Fortunately, this problem has an easy solution.

IEEE 802.15.4 UWB (Ultra-Wideband) radios with timestamping functionality allow measuring the time of flight (well, not directly, but it can be inferred from an exchange of messages) of your signal. With some added crypto, it isn't difficult to build a solution which is limited to a specified distance. You can get as precise as ±20cm.

This means that you can build a system which will not work beyond a certain distance, because signals will take too long to travel.

I'm surprised this hasn't been picked up by car manufacturers yet. Perhaps there is too little market pressure.