Taking Shape

Ever wonder what speed you would need to drive on a city street in order to make it through every traffic light without stopping? Ever wonder why you can be the only car stopped waiting at a traffic light intersection and the light won't change green? As it turns out, it’s not based on luck. There's actually a science behind it.

Like most sciences, it's a highly specialized field with a lot of technical jargon and sophisticated systems to make it all work, efficiently and safely. Traffic engineers have toiled for years to perfect light signals in order to control the right-of-way for vehicles arriving at intersections, reducing traffic delay and other accident-producing conflicts.

Indeed, there are many benefits to traffic light management systems. The more technologically advanced systems offer the most benefits. Fewer cars stopped at red lights can reduce idling emissions; and by allowing big rigs to pass through intersections without stopping also helps the environment. Traffic light timing generally results in fewer rear-end accidents. And best of all, everyone can get to where they're going quicker and more efficiently.

The first automatic traffic light signal — using red, amber and green railroad lights — was installed in 1920 on a busy corner in Detroit. Within several years, many other major U.S. cities began to install these signals. Back then, the traffic lights worked from moving gears (like that of a clock), clicking the lights on and off.

Today, most signals operate from computer-driven controls that allow for increased flexibility in keeping a green or red light turned on for a specific length of time. Most can respond to the number of cars entering the intersection from different directions. In short, these signals are not blind; they can 'see' cars through one of three ways: through radio waves that can be bounced to detect moving objects; cameras; or through sensors that are strategically placed under the pavement. The latter is the most commonly used.

The computers that monitor these signals (waves, camera or sensors) are housed in a four- to five-foot tall control box, usually positioned near the intersection. When cars continuously go through an intersection, it keeps triggering the signals, indicating that the light should remain green. When the triggers stop firing, it means there are no more cars and the light should turn red. Usually, there’s a maximum amount of time (about 30 to 40 seconds) that the light will stay green. But if there are no cars coming, that time may be cut in half.

If you really want to know the nitty-gritty and how the timing works, check out the U.S. Department of Transportation’s downloadable Traffic Signal Timing Manual.

Unfortunately, not every city has sophisticated traffic light systems. Some lack the technology, while others lack the personnel to oversee the systems. But by installing more technologically advanced systems, cities can better control the flow of traffic, reduce accidents and help the environment. Talk to your city planners to find out what kind of traffic light systems your streets have—and if the technology is a little outdated, consider what your city can do to improve it.

One thing to keep in mind about traffic signals is that they do not control speed. In fact, sometimes traffic signals can result in greater speeds as drivers accelerate to get through the signal before it turns red. If you're looking to control speed in your neighborhood, there are other more effective measures such as speed bumps, speed limit signs and good 'ole traffic enforcement.