i have a friend that is a indiana dot civil engineer. all you need is a strobe light that flashes at the correct frequency. i know rocco has seen the white flashing light that is on teh front of every emergency vehicle. next time an ambulance fire truck or cop car is coming towards you, look at the roof of teh car or on its light bar and you will see a white strobe flashing. that is what signals teh system to change the light, it also turns on teh big bulb on the traffic signal itself signafying that the sytem is engaged. i cant remember teh frequency right now but next time i talk to mike i will ask. also some systems are coded and can report to the local pd what vehicle has set off teh system by a certain flash sequence. and that folks is how it works