TMC - Traffic Message Channel

The Traffic Message Channel (TMC) is a specific application of the FM Radio Data System (RDS) used for broadcasting real-time traffic and weather information. Data messages are received silently and decoded by a TMC-equipped car radio or navigation system, and delivered to the driver in a variety of ways. The most common of these is a TMC-enabled navigation system that can offer dynamic route guidance - alerting the driver of a problem on the planned route and calculating an alternative route to avoid the incident.

Here is how it works: data related to traffic flows, incidents, weather etc. are gathered from traffic monitoring systems, emergency services, motorists’ calls etc., and are collated at a central traffic information centre. They are then passed to the TMC traffic information service provider, who generates TMC messages according to the ALERT-C coding protocol.

Standard TMC user messages provide five basic items of broadcast information:

* event description, details of the weather situation or traffic problem and its severity
* location, the area, highway segment or point location affected
* direction and extent, identifying the adjacent segments or point locations affected, and the direction of traffic affected
* duration, how long the problem is expected to last
* diversion advice, whether or not drivers are advised to find an alternative route.

The service provider sends the coded messages to the appropriate FM radio broadcaster for transmission as an RDS (Radio Data System) signal within normal FM radio transmissions. The TMC data are received by the vehicle radio and antenna, and decoded by a TMC decoder. This reconstructs the original message, using a database of event and location codes, which is presented to the driver as a visual or spoken message.

It takes typically about 30 seconds from the first report of a traffic incident to the traffic information centre until the same information is available in the vehicle.

The user can select the language used to present the traffic information by the TMC receiver, typically a navigation system or car radio. The user can also opt to filter messages, so that only those concerning the immediate route are selected.

The service is free in almost all of Europe but you need pay a subscription fee in the U.S. and the U.K usually around $60 per year.