By Peter DeMarco
Most modern traffic signals don’t operate blindly; they actually respond to the number of cars entering the intersection from various directions. There are several ways traffic lights can “see’’ cars: radio waves or microwaves can be bounced to detect moving objects; cameras can perceive changes; cars can pass over pressure-sensitive plates. But often, traffic lights know how many cars are in an intersection thanks to sensors placed strategically under the pavement.
It works this way: Picture the first car in line at a red light. About an inch underneath the pavement where the car’s idling, an electrically charged wire has been spread in a 6-by-6-foot loop to form a magnetic field. Whenever a car passes over the loop, it disrupts the field.A record of every disruption is sent to a control box, a metal cabinet about 4 feet tall positioned nearby. You may have never noticed the boxes, but Gillon assured me they exist at just about every signaled intersection around here. A tiny computer inside the control box analyzes the disruptions. If they keep happening, it means that cars are still entering the intersection from that direction, so the light should stay green. If the disruptions stop, it means there are no more cars and the light should switch to red.
A traffic light can’t stay green forever, so there’s usually a maximum number of seconds allowed before the computer switches the light to red. On a main street, it might be 30 or 40 seconds, Gillon said. But with no vehicles coming, the light might switch to red in half that time.