Traffic message channel

Traffic Message Channel (TMC) is a technology for delivering traffic and travel information to motor vehicle drivers. It is digitally coded using the ALERT C or TPEG protocol into Radio Data System (RDS) carried via conventional FM radio broadcasts. It can also be transmitted on Digital Audio Broadcasting or satellite radio. TMC allows silent delivery of dynamic information suitable for reproduction or display in the user's language without interrupting audio broadcast services. Both public and commercial services are operational in many countries. When data is integrated directly into a navigation system, traffic information can be used in the system's route calculation.

Development Detailed technical proposals for an RDS-TMC broadcasting protocol were first developed in the European Community's DRIVE programme research project RDS-ALERT, a partnership of the BBC, Philips, Blaupunkt, TRRL and CCETT led by Castle Rock Consultants (CRC). The main goal of the project was to develop and build consensus upon a draft standard for broadcasting RDS-TMC traffic messages in densely coded digital form. An initial proposal for defining RDS-TMC data fields had been made to the European Conference of Ministers of Transport (ECMT) in Madrid, based on a scheme developed by CCETT and Philips in the Eureka-sponsored CARMINAT research project. This proposal required the use of at least two 104-bit RDS data groups for each message. Within these RDS Groups, 32 bits per group would be used for traffic data, giving a total traffic message length of 64 bits. A second proposal, by Bosch-Blaupunkt and the German Road Research Institute BASt, sought to use just a single RDS Group per traffic message. Then, in 1987, the CEC invited Castle Rock Consultants to lead a joint team that would take TMC development a stage further. CRC produced a proposal for a modified BASt/Blaupunkt single group message definition, which became known as the ALERT A coding scheme. Tests also continued at CCETT and BBC on the CARMINAT approach, which formed the basis of an alternative ALERT B coding proposal. A major question addressed in the Alert A scheme was the total number of traffic event locations to be coded. Initial estimates suggested that, in Europe, a maximum of 65,000 significant junctions might be needed for the Federal Republic of Germany. An efficient coding system would require only 16 bits to code these, simply by numbering each intersection from 1 to 65535. Calculations for France, Britain and elsewhere suggested that around 30,000 to 40,000 locations should be enough for most European national or U.S. statewide systems. A standard 16-bit location code was, therefore, adopted for inter-urban networks. The Madrid proposal of 1987, by comparison, had required 33 bits to code problem location, with separate fields for road number, road class, area of the country, etc. These 33 bits gave a theoretical total of 8.5 billion location codes, most of which could never be used. After consultation with ECMT, a combined approach was developed called the ALERT C Protocol that aimed to combine the best features of each approach. ALERT A and C replaced the CARMINAT message categories cause, effect and advice with a single 11-bit basic message code. This permits up to 2048 basic message phrases to be broadcast. The new ALERT protocols significantly increased the efficiency of message coding, shortening the basic message content from 18 to 11 bits. In conjunction with the revised location codes, which saved 17 of the 33 bits previously assigned, this allowed the great majority of traffic messages to be broadcast using a single TMC data sequence. In 1991, ECMT recommended moving forward with further testing of the protocols. The work continued with a larger consortium including Volvo and Ford Motor Company in the European Commission's DRIVE II project ATT-ALERT.

Operation Each traffic incident is binary-encoded and sent as a TMC message. Each message consists of an event code, location code, expected incident duration, affected extent and other details. The message contains a list of up to 2048 event phrases defined by 11 binary bits (of which 1402 were in use as of 2007) that can be translated by the receiver into the user's language. Some phrases describe individual situations such as a crash, while others cover combinations of events such as construction causing long delays. In Europe, location code tables are maintained on a national level. Those location tables are integrated in the maps provided by in-vehicle navigation system companies such as HERE Technologies and TomTom and by vehicle manufacturers such as Volvo. In other countries, such as the U.S. and Canada, private companies maintain the location tables and market TMC services commercially. Sources of traffic information typically include police, traffic control centers, camera systems, traffic speed detectors, floating car data, winter driving reports and roadwork reports.

Coordination TMC-Forum, a non-profit organization whose members included service providers, receiver manufacturers, car manufacturers, map vendors, broadcasters (public and private), automobile clubs, and public authorities, was a forum to discuss traffic information related matters. It maintained the TMC-Standard (ISO 14819). On 11 November 2007, the TMC-Forum and the TPEG-Forum merged into the Traveller Information Services Association (TISA). TISA has taken over all of TMC-Forum's activities and responsibilities.

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