A device which knows the speed limit of any road and automatically slows speeding cars is being tested in London.

The Intelligent Speed Adaptation (ISA) is being developed by Transport for London and uses new digital speed maps of the city.

Using GPS technology is 'knows' the speed limit and display it on a dashboard screen, when a button is pressed the vehicles speed is also limited.

Initially the gadget will be tested in taxis and busses but bosses hope motorist will be able to have the speed limiter installed within a year.

When traveling at the correct speed a smiling green face is displayed, if speeding it turns into an angry red face ... if going ridiculously fast it probably turns into Jeremy Clarkson.