RDS-TMC has to rely on the use of relatively lean location tables, pointing only to significant road junctions along defined national and regional highways. The precision of each traffic event's location is low compared to modern GPS devices. The user's navigation system locates a driver to about 3 metres, but only knows that the crash is between Exit 3 and Exit 4, northbound on the motorway. This limitation is because traffic events (accidents, congestion, burst water mains, faulty traffic lights, etc.) have to be superimposed onto the maps in users' GPS devices by matching the reported location into the location table. If the nearest location table point (nearest significant junction) is located some distance from the point of the crash, then the traffic event might be shown on the device as being on a section of main road between two junctions instead of its real location. Even if the lack of accuracy causes an error of only a short distance, this can make a significant difference as to how the program in the GPS device will interpret the event in relation to the user's planned route. For example, if there is roadwork close to a junction, the position of the roadwork may be interpreted as being a short distance away from its true location. This could place the apparent roadwork on the other side of the junction. The consequence would be that the GPS device might not divert the user from the roadwork because it was assumed to not affect the route.
In the US and elsewhere, systems such as CARS already exist that could track and pinpoint event locations with accuracies of a meter, and these real-time data are already being published in XML for access by companies such as Google and TomTom. These incident reports are also being received from Google on mobile phones and handheld devices that are in vehicles already. These are not currently in use in most navigational systems, for reasons that are unclear.
Read more about this topic: Traffic Message Channel
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