Why not use the sensing radar that is built into new vehicles to identify a person or persons that it deems too close or a danger by virtue of them either standing, walking or moving by other means in or into the path of the moving vehicle (both forwards and in reverse) and, if recognised as a potential danger,use the data to get the onboard electronics to produce an ever increasing warning sound the closer the vehicle gets to the to the person(s) who are judged to be in danger (by the car). Does the sound need to be “on” up to 12mph if there is nobody around!