Lots of people listen to musich while walking or running. And this creates a dangerous situation - since you cannot hear the cars approaching from behind. Not until it is very close.
Could not the music players warn the runner/walker that a car is coming?
Could possibly pick up the sound of something approaching fast (sound getting stronger). And then play a warning signal so that the runner knows that something is coming from behind - fast.
That could give the runner/walker a few extra seconds to verify what is coming. Step (more) out of the way. Or even stop and let the car pass.
Any music device inventor/innovators taking on this challenge?