Traffic telepathy? Turn signals have way of knowing we're there


May 09, 1994

Perhaps Intrepid Commuter has been writing this column too long, but lately we find ourselves communicating with traffic signals.

They tend not to be prolonged interludes. In fact, there's not much talking involved at all. But somehow the lights always seem to acknowledge our presence and allow us to continue on our way.

That raises a question (besides the obvious inquiry regarding recent blows to the head): How does a traffic signal know we are there? Typically, the left-turn arrow won't even bother to turn on if no vehicles are in the left-turn lanes.

Faithful reader David Ptak has given this matter some thought. The Carney resident recently sent us a letter seeking an explanation.

"I have never seen a story about how turn lane signals know that cars are in the turn lane," Mr. Ptak writes. "A lot of people I know wonder about this. Is it a laser or weight activated?"

Neither, Dave. Much to our surprise, we have learned that most traffic signals are run by clever bioengineered creatures much like the prehistoric animals that ran the household appliances on The Flintstones.

Actually, we wish traffic signals operated that way because it's a simpler explanation. But bear with us, and we shall try to shed some light in a manner any simpleton (sorry, Dave) can understand.

We're not talking lasers or weights, but magnetism. Buried in that left-turn lane is something called a "loop detector," which is nothing more than 14-gauge insulated wire (similar to the stuff in the walls of your home) strung in a 30-foot by 6-foot pattern 3 inches below the surface.

An electrical current running through that loop creates a magnetic field. When a car (or any other large metallic object) enters the field, it creates a disturbance that registers within an amplifier.

The amplifier sits in a metal cabinet positioned near the intersection along with a lot of other fancy electrical equipment. It sends a message to the traffic signal's computer. The message -- traffic engineers refer to it as a "call" -- can be translated as: "Hey, I'm sensing a car out here."

Now the computer, called a controller, runs the intersection's traffic signals. When it gets the message from the amplifier, it makes sure a left-turn arrow is included in the next sequence or "cycle" of lights.

But wait, there's more. After the left-turn arrow is turned on and the car makes its turn, the computer will keep it on to accommodate additional cars if the detector keeps registering them.

If there aren't many cars, the light may last only five seconds. If there are a lot of cars, it may stay on up to 20 seconds.

A state highway intersection may have 14 loop detectors keeping track of traffic in every direction. If a loop is severed or short-circuits, the computer assumes traffic is there. If a car breaks down and keeps the loop tied up, the computer eventually ignores it and follows a prerecorded program governing that situation.

On the other hand, if the computer ever breaks down, the light simply reverts to a flashing mode, typically slowing traffic in one direction with a blinking yellow and stopping it in the other with a blinking red.

"I could sit here for hours and talk timing," Paul Robinson, a traffic control technician with the State Highway Administration, told Intrepid Commuter as we fought back a sudden urge to flee from the room.

"Loop detectors are sensitive enough to detect a bicycle. The only problem we've ever had is with the Amish and horse-drawn wagons."

Loop detectors are capable of determining a vehicle's speed and mass. They can, for instance, tell the difference between trucks and cars and figure out how many vehicles on a given road are breaking the speed limit.

A single traffic signal control system with loop detectors and a computerized controller costs between $10,000 and $12,000. They can be found at all SHA intersections and some county roads, although some political subdivisions, including Baltimore, don't use many of them.

Loops replaced older technologies such as pressure pads, which measured a vehicle's weight. In the 1930s, Baltimore was the birthplace of the first traffic signal detector -- a noise-based system that required motorists to honk their horns.

While loop detectors have become the industry standard, experts foresee them being replaced within the next decade. The newest technology is optical. Computers can read a video image to figure out what's going on with traffic.

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