Reboot time: When traffic signals go out of sync

Street Smarts reader Skip Darimont, of Everett, wondered about the signal timing on Meadow Road for drivers looking to get onto 164th Street SW. The green lights were awfully short, allowing just two or three cars to get through.

“We know this is a busy commuter corridor because of the nearness of I-5 and traffic engineers want to keep cars moving on 164th,” Darimont said.

Still, it was taking two light cycles to get through — and sometimes longer.

“Sometimes people take advantage of the red light while waiting on Meadow Road and use their phones. When the light changes, all the cars behind them sit until the first car moves thereby reducing the ‘window of opportunity’ to get on 164th,” he noted.

Snohomish County heard from others about the problem, too, and the signal timing was soon fixed.

So how did it happen?

“Traffic signal controllers are computers and, like most computers, they sometimes malfunction,” said Jim Bloodgood, the county’s traffic engineering manager.

The problem in this case was traced to equipment that was not properly accepting vehicle detection inputs, thus giving shorter green times to some traffic movements.

“To fix this, the controller was, for lack of a better term, rebooted, to clear the memory of corrupted data. This appears to have corrected the situation the reader commented on,” Bloodgood said.

The county operates the traffic signals along 164th Street on both sides of I-5. They are designed to operate in coordination on both weekdays and weekends. Cycle times vary between 90 seconds and 140 seconds depending on the time of day. Cycle lengths also are coordinated with state-operated signals at the I-5 freeway ramps.

There are other reasons for short green times — or times when a green light is even skipped for a cycle.

For example, when emergency vehicles leave Martha Lake Fire Station 21 on 13th Avenue W on emergency calls, they preempt regular traffic signal cycles. That can take the signals out of coordination and take up to three cycles before things are back in their regular rotation, Bloodgood said. When a signal is at a 140-second cycle, that can take as much as 7 minutes.

The one delay no computer can speed up? That driver ogling their phone instead of the light.

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