Her ein my town, we have something called an opticom on the fire department's trucks. There is a sensor near the traffic lights. When the opticom button is pushed, the sensor sees it allowing the light to turn green, while in sight. When the vehicle moves away, the light turns red again. And Xplorer, this is THE most creative modding team in Emergency4 history. They probably hhave already thought about your idea. Hell, they must have millions of ideas we don't know about!
Yeah, the town next to me has a traffic light control system on almost all of their stoplights. Their system works on the sound of the siren, so any emergency vehicle can use it from any department.