I wonder if we could have radio beacons for pedestrians, in addition to the current braking systems which use radio waves reflected from electronically passive

I wonder if we could have radio beacons for pedestrians, in addition to the current braking systems which use radio waves reflected from electronically passive pedestrians. It might not even require GPS or anything complicated; just respond to car’s radar squawks, so that it can measure the distance and relative speed from the delay and Doppler shift. You’d need to use frequency that bounces around nicely, to “get around” other cars blocking the line of sight. We can’t do that with passive pedestrians, because we can’t tell the echo from pedestrian from the (also moving, like other cars) background. This could work more reliably and at much higher distances. Of course someone must have tested that already; I’d be grateful for a link :-)