Although the two-year closure of the Seattle bus tunnel won't start until September, bus riders will get a taste of what's to come as early as June.

The tunnel will be closed Saturdays beginning in June so that Sound Transit can begin work on retrofitting the tunnel for its light-rail trains.

The closures should give riders, as well as officials with Metro Transit and Sound Transit, an early glimpse of how public transit downtown will be impacted. The June closure will allow tunnel contractors to do early work sawing concrete before the roadway is lowered for installation of the rail, said Rebecca Roush, Sound Transit spokeswoman.

Despite months of planning by both agencies, Metropolitan King County Councilman Dwight Pelz says the change could result in chaos starting Sept. 26, when the tunnel will be closed for two years. The Saturday closures starting in June, for example, could affect fans headed to Mariner games at Safeco Field.

Worries that a closed tunnel will choke downtown streets led Pelz, at a meeting of the council's Transportation Committee yesterday, to ask Metro to return with additional contingency plans to ease traffic if its other plans don't work. For now, Metro is relying on reconfigured streets and dedicated bus lanes, as well as police, to keep traffic moving.

"When downtown gridlocks, the only way to beat it is the tunnel," said Pelz. "There needs to be a written plan if traffic doesn't move that Monday [Sept. 26] or Tuesday."

So what will it mean to commuters who have relied on the tunnel to move under downtown Seattle, avoiding street traffic? In short, bus trips are expected to take longer.

Councilman Bob Ferguson, who rides the Route 41 bus from North Seattle, asked how much longer his ride would be to the King County Courthouse. Victor Obeso, Metro supervisor of service planning, said the closure could add 4 to 6 minutes to the trip.

When the tunnel is closed in September, Metro and Sound Transit plan to:

• Limit traffic on Third Avenue almost exclusively to buses during the morning and evening peak periods.

• Add a northbound bus-only lane to Ninth Avenue, now one-way southbound, between Olive Way and Stewart Street to help buses get downtown from Interstate 5.

• Reserve one lane of one-way, eastbound Olive Way for buses and right-turning traffic during peak periods.

• Add a northbound bus-only lane to Fifth Avenue South between South Jackson and South Washington streets, where it's now one-way southbound.

• Straighten one-way northbound Prefontaine Place South, which will be reserved for buses during peak periods.

All of the bus-tunnel mitigation, from street modifications to placing traffic officers at key intersections, is expected to cost $16 million, with Sound Transit paying $13 million. The rest will come from Metro, Seattle and Snohomish County's Community Transit.

Whether all the mitigation will work is still a question. Service will be subject to more daily fluctuations, Obeso acknowledged, although he said he thinks a transit-only Third Avenue should keep bus traffic moving through downtown.

What Metro couldn't answer is whether the tunnel closure will cause more of today's bus riders to take their own cars downtown, with Metro losing riders.

As part of the changes, Third Avenue will switch to a "skip-stop" pattern. Now buses on Third Avenue stop at every bus stop; in September they will skip stops to make the trip faster.

The mezzanine at Westlake Center, which connects the downtown department stores, will close when the tunnel is shut down.

Construction crews have already started tearing up the pavement near the Paramount Theatre, the first step toward digging a new stub tunnel where trains will turn around at the rail lines' north end.

Construction in the tunnel involves lowering the roadway and installing rail and a power system. The tunnel is expected to be closed for two years, then reopened to buses. But not all the buses will return.

According to Obeso, 85 percent of the current tunnel buses will return. The tunnel can now handle 70 buses an hour during peak periods, but that will drop to 60 when the tunnel opens to rail. And as light-rail demand grows and trains run closer together, that number could shrink to 45 buses an hour.

Eventually, said Sound Transit official Michael Williams, the demand may be so great, there won't be room in the tunnel for buses.

But that wouldn't occur until light rail was extended north to the University District, and it wouldn't happen for at least 10 years, said Pelz.

In other developments yesterday, Metro announced it was splitting its No. 7 bus route, the route with the highest ridership.

The bus from downtown Seattle to Rainier Beach will remain No. 7, but the continuation from downtown to the University District will be a new bus, No. 49. The change begins in June.

Metro said the change was made because the schedule on the heavily traveled route is very unreliable.

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