Red Light, Green Light: Northern Virginia’s Traffic Lights

J.P. Knight’s traffic signaling system, "a semaphore," was an adaptation of the train signals used to let conductors know if a stretch of track was available or occupied. His street device became a simple post with a gas light on top. The post, operated by police traffic officers, used movable arms; in the horizontal position, the arms indicated "stop" and when pulled up, in the vertical position, it meant "go." At night, the gas light used a red lens for stop and a green lens for go.

The next innovation became the traffic tower (a booth) wherein an officer stood in order to see above the traffic and thus decide when to change the semaphore or operate the lights. In 1917, Detroit installed the first traffic tower in the United States.

Soon after, a third color was added to the traffic light system. Yellow was used to signify that a change from green to red was about to happen, thus allowing added time for traffic to pass or stop safely. The three-color system was standardized nationally in 1935, making it easier and safer for drivers in all parts of the country who would encounter the same system wherever they drove.

You can see the latest version of the full technical standards in the Manual on Uniform Traffic Control Devices, (MUTCD) at this U.S. Department of Transportation Federal Highway Administration site http://mutcd.fhwa.dot.gov/. You can also peruse Dr. H. Gene Hawkins, Jr.’s informative website at https://ceprofs.civil.tamu.edu/ghawkins/MUTCD-History.htm.

The first automated signals, one of which was installed in Baltimore in 1928, used different systems to set the length of time the lights would be on. That curious precursor detected a car’s horn honk. The driver stopped at the red light and honked. The honk’s sonic vibrations caused the signal’s electrical circuits to change the light and a timer allowed about 10 seconds for the driver to cross the intersection.   

Today’s lights are mostly placed up high on wires or poles above our roads so that can see them at a distance and drivers can begin to adjust their speed accordingly. All of our current signals have the standard red on top, yellow in the middle, and green at the bottom, and most use LEDs with lenses that are 12 inches in diameter that require only about 25 watts of electricity to operate. The consistent placement of the three colors and the strength of the light they emit have allowed those with color vision confusion to drive safely knowing the bottom light is "go" and top one "stop."

Most of our lights are on timers that are calibrated to the time of day so that they are on longer in peak traffic periods and change more often during off peak hours. This set-up works most of the time, but can be a pain when you are the road’s lone driver in an off-peak period with no cross traffic but are still detained at a red light. And you should also know that you are not crazy—yes, our area is known for having some of the longest lights in the country (that is they last long because the traffic is . The intersection at Huntington Avenue and Telegraph Road is perennially among the most congested.

But you should also know that changing the timing on a series of traffic lights can be a time-consuming and costly endeavor. In 2008, Virginia Beach began changing the traffic-light timing on about 250 intersections. When their project ends this October, it will have taken over 40 months and cost $1.7 million to complete the job.

Many signal lights also have detectors that can tell when you have reached an intersection and when a lot of cars are stacked up at that intersection, so that the length of the light can be adjusted both once it is on (while traffic is passing through an intersection) and before, when it is about to turn green.

Similarly, the detectors can tell when cars have entered a turn lane and when there are too many waiting there in order to activate the turn arrow. These types of detectors can also activate red light cameras which will photograph you as your car trips the sensors when you go past them during a red light interval. You will then receive a ticket and a handy memento of your moment of madness.

Traffic lights are but one of the many technologies that help us around our community and we will be exploring others in forthcoming pieces.