I am not assuming they RFID would have personal or identifiable information on them... just a cheap technology that emits a signal that can be detected... the key word in there is "cheap". And the detectors could also be used by joggers, bikers, or even installed in children's bicycle helmets. I think the major hurdle would be vehicle manufacturer buy-in. There has to be a profit based reason, like a tax write-off or something to make them want to add it as a feature.
BTW... my middle child was hit by a car... a very very slow moving car backing from a driveway. Something like a proximity detection device installed in his helmet would have allowed him to skip the bruises and tears (no real damage, luckily).
Anyone want to help me build a company? :)