Opening car-pool lanes to solo drivers at night along four of Puget Sound's five major transportation corridors has traffic traveling ever-so-slightly faster along two packed Eastside...

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Opening car-pool lanes to solo drivers at night along four of Puget Sound’s five major transportation corridors has traffic traveling ever-so-slightly faster along two packed Eastside routes while leaving mass transit unhindered.

An evaluation to be presented today to the state Transportation Commission in Olympia also shows that 36 percent of drivers surveyed by a team from the University of Washington were unaware they could use the lanes, though most thought the policy was a good idea.

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Despite the information gap, the state’s transportation experiment is easing congestion, said Charlie Howard, director of strategic planning and programming for the state Department of Transportation.

“The bottom line is the pilot project is doing pretty much what we expected it to do,” Howard said of the report assembled by the UW’s Washington State Transportation Center.

Two locations showed a slight speed increase: Interstate 405 south of Interstate 90 at Newcastle, and southbound State Route 167 at Renton and Auburn. Drivers traveled 1 to 3 mph faster.

It’s been more than a year since the state began to allow solo drivers into car-pool lanes between 7 p.m. and 5 a.m. along Highway 167, Interstate 405, Highway 520 east of Bellevue Way and Interstate 90 east of Mercer Island.

The two-year pilot program does not include Interstate 5 because a DOT study showed I-5’s car-pool lanes are heavily traveled during off-peak hours, and preparing the lanes for use at night would have cost an estimated $11 million.

Signage and safety improvements for the project have cost around $1 million, Howard said.

The Transportation Commission voted for the project in January 2003. Advocates called it a chance to make available underused lane space and boost the spirits of commuters weary of watching buses and carpools whiz by in the sometimes-empty diamond lanes.

Opponents, including the pro-transit Transportation Choices Coalition, said the experiment would increase the incentive for commuters to drive alone rather than seek alternatives.

Peter Hurley, the coalition’s executive director, said yesterday he wonders whether the $1 million could have been better spent providing mass-transit incentives, given the limited impact he thinks the pilot program has had on congestion.

The report found no change in the accident rate. The number of car-pool lane violators rose slightly, typically around the hours when the rules change.

A survey this year of 1,209 commuters found that most respondents thought the project was a good idea, though most were not aware they could use the lanes and many said they perceived little or no change in congestion.

The study notes it’s difficult to gauge improvement because congestion typically dissipates after 7 p.m. anyway.

Howard said he hopes the progress, however slight, will be sufficient to persuade the Legislature to support another pilot project along nine miles of Highway 167 from Auburn to Renton.

This time, the state wants to charge solo drivers for the convenience of using the diamond lanes. Transponders in their cars would signal a monitor to charge their prepaid accounts. As a way to maintain space in car-pool lanes for mass transit, solo drivers would be charged more to use the lanes when traffic is congested, less when it’s not.

Karen Gaudette: 206-515-5618 or kgaudette@seattletimes.com