A congestion-relief project on Interstate 405 near Renton City Hall reaches a major milestone this weekend with the opening of the new Benson Road bridge over the freeway.

The switch-over to the new bridge is weather-dependent as crews will finish paving and striping the bridge before its scheduled opening at 5 a.m. Sunday.

The road is closed between City Hall and just to the south of the new bridge on Benson Road, which is a typical closure during the project, starting at 6 a.m. Saturday.

A detour takes drivers around the closure on South Puget Drive, Talbot Road South and South Grady Way, back to Main Avenue.

Moving traffic to the new bridge also sets the stage for tearing down the old one, scheduled for July 10 and 11. That work will require the full closure of the freeway and is similar to the one done in July 2008 when the Wilburton Tunnel in Bellevue was torn down.

(The Renton Reporter will have full details and maps of detours around the freeway closure in July in next Friday’s print edition and online at rentonreporter.com.)

Eventually, the old stretch of Benson Road between City Hall and the freeway will be removed and landscaped.

The entire $83.6 million Stage 2 Interstate 405 widening project is nearly a year ahead of schedule, with completion scheduled for this fall. Originally, that completion date was July 2011.

That early completion date is also weather-dependent as the state and its contractor are planning to pave the freeway project, which includes a new lane and an onramp and offramp serving Talbot Road, this September.

The relatively mild winter weather allowed the state to do more work than anticipated during the winter, said Kory Voldman, deputy construction manager for I-405 Corridor Design-Builders.

“This spring and summer have not been as favorable,” he said.

There is no financial incentive to finish the project early, he said. The contractor works for the Washington state Department of Transportation which awarded the contractor a design-build contract.

The new bridge is 40 feet wide and about 510 feet long and sits higher than the existing bridge.

The Benson Road was realigned about 30 to 40 feet closer to the freeway to make room for the new offramp from southbound 405 to Talbot Road. The offramp will go over Benson Road near City Hall on a new bridge now under construction. The piers for that bridge are visible.

The road was raised about 10 feet to accommodate the need for a higher bridge to make way for the wider freeway.

The onramp from Talbot Road to northbound I-405 is visible on the east side of the freeway. That project involved extensive work in the freeway median.

THIS WEEKEND:

Closes 6 a.m. Saturday and reopens at 5 a.m. Sunday, when new Benson Road bridge opens to traffic.

JULY 10-11

The removal of the old Benson Road bridge will mean the entire closure of Interstate 405.

Both projects are weather-dependent.