Car-to-car radio 'ahem'

The main horn would remain the same noisy apparatus that everyone hates. In an emergency, your hand would go there.

But: when the guy ahead is asleep, you would press a button on the dash and a very weak radio signal would sound an alarm in the car ahead. Make it as focused as possible, so that it would be unlikely to bother more than the people just in front of your car. (Maybe infrared would be better, but could you keep the transmitter and receiver clean enough in all weather?)

Make the signal more of an 'ahem' than a 'blat'. You want only to lightly remind your fellow driver that it is indeed time to proceed through the intersection.

1) Make it a legal requirement that the thing cuts the other car's sound system (and yours, to be fair) for the duration of the signal.

2) Put a one- or two second delay on it so idiots wouldn't use it to harrass all drivers who aren't used to driving dragstrips on weekends.

3) Perhaps add a standard set of signals that drivers could transmit: "Green light" would be standard, but maybe you could quickly reset it to transmit "Lights out" or "Trunk open" or "Baby still on roof" or whatever.

This would be a boon to non-drivers in the area -- residents, workers, pedestrians -- who would not have to listen to the horns of drivers who just want to communicate with other drivers.