Construction is about to begin on Mercer Street, to rebuild the hectic eastbound corridor into a tree-lined, two way boulevard, but traffic won't be affected for another year.

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Construction is about to begin on Mercer Street to rebuild the hectic eastbound corridor into a tree-lined, two-way boulevard.

Drivers are not expected to face delays and detours during the first year, because the initial phase will be to create the westbound lanes, across a string of vacant lots where several buildings were demolished this year.

The three-year, $164 million project, which includes $30 million in federal stimulus aid, also will convert Valley Street — the current westbound thoroughfare — into a two-lane arterial with bicycle lanes.

Government officials held a ceremonial groundbreaking Wednesday morning in a place where drivers have grumbled for four decades about traffic gridlock.

“I think there are some adults here whose very first words were ‘Mercer Mess,’ from hearing their parents say it,” said Sen. Patty Murray, D-Wash.

Murray, who has worked to secure federal infrastructure grants, is being challenged by Republican Dino Rossi, whose campaign theme is to reduce federal spending on pet projects.

The project has been controversial because studies predicted it wouldn’t reduce congestion eastbound.

Westbound drivers would have an easier trip because they could go straight toward Seattle Center, instead of making a pair of confusing turns from Interstate 5 to Valley Street.

After the ceremony, U.S. Transportation Secretary Ray LaHood told reporters that stimulus funds are meant to benefit not just car drivers but “multimodal” use, including walking, bicycling and transit.

Part of the allure is to make the neighborhood more pleasant for biotech workers and for, which is building a new headquarters there. Critics have said other, less-fashionable neighborhoods have more urgent street needs.

As LaHood left town Wednesday, staffers said he detoured to view the now-closed South Park Bridge. LaHood said backers are urging him to release federal aid for a new drawspan.

The Mercer project was originally estimated at about $190 million, but the city lowered the figure after receiving low bids from prime contractor Gary Merlino Construction and others, said spokesman Rick Sheridan.

Seattle is trying to fund a follow-up project called Mercer West, from Dexter Avenue North to Elliott Avenue West. Among other goals, the city would add street crossings over and under Highway 99, for better bicycle and local car traffic.

Mike Lindblom: 206-515-5631 or