All lanes of Interstate 405 near City Hall will close July 10 and 11 so the state can remove the old Benson Road bridge over the freeway.

And that’s not the last of the road closures that will affect traffic near City Hall.

The next two nights, July 12 and July 13, the state will close just Benson Road through the construction zone to place girders for the new offramp from 405 to Talbot Road South. Those closures are 6 p.m. to 6 a.m. both nights.

The two projects are major milestones in the $83.6 million, I-405 Stage 2 widening project that will add two freeway lanes between state routes 167 and 169 and an onramp from Talbot Road to northbound 405 and the southbound freeway offramp to Talbot.

But it’s the total I-405 closure July 10 and 11 that will disrupt travel regionally and add traffic to downtown Renton. The state hopes to see about 100,000 vehicles avoid the area, including about 20,000 in downtown Renton.

Some concern has been expressed, including by the City of Renton, about the impact on the Return to Renton Car Show Sunday morning, July 11. The state’s working timeline is to have the freeway reopened by 11 a.m. The car show starts at 10 a.m. and is centered around the Piazza on South Third Street, which is not part of the freeway-closure detour route.

The freeway is closed between State Route 167 and just to the north of State Route 169.

The detour route for traffic detoured off northbound I-405 includes Grady Way, Main Avenue, Bronson Way and Sunset Boulevard, all of which skirt the freeway. Bronson Way provides access to SR 169.

The detour route for southbound traffic on 405 uses Northeast Park Avenue, Logan Avenue North, Airport Way and Rainier Avenue, which returns travelers to the intersection of I-405 and SR 167.

The state is hoping that regional travelers will simply avoid the area, using Interstate 5 northbound and southbound. The last place for through traffic to get off southbound I-405 is Interstate 90.

The City of Renton has worked with the state to prepare detours for the freeway closures and keep traffic moving.

“The city is not closed,” said Lisa Hodgson, the Stage 2 project engineer for the state Department of Transportation.

The city will change the timing of signals on the detour routes to help speed traffic, although that could delay traffic on some intersecting streets, said Rich Perteet, the deputy public works administrator for transportation.

The city recently tested the signal retiming.

“You can just fly down our detour streets,” Perteet told the Renton City Council during a transportation briefing Monday. In an interview, he said travelers should expect significant congestion throughout downtown Renton during the day Saturday.

Those coming to downtown Renton on Saturday should consider doing so earlier in the morning and starting in the early evening, he said.

The process to remove the Benson Road bridge – and the I-405 closures – is similar to the one used in July 2008 to tear down the Wilburton Tunnel in Bellevue.