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17 Oct 97

Traffic Lights on the Pass

I drive Highway 154 to and from my home on San Marcos Pass everyday, so I've had lots of time to contemplate the stoplights that control traffic across the single lane on Cold Springs Bridge during the $%!&# construction. The light is almost always red when I get to it, so it's definitely not red and green for equal times. Morning traffic is much heavier going towards Santa Barbara than towards school, and the reverse in the evening, so it's not programmed in any simple way. So what is the method or algorithm used to control the signals?

How do the lights at Cold Springs Bridge work? John Norris points out that the whole bridge is now an intersection. The lights must stay red (instead of yellow) both ways to clear the lane, and so the lights are usually red when we get there. There's a blue sensor on a green box under the orange warning light about 100 meters from the bridge on each side. I believe the light turns green and stays on for a minimum time of several minutes. Then the sensor waits for the first break in traffic and turns the light red. Two weeks ago Highway 101 was closed by a fire at El Capitan beach, and all traffic was routed over the Pass. We learned the hard way that if traffic is continuous, the signal waits for a fairly long maximum time and then turns red anyway. The result was traffic backed up 25 miles to Solvang. I think this algorithm with the sensor covers all cases, so it's not necessary to program exact timing for the signals. Hopefully this will soon be over!

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Marc Kummel / mkummel@rain.org