Traffic Signal Operation 101

If we drive in a populated area, like most of our villages, we cannot avoid intersections controlled by three-color traffic signals. But do you know what makes them change? Here’s a brief tutorial on their operation.

Let’s start with a simple four-legged intersection, which allows green on one street while the other is red, and vice versa. Most signals “rest” on green for the busiest street, and if the signal’s detection system — which may be loop sensors embedded in the pavement or video cameras on the mast arms — detects a vehicle stopped on the street with the red signal, it will change to green after a pre-determined time, assuming the vehicle that was there remains. A delay is normal to give the vehicle a chance to turn right on red.

At more complex intersections, there may be turning lanes, with associated turn arrows controlling traffic turning movements. Throw in pedestrian signals, and it complicates the signal system even more. At some intersections, all traffic is held in place while pedestrians cross, and in other intersections, pedestrians and traffic move simultaneously.

At most intersections with traffic lights controlling the orderly movement of vehicles and pedestrians, the phases of the signals are controlled by computers. Based on traffic volumes, the allowable “green” for any direction or lane has a pre-determined maximum allowable time. If there is enough traffic to reach this maximum time, the signal will change, even if there are more vehicles wanting to proceed. Should there not be enough vehicles to maximize a phase, the light will change sooner.

If you leave too large a gap with the vehicle in front of you, the signal may change back to red before you get to it, as the sensors may not detect your vehicle in time, forcing you and any vehicle behind you to wait for the next cycle.

In all cases at intersections, drivers need to be attentive, even if they are facing a red signal. A pet peeve of most drivers is to be waiting at a signal behind another car that doesn’t go when the signal first turns green. It’s tempting to look around at the sights while waiting for the light to change, but your task is to pay attention to your driving and that includes watching for the signal to change.

Furthermore, if a driver fails to “go” when a turn arrow allows it to proceed, drivers several vehicles behind may not make the light if the signal reaches the maximum allowable number of seconds for the green.

The next time you are waiting at a red light, stay focused on the signal, and when it allows you to go, confirm other traffic and pedestrians are clear and it is safe to proceed — then go! Your fellow drivers will appreciate it.