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 protocol into RDS Type 8A groups 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.

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.

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