No one likes to be backed up at a traffic light, but sometimes that’s as good as it gets. If it happens on Mendocino Avenue, College Avenue or Guerneville Road, it’s best to sit back, relax and leave things in the hands of the “smart signal.”

Two Road Warrior readers had similar questions, answered by Santa Rosa’s traffic engineer, Rob Sprinkle.

Question: As a Santa Rosa driver, I’m amazed and frustrated by the traffic lights in town. They seem designed to impede traffic, not facilitate it. In most cities, if you are driving on a major thoroughfare, going the speed limit, you make a number of lights and then all the lights turn red at once and the cross street lights all turn green. In Santa Rosa, it’s possible and frequent to miss two, three, four lights in a row on major streets like College Avenue, Fulton Road, Guerneville Road. Is there something about the lights in Santa Rosa I don’t know? Is there something the traffic engineers don’t know? – Craig

Question: Could you ask the SR city traffic folks why the heckypeck they can’t adjust the timing of the left turn light from Mendocino southbound onto College eastbound so that more than 3 or 4 cars can make it before the light turns yellow? It seems that if the first car doesn’t start moving the second the light turns green, then it moves to yellow pretty quickly. And since that intersection has such a long cycle, it seems like drivers waiting in the left turn lane somewhat zone out. Yesterday I was the 4th car and had to sit through two cycles before I could turn. –Kathy McMorrow

The simple answer is that a computer decides how long the turn light stays on, says Sprinkle. “When Kathy went through, traffic may have been so busy it stole time away from the turn light to help keep through traffic moving,” he said.

The traffic signal at the Mendocino/College intersection is one of 46 in the city that are smart enough to decide for themselves how many cars are in each lane and how much time to give them. (The others are strung out at other locations along Mendocino, College and Guerneville Road.)

Using adaptive traffic control technology, the signal measures volume in each direction and, within a cycle that can last up to 120 seconds, determines how many seconds to give in each direction. In the left turn lane, if it doesn’t detect a car after 2.5-3 seconds, it moves on to the next phase and adds a little extra time there.

As the volume goes down, so does the length of each cycle. The day Sprinkle answered this question, the cycle was down to 111 seconds while remaining synchronized with the light at College and Humboldt Avenue.


Follow the Road Warrior on Twitter @PDRoadWarrior.