The TMC data adheres to the "Alert C" standard which can encode up to 2048 individual messages which are then translated into the chosen language of the device. Each incident is given a unique locator, each country has a look-up table for these locations and they are typically road numbers and junctions, in addition there is then some textual data about the incident and also the time and an event code.

Alert-C is closely related to the Datex standard but is limited in order that it can run over the RDS (Radio Data System) that works on VHF/FM radio frequencies and is in itself an EU standard. Almost all implementations see the TMC data transmitted over VHF/FM radio although some navigation system manufacturers like TomTom download the data and make it available over a GSM/GPRS data link.

So just how does TMC work?

Firstly we need some data, some information about traffic flow on the roads, we'll use the UK as an example. We have 2 TMC data providers, iTIS holdings and RAC Trafficmaster (RTT). Both of these providers need data from vehicles using the roads but they use differing methods to get it. iTIS use floating car data (FCD) this uses either GPS or cellular (mobile phone) tracking of a large fleet of vehicles to create a statistically accurate picture of road traffic. iTIS use Eddie Stobart's haulage fleet, AA patrol vehicles and National Express coaches, all in all about 50,000 vehicles. Trafficmaster use their own network of road side sensors which already provide accurate flow data to provide  their TMC data feed. These sensors (blue cameras on blue poles) are often mistaken for speed cameras, in fact they are reading the 4 central characters of your number plate and then calculating your average speed as you arrive at the next camera. Trafficmaster do not store the whole number and as such cannot track you and cannot provide information to the Police.

Now this data needs to get to you and me via a transmission over the air. To do this both companies use the RDS FM system and iTIS transmit on a silent data channel as part of Classic FM's signal and Traffic master use a commercial network of stations from Gcap and Chrysalis.

This RDS data is an always on channel and the individual TMC messages are broadcast to any listening receivers. It is not a free service, the device that receives the signal needs to be licensed and all UK systems license per unit from one of the 2 companies. The data is then decoded and shown alongside mapping data from either Navteq or Tele Atlas within the GPS navigation system.

The data is then translated and displayed on your device, the manner of which is entirely up to the manufacturer. Some opt for a simple list and nothing more, others like Garmin and TomTom show the traffic using symbols on the maps themselves and systems like those installed in top end cars like Porsche will allow you to automatically re route around a congested road. Some GPS navigation systems and car makers systems come with TMC "free" others make it a chargeable option or even require you to buy a TMC receiver to plug into the main unit.

In practice the system is only as good as the data it relies on and TMC is often criticised for being out of date or slow to react to an incident. This is merely a result of the sample size and talk of allowing road users with suitable systems to send data to each other surfaced recently in the UK press, a kind of Wikipedia for traffic data!

So that's how TMC works.

See our Navigation System Reviews - Including TMC

Published - 15/04/2007

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