Harry, you'll want to explore 3M's Opticom traffic control systems. My father's department in Ontario had it back in the '80s and a lot of departments have it now. I should say - that is one of, and the oldest, traffic signal pre-emption system in use. There are others that may use different technology, but as I say, Opticom is the most prominent.
Opticom systems use a strobe flash with a specific frequency and wavelength, with or without an IR component, mounted on vehicles. Fire and ambulance definitely have them, some police have them, and in Alberta I have even seen a few gas company trucks with them. Some places also give them to transit buses. I believe you can have several levels of priority, so the transit buses don't pre-empt if a fire truck has already captured the intersection from another direction.
The receivers on the equipped intersections (often just major intersections or along routes that lead to/from fire stations - it's not cost effective to equip all signalized intersections) look like tiny steam whistles turned on their sides... not much bigger than a Sharpie marker. Below is an example from Wikipedia - the floodlight is lit up when the Opticom "captures" the intersection, so the approaching crew knows they have the light. (If they don't get the floodlight, they are to treat the intersection as a red light, because either the Opticom has failed or another crew has captured the intersection from another direction.)
Several years ago, someone was flooding eBay (etc) with schematics and/or built devices that would allow the general public to take control of an Opticom-equipped intersection. These were quickly made illegal. Contrary to popular belief, though, flashing your high beams really fast or using a Radio Shack strobe lamp or things like that won't cut it. You need a legitimate Opticom sender.