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A four-day closure that starts late Friday night on Seattle’s Highway 99 may end up wreaking havoc not only on drivers, but on bus passengers coming in and out of downtown.

The construction shutdown near Mercer Street begins Friday night and wraps up at 5 a.m. Wednesday.

About 36,000 weekday bus passengers, who use 17 different routes north and south of downtown, could face delays, alongside tens of thousands of motorists.

It’s expected to be so bad that even Metro Transit is telling bus riders to consider alternate forms of transportation.

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The incoming RapidRide E Line and four other North End bus routes will be diverted onto minor arterials, and onto Fifth Avenue North past Seattle Center, while the highway is blocked near Mercer Street.

Metro can take advantage of the bus-only Aurora lane, but buses then must slog through general traffic until reaching the Third Avenue busway.

The most jarring feature will be a Valley Street detour, where southbound traffic on Aurora Avenue North must turn right toward Seattle Center or left into South Lake Union, where traffic police await.

The routes that must detour are the E Line from Shoreline into Seattle, Route 5 from Greenwood, the 16 from Northgate, and the 26 and 28 express runs, from east Green Lake/Wallingford and from Crown Hill, respectively.

Routes that normally travel Fifth, Westlake and Dexter avenues north will be slowed by the detoured traffic.

The disruption will allow work related to construction of the Highway 99 tunnel — including removal of an Aurora overpass, across now-defunct Broad Street.

“We do expect spillover onto I-5 and onto city streets. That’s why we’re asking drivers to make alternate plans, to telework,” said state Highway 99 spokeswoman Laura Newborn.

Here’s the timeline:

• Aurora will close from 10 p.m. Friday until 5 a.m. Wednesday, from the Battery Street Tunnel to Valley Street. Traffic arriving from the north must exit upstream to take Dexter Avenue North, or turn west or east at Valley Street.

• The Alaskan Way Viaduct will close from 10 p.m. Friday until 5 a.m. Monday, except for traffic leaving the Friday night Seahawks game, which can leave Sodo via the Royal Brougham Way ramps or the southbound Columbia Street ramp, until 11:59 p.m.

• Traffic on the viaduct resumes Monday morning, but northbound cars entering the city must exit at Western Avenue, or sooner at Seneca Street, because the Battery Street Tunnel will still be closed. Because of this, buses heading downtown on the viaduct are likely to be delayed, said spokesman Jeff Switzer of King County Metro Transit.

As Aurora buses are delayed, Metro will deploy standby buses to help maintain normal schedules, he said. The agency is urging riders to travel on off-peak hours.

After the Aurora overpass is removed at Broad Street, a new roadbed will be paved there on top of fill dirt and concrete rubble. This will allow room to build ramps to the future Highway 99 tunnel.

The weekend projects also include replacing 81 concrete road panels in Sodo and fixing a damaged expansion joint at the Seneca Street offramp.

Mike Lindblom: 206-515-5631 or

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