that will sense if a car is approaching from behind and alert you with either a flashing light or sound alert? Like some kind of miniature doppler sensor?

I haven't found a bike-mounted or helmet-mounted mirror I like, and I don't want to ride in the middle of the road when there are cars coming up from behind. But if there are no cars behind me, I prefer not to ride in the gravel and broken glass that accumulates on the side of the road where bikers are always told to go.