People seem to listen to the radio in their cars a lot. This is despite the adverts which both plague and enable the medium.
Today’s invention is a light which is visible externally on vehicles and which changes colour, depending on the station to which their radio is tuned.
This would allow people listening to the same programme to share eg a broadcast joke whilst in traffic jams and develop more of a feeling of community. It might also allow audience members to try listening to the favourite station of people driving a vehicle which they admire.
The most useful aspect of this would be to enable automated, realtime collection of listening statistics by area (using roadside, colour-sensitive cameras) so as to tune the costs and timing of advertising (it might even allow for a new form of social segmentation: ie are those Bentley owners really listening to Radio 3?).