The GTM 20 is an integrated FM TMC (Traffic Message Channel) traffic receiver, antenna, and cigarette lighter adapter. It receives digital information (where available) on traffic and road construction tie-ups that your compatible Garmin unit uses to offer an alternate route.
TMC data, gives prompt, accurate notification of accidents, road construction, police, or emergency action, etc. So you can visually monitor traffic flow, and if necessary, avoid traffic tie-ups.
With the GTM 20, you see traffic developments in real-time, as they happen, so you can make the right decision sooner. This system alerts you when a traffic problem is on your route, and proposes an alternate route.