When you arrive at an intersection and the lights change, did you ever wonder why? I did. This is the very thorough answer I received from the RCMP:
Traffic lights are activated by the presence of vehicles by two different systems. One system uses wire loops in the roadway that sense the presence of a vehicle. When the vehicle is above the loop a signal is sent to the traffic signal control computer which is located in a large metal box at the intersection, either on a pole or on the shoulder of the road. Where the loops have been installed in existing asphalt you will see a loop or rectangle of tar on the surface. Where the loops are placed under new asphalt there is no visible sign of the location of the loops.
A second more modern system uses black and white cameras placed on the signal arm near the traffic lights. The camera is directed at the lanes approaching the intersection. When a vehicle enters the lane near the intersection the presence changes the grey scale or image in the camera. The change in the grey scale sends a signal to the traffic signal control similar to the signal sent by the wire loop in the road surface. The camera senses which lane the vehicle is in and the signal determines whether an advanced left turn or green light is required.
Intersections with red light enforcement cameras have several signal loops in the roadway. These loops measure the speed of the vehicles as they enter the intersection and the movement of a vehicle into the intersection when the light is red, which activates the red light camera. The camera takes two pictures, one of the vehicle entering the intersection and one of the vehicle traveling through the intersection, both which also show the phase/colour of the light The photo captures the licence plate and the computer records the time, date and speed of the vehicle.
To my knowledge radar is not used to control signals.
The police do not have the ability to change the traffic signals as they approach. Fire trucks have the ability to change the lights as they approach by sending a signal to a receiver on the signal arm. Not all traffic signals have this receiver.