There are cities that allow ambulances and other emergency vehicles to control the traffic lights (turning them green so the ambulance can get to the emergency faster). This is done with a device called a MIRT (MIRT = mobile infrared transmitter). Eventually troublemakers get a MIRT and start causing trouble. People love to drive around and never get a red light.
It occurred to me that the problem is that all the MIRTs are the same. If each MIRT used some sort of encryption, where each ambulance had its own private key, then each traffic light would know which ambulance was passing by. When a device was stolen, its key could be removed from the list of valid keys, and all copies of that device would be disabled at once.
To make this work, you need all the traffic lights to receive updates periodically. This could either be networking (WiMax?) or it could be someone driving around with a special device that can update the keys through the MIRT. Of course, if someone stole that device, it's trouble!