Wed04162014

On The Road

Multiple jurisdictions handle complex system of local traffic signals

When the lights change

Let's give traffic signals credit for being intelligent and able to move traffic along at a safe pace. That's because of a computer chip in the controller that tells the traffic light when to change from red to green.

Dave Donahue, the Los Altos City traffic engineer, maintains 14 traffic signals in Los Altos that keep traffic moving. He can remember working with many different signal systems in past years.

"We used to coordinate signals with a steel pressure plate buried in the road. When a car ran over the plate it would determine the amount of time the light would stay green before it went red," Donahue said. "And when people pulled up to a stop light and waited longer than they thought necessary, they would wonder if they were on the pressure plate."

Today, most traffic signals are timed by the number of vehicles that run over an insulated copper wire in the shape of a loop. The loop detector is made into a six-foot loop buried in the asphalt. When a car runs over the loop, the magnetic field in the controller picks up the detection.

The loop also tells the controller when the car leaves. That information is relayed to the next signal like a "coming attraction."

Depending on traffic demand, the controller allows a car to travel down Foothill Expressway without stopping late at night, but it also stacks up traffic at Magdalena Avenue or El Monte Avenue to allow safe traffic movement.

Of the 36 traffic signals in the Los Altos area, only 14 are maintained by the city of Los Altos. The signals on Foothill Expressway are maintained by Santa Clara County and the signals along El Camino Real are controlled by CalTrans. Three signals in Los Altos Hills are maintained privately and the one signal at Foothill Boulevard and Cristo Rey Drive is maintained by Cupertino.

Mountain View has a coordinated signal system along Grant Road and Los Altos coordinates two signals with the Mountain View system.

"When people call the city and say a signal isn't working, we have to know what signal and who has jurisdiction," Donahue said. "Usually it's the loop that dies, and we replace it if it's in our jurisdiction."

"Occasionally a resident asks for a signal to be placed in a certain location," said Bruce Bane, director of public works. "The city does not believe in having signals in residential areas and signals that cost more than $100,000. The recent signal at Grant and Oak cost more than that because of coordinating with another system."

"We had problems with the signalization at the Rancho Shopping Center until people found out how to handle it," Donahue said. "Since Santa Clara County maintains the signal, the intent is to make the expressway as attractive as possible so the commuter sees it as the fastest way to where a driver is going.

"There are two kinds of traffic coordination. One is for safety when the signals are close together like at Rancho, and the other is to move volumes of traffic," Donahue said.

Here are some numbers to call when traffic lights malfunction:

City of Los Altos, 948-1491 ext. 221; after hours (408) 988-5541. County of Santa Clara, (408) 299-3604; after 4 p.m. (408) 299-2507. CalTrans (state) 330-6500; after 4 p.m., (510) 286-0315. Los Altos Hills 941-7222; after hours, (408) 988-5541.

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