It’s actually pretty complex. Traffic light patterns vary considerably from country to country, although traffic lights in the United States typically follow the National Electrical Manufacturers Association TS-1 or TS-2 standards.
So, let’s say you’re an engineer, and your job is to set up patterns for a city’s traffic lights. You want to keep vehicles moving wherever possible while allowing sufficient time for pedestrians to cross the street. It’s an important job—poorly timed traffic lights can lead to inaccurate citations for drivers and hundreds of thousands of dollars in lost revenue for cities. How will you make sure you’ve got the best possible system?
You could simply go with a fixed time control. In this system, the signals cycle after a set number of seconds and cannot change the length of these cycles based on current traffic. At a small intersection, that might be sufficient, but at heavy-traffic intersections, more attention might be required.
Here are some of the factors you might consider:
- Speed Limits and Clearance Intervals - The most obvious (and arguably the most important) factor is the speed of the cars moving past the signals. The faster the cars, the more of them can move through on the green cycle.
When the light starts to change, it will take a few seconds for the intersection to clear. Traffic controllers need to know these "clearance intervals" when setting the signal cycle lengths—obviously, you don’t want cars entering the intersection while other vehicles are still there.
- Physical Characteristics of the Intersection - How large is the intersection? Is it on a grade? Is the road made of asphalt or another material?
No two intersections are alike, and traffic controllers need to consider every unique factor to optimize times properly.
- Traffic Light Triggers - Intersections often need different cycle lengths during different times of day. After all, if most traffic moves through an intersection in one direction at noon but in different directions during rush hour, you can’t rely on the same traffic signal cycles to keep vehicles moving. Controllers need to consider traffic patterns to work effectively.
Many signals use triggers to determine when vehicles or pedestrians are in position. Traffic light patterns might be triggered by sensors, video cameras, or buttons that pedestrians press to indicate they want to cross (although in some cities, those buttons might not be hooked up to anything at all).
So, how do you negotiate all of those factors when setting up your traffic light patterns? You don’t—you let a computer handle it. Most modern traffic signals utilize sophisticated algorithms, and computer software makes changes to the patterns to keep the signals running smoothly.
Of course, engineers still need to develop those algorithms in the first place, and they’ll look at the unique characteristics of each intersection to keep drivers moving. It’s an extremely complicated science, and it’s something that most drivers take for granted.
The next time you find yourself hitting a series of perfectly timed green lights on your morning commute, be sure to take a moment to think of the engineers (and computers) that made the experience possible.