Starting Monday night, work to move underground utilities for the new Highway 99 tunnel will mean congestion for drivers entering downtown Seattle from Aurora Avenue North.

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Drivers entering downtown Seattle on Aurora Avenue North can expect six weeks of extra congestion, starting Tuesday morning, as workers move underground utilities for the new Highway 99 tunnel.

The right-hand lane southbound will be closed until about April 17, from Republican to John streets.

The closure will probably clog traffic entering the Battery Street Tunnel on the way to Sodo and the First Avenue South Bridge. Instead of both the right and center lanes of Aurora exiting to Denny Way downtown, only the center lane will carry exiting traffic for six weeks.

Slowdowns that now begin along Queen Anne Hill will be somewhat longer, said spokesman Travis Phelps of the state Department of Transportation. About 31,000 vehicles a day use the southbound lanes there.

In addition, 385 transit buses per day will be detoured. Buses will make a sharp right-turn exit at Valley Street and then turn toward downtown using Fifth Avenue North at Seattle Center. (Two Metro Transit stops on Aurora will be closed.) People commuting to South Lake Union will have to get off at Fifth Avenue and walk an extra two or three blocks.

During the shutdown, contractors will install a new underground vault in the closed lane of Aurora Avenue. Utility lines will be placed there — freeing space to construct highway ramps, the tunnel’s north portal, and an operations building just west of Aurora. The tunnel is scheduled to open for traffic at the end of 2015, and Aurora next to the portal will become a four-lane arterial to Belltown.

This spring’s utility work is scheduled to be done a few days before tourist season begins. Seattle Center, born of the 1962 World’s Fair, will hold its “The Next Fifty” opening celebration April 21, as well as Earth Day festivities, a Rat City Rollergirls bout and Pacific Northwest Ballet performance the same weekend. Between 30,000 and 40,000 people are expected to visit.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com. On Twitter @mikelindblom.