A road sensor detects whether the traffic that is passing belongs to a certain category, and this signal is sent to the traffic crossing controller to run the program signals corresponding to this situation. For example, supposing that traffic priority is given to trucks, when a passing truck is detected it must be allowed to pass through the intersection.
Signal priority systems reduce the amount of time that a vehicle of a certain category is stopped at the lights by extending the duration of green lights and reducing the duration of red lights. As a result, travel time is reduced for these vehicles.