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Originally published Monday, May 25, 2009 at 12:00 AM

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Bumper to Bumper

Timing is everything for harried driver

Q: How does Colleen Harris know when baseball is back in town? Simple: when baseball traffic backs up her regular commute from Mercer Island...

Seattle Times staff reporter

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Q: How does Colleen Harris know when baseball is back in town? Simple: when baseball traffic backs up her regular commute from Mercer Island to her West Seattle neighborhood.

On game-day afternoons, as Harris is headed home from work, Interstate 90 traffic to Safeco Field gets so congested, she says, that it's time-consuming to reach the Interstate 5 interchange so she can head south to the exit to the West Seattle Bridge. On those days, she reverts to her shortcut: I-90 to southbound Rainier Avenue South, then across Beacon Hill to the South Columbian Way approach to the West Seattle Bridge.

That route would be easier still, she says, if it weren't for two traffic lights on South Spokane Street — they're about a block apart — before she jumps on Columbian Way.

She vaguely recalls reading somewhere that the timing for those lights had been synchronized. "But it seems worse than usual," she noted a few days ago. "We can't get more than five cars through at 15th Avenue South because the light is red at the bottom of the hill [South Spokane Street at South Columbian Way], and doesn't turn green until after the first light is red."

Simply put, she said, that hampers traffic flow. "That gets to be a pain," she complained. "Shouldn't those two lights be timed better?"

A: Brian Kemper, manager of the Seattle Department of Transportation's traffic-signal operations, says a solution is not simple. The signal timing operates that way because eastbound South Spokane Street is a steep grade uphill, providing limited visibility for vehicles making left turns.

There are no left-turn pockets and no space to add them, he said.

"So drivers turning left would block drivers behind them if they had to wait for a gap in oncoming traffic."

As it is, traffic headed east is timed differently from traffic headed west. The city calls that split-phase operation.

Because of minimum timing requirements, says Kemper, the split-phase operation cuts down on the time each direction has a green light.

"We strive to find a balance that best serves all users," he said, "but on occasion we must favor one direction over another for safety reasons."

Q: Is Northeast 75th Street between Lake City Way Northeast and 35th Avenue Northeast supposed to be one lane, or two? "I don't see any markings, but people pass on the right all the time," one reader complained.

A: A portion of that stretch is about 20 feet wide in each direction, and a portion about 22 feet. City code says a single lane needs at least eight feet of space.

Eric Widstrand, SDOT's city traffic engineer, says that means that entire stretch can handle more than a single lane of traffic in each direction.

The city doesn't typically mark multiple lanes on streets where traffic volumes are relatively low, he said, and that's the case along that stretch of Northeast 75th Street.

Copyright © 2009 The Seattle Times Company

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