Our traffic lights (and the gates to my complex) are triggered by some kind of elctrical hoobajoob that detects the presence of a vehicle sitting at the intersection.
Traffic generally flows until a car stops at another part of the intersection, which will cause the light for that car to eventually change to green.
Larger motorcycles produce enough electric "field" (hoobajoob) to trigger the sensors. My little 300cc Honda did not for many of them, putting me in a situation where I had to wait until the intersection was clear enough to squirt through on a red light. Scores of articles have been written on how to get a motorcycle to trigger these sensors, from where to place the bike over the sensor, putting the sidestand down, etc.