You're cruising along a slick, rural highway when you notice a green light 50 yards ahead. Fearing that you can't stop in time, you press harder on the accelerator, hoping the signal will stay green.
Seconds later, you blow through the intersection, thankful your luck held.
But luck may have had nothing to do with it. Chances are, the signal knew you were coming. What's more, it may have held the green light for you.
Though it sounds futuristic, traffic signals such as those are not uncommon. Today's signals are capable of detection, coordination and decision-making.
They regularly watch through-traffic, detect cars in turn lanes, wait for pedestrians, "listen" for railroad signals and monitor traffic volume.
Based on that information, they change sequencing patterns to give preference to rush-hour traffic going in a particular direction. They activate and deactivate green turn arrows. And in some cases-especially in uncongested rural areas-they even extend green lights.
"People expect signals to be intelligent," says Nagui Rouphail, a professor of civil engineering at North Carolina State University, explaining that impatient drivers want the lights to determine how many cars are waiting and adjust their cycles to let most of them pass. "As a result, the state of the art keeps advancing."
Most important, however, signals work to keep traffic moving along a city's major arteries with the least stoppage. That's a critical task in big cities such as Chicago, where traffic volume at certain times exceeds the streets' capacities. According to Rouphail, some 600 cars an hour will travel in a busy lane in a given intersection. To show how congesting that can be, if the light at that intersection remained green for one hour, a maximum of 1,800 cars would be able to pass.
It's also difficult. To accomplish it, engineers synchronize the lights so a street's traffic system passes clumps of vehicles from intersection to intersection like a relay runner handing off a baton.
None of this is new. For more than 40 years, states and municipalities have used actuated signals, which can detect traffic and alter their operation to match volume and flow. They've also long used arterial coordination. For decades, Chicago's intersections have been synchronized to move motorists from light to light.
"There's probably a million-and-one exceptions, but theoretically, if you are traveling at 28 miles per hour, you should hit all the green lights," notes Tom Kaeser, assistant chief engineer for the Bureau of Traffic with the Chicago Department of Transportation.
Don't feel left out if you haven't experienced it, though. Traffic engineers say it doesn't often happen because of congestion.
Before choosing today's methods, engineers tried all kinds of automatic control. Simple signals were used as far back as 1868 in Westminster, England. They were illuminated by gaslights, which abruptly lost favor when one exploded less than a year after they were installed.
Nearly a half-century later, the first American traffic light appeared on Euclid Avenue in Cleveland in 1913.
By the 1930s, engineers began experimenting with traffic-actuated or "responsive" signals. One of the first used a series of microphones along the street. Motorists were told to toot their horns so the microphones could detect them and activate a green light.
By the 40s, the run-of-the-mill traffic signal operated much as today's lights do. But instead of computerized controls, the light cycle was timed by electrical relays with discs much like an electric meter totes up a household's usage.
In the last 40 years, traffic engineering has become a discipline unto itself. Now a recognized branch of civil engineering, it grew out of the efforts of surveyors who built the first roads. Today, some engineers earn Ph.D.s in the subject.
"It's quite a complex process," says Rouphail, a specialist in traffic engineering. "As opposed to other areas of civil engineering, here you are dealing with a human element."
Since the 1950s, states and municipalities have used four major types of signals. They are:
- Fixed-time signals. Many of Chicago's 2,685 signal intersections use these. In this scheme, the traffic signal turns the lights on and off at prescribed intervals. Most fixed-time lights contain up to 10 patterns that are used at various times of day. Patterns are stored in a computer in a control box (a green box in the city, yellow in the suburbs) near the intersection.
Fixed-time systems are based on predictions based on actual counts of the traffic passing through an intersection rather than using detectors to determine what's happening at the moment. A timer in the computer determines what pattern to use.
The advantage to fixed-time systems is that they can be synchronized. Most use a set cycle-the amount of time it takes a green light to return to green. In Chicago, the most common cycle lasts 65 seconds. With every light operating at the same cycle time, traffic flows more smoothly.