Beginning next week, cars on Third Avenue in downtown Seattle will be allowed to travel only one block during rush hour before being rerouted...

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Beginning next week, cars on Third Avenue in downtown Seattle will be allowed to travel only one block during rush hour before being rerouted off the street to make way for buses that will be diverted when the bus tunnel closes.

While police may stop cars that violate the one-block rule next week, no one will be ticketed until the first week of October.

This may be the most-visible change when the tunnel closes for two years Sept. 24 to be retrofitted for Sound Transit’s light-rail system.

About 140 buses that use the tunnel each weekday between 4:30 p.m. and 5:30 p.m., the busiest hour, will be rerouted to the surface. That will increase the number of north-south buses on downtown streets by more than 20 percent.

The bus tunnel was built under Third Avenue in the late 1980s. During the retrofit, Sound Transit will lower the roadbed in the stations by 6 inches so passengers will be able to walk or wheel themselves directly from the platform onto rail cars and low-floor buses.

Buses are scheduled to return to the tunnel when it reopens in 2007 and share it with trains when the light-rail line opens two years later.

At a meeting yesterday before the Downtown Seattle Association, city, Metro and Sound Transit officials said letting commuters know of the changes could smooth the transition.

In all, 21 tunnel bus routes that carry 36,000 riders every weekday will be diverted to downtown streets.

Grace Crunican, head of the city’s Department of Transportation, said the change is expected to add one minute to the car commute between Stewart and Jackson streets and two minutes to the bus trip. The restrictions on Third Avenue will be in place from 6 a.m. to 9 a.m. and from 3 p.m. to 7 p.m. Jon Layzer, Sound Transit project manager for the city, said at a briefing that pedestrians and motorists can help keep traffic flowing by not blocking intersections.

Four motorcycle officers and a sergeant will be assigned to traffic control downtown during the two-year closure, he said, and they will be supplemented by nine more officers for about a month starting Sept. 19.

Also on Sept. 19, “street teams” of Sound Transit, Metro and Seattle city employees will fan out to downtown bus stops, tunnel entrances, and tunnel-station platforms (before they close) to answer questions and provide information about the closure and new bus routes.

Susan Gilmore: 206-464-2054 or sgilmore@seattletimes.com