Editing Traffic message channel (section)

== Functionality ==
RDS-TMC is a low-bandwidth system. Each RDS-TMC traffic message comprises 37 data bits sent at most 1–3 times per second, using a low capacity data channel primarily designed for [[FM radio]] station name identification and tuning. Compressing traffic incident descriptions in multiple languages into 16 bits for a location, 11 bits for an event description code, plus 3 bits for the event's extent and a few extra bits for the duration/system management was necessary due to pre-existing constraints in the [[Radio Data System|RDS]] standard. Almost all the other broadcast data bits were already assigned from each 104-bit RDS Group.

A major design challenge of RDS-TMC was to find a way of describing traffic event locations across an entire state or country.<ref name=CRC /> Such a system could not convey precise [[latitude]]-[[longitude]] data (available 25 years later using [[GPS]] in applications such as [[Waze]]). Instead, RDS-TMC relies on the use of location tables that point only to significant highway junctions. The precision of each traffic event's location is low compared to that of modern smartphone devices. The user's navigation system locates a driver to about 3 metres (10 feet), but only knows, for example, that a crash took place between Exit 3 and Exit 4, northbound on a particular motorway. This limitation requires that traffic events (accidents, congestion, burst water mains, faulty traffic lights, etc.) have to be superimposed onto maps by mapping the reported location to the TMC location table. If the nearest location table point lies at some distance from the exact position of the incident, then the report appears on a section of main road between two junctions instead of at its exact location. The limited precision can make a significant difference as to how navigation devices interpret the incident, potentially leading to an occasional poor route choice.

In the US and elsewhere, systems such as CARS (Condition Acquisition and Reporting System)<ref>P. Davies, K. Virshbo, Castle Rock Associates, Portland, Oregon, USA. ''CARS (Condition Acquisition Reporting System) and the CARS Group.'' Archives of Transport System Telematics, Volume 2, Issue 1, February 2009.</ref><ref>Dennis W. Johnson.''Congress Online: Bridging the Gap Between Citizens and Their Representatives'' p41.</ref> can pinpoint event locations or their start and end points with one-metre precision. These real-time data are published in XML for access by companies such as [[Google]] and [[TomTom]].<ref>{{cite web| url = http://www.iowadot.gov/511/xmlfeed.html| url-status = dead| archive-url = https://web.archive.org/web/20120719072657/http://www.iowadot.gov/511/xmlfeed.html| archive-date = 2012-07-19| title = 511 Information - Iowa DOT}}</ref> These incident reports can be delivered to mobile phones and handheld devices in vehicles.<ref>{{cite web|url=https://play.google.com/store/apps/details?id=crc.carsapp.id|title=Idaho 511 – Android Apps on Google Play|author=Castle Rock Associates|work=google.com}}</ref><ref>{{cite web|url=http://www.dot.state.mn.us/newsrels/14/07/30mobileapp.html|title=Minnesota Department of Transportation|work=state.mn.us|url-status=dead|archive-url=https://web.archive.org/web/20150402195047/http://www.dot.state.mn.us/newsrels/14/07/30mobileapp.html|archive-date=2 April 2015}}</ref> However, major real-world traffic incidents usually spread from hundreds of metres up to many kilometres, once traffic backups have developed. On motorways and other major roads, there are typically few or no detours available between significant junctions, which are all included in the TMC location tables. Many traffic report locations are only approximate, and as queues grow, locations can change swiftly. So GPS-based systems are more ''precise'', but are not necessarily more [[Accuracy and precision|''accurate'']].