Greener GPS

A great idea over dinner from Rajendre Khargi, Chair of OneWorld Netherlands. Why not add a third option to the in-car navigation systems? As well as choosing the fastest route or the most fuel efficient, why not have the option of the route that saves the most CO2? This would presumably be based on estimations of cruising speeds, traffic etc. Then at the end of the journey it would show how much carbon you had saved. It sounds to me like something people would be glad to have and to use.