Emergency Beeps

Today I saw a fire engine, on the way to some emergency, attempting to cross a 4-lane road in traffic. Trying to be a good citizen, I pulled my car onto the roadside but I noticed several other cars apparently oblivious to the rather large vehicle with flashing lights and a horn heading towards their general direction. And I noticed that all of those cars who didn't seem to notice the truck had their windows up.

Pure speculation on my part, but it seems to me that the drivers of at least some of those cars had something coming out of their car speakers and so couldn't hear the horn (and didn't look for the truck).

So my idea is some sort of transmitter, affixed to emergency vehicles such as firefighters' trucks, ambulances and police cars, that sends a signal along a certain radius. That signal would be picked up by the cars within that radius (500 metres/yards?) by a receiver in each car and a distinct, identifiable (but not overwhelming) code would be broadcast from the stereos/radios in those cars to their respective drivers. The code/signal would be something simple like a short "beep beep beep" that is repeated every 5 seconds or so, as long as the emergency vehicle was within 500 yards/metres. The code would be broadcast even if CDs/MP3s/cassettes are being played.

If the radio/CD/etc in a car is not switched on, no signal would be broadcast through the speakers even if the emergency vehicle is inside the radius. When the emergency vehicle arrives at its destination and is put in "park", the signal transmissions end.