Traffic alert: Plan to be on the road a little longer this weekend. Several construction projects and events are expected to snarl traffic.

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Ongoing carpool-lane construction on the Interstate 90 bridge corridor will cause another round of weekend traffic delays and detours, where drivers go westbound from Bellevue onto Mercer Island.

The mainline freeway will close westbound, from 11 p.m. Friday until 5 a.m. Monday — overlapping with two Mariners baseball games and a Snoop Dogg concert next to the western terminus of the freeway in Sodo.  Drivers entering Seattle may only use the express lanes.

Unlike a similar closure three weeks ago, two lanes of traffic will stay to the right across the East Channel Bridge, so drivers arriving from Bellevue and I-90 won’t need to suddenly veer left. Instead, the left-side split entering  the express lanes will be positioned farther ahead, on Mercer Island, as shown on the diagram.   As before, the Rainier Avenue South exits on the Seattle side will close, so that drivers continue to South Dearborn Street, and backtrack into Rainier Valley.

Workers will continue to retrofit the fire-protection systems, streetlights, and electrical components in the Mercer Island lids and Mount Baker tunnel. This is the third of  about 30 closures in a $188 million project to add I-90 carpool lanes and improve safety systems, before Sound Transit light rail construction takes over the express lanes in 2017. Train service from the International District/Chinatown Station to Mercer Island, Bellevue and Overlake is scheduled to begin in 2023.

In the late-March detour, traffic backups stretched as far as Factoria and Eastgate, as well as feeder ramps from I-405, said Annie Johnson, spokeswoman for the Washington State Department of Transportation.  Also, traffic tends to increase on the Highway 520 toll bridge.  But unlike March 27, the Alaskan Way Viaduct in Seattle will be open this weekend, relieving some regional pressure.

Another eastbound I-90 detour into the express lanes is scheduled May 2-3, followed by a westbound detour May 16-17, but there won’t be other diversions until fall, said Johnson.

In other weekend construction, the Seattle Department of Transportation will close Dexter Avenue North on both sides of Mercer Street  from 8 p.m. Friday until 5 a.m. Monday — to restore Dexter to its permanent, straight alignment.  For several months, a construction curve made the intersection more hazardous to bicyclists and baffling for motorists.  Two miles away, the  north end of bumpy Eastlake Avenue East,  near University Bridge, will be repaved over the weekend, though one lane in each direction will stay open.

On Interstate 405, the state will close several ramps and close from one to three lanes in spots between Bellevue and Lynnwood during the weekend,  mostly during overnight and morning hours, to finish building the express-toll lanes to open this fall.  These include an overnight closure of the northbound exit to Highway 527, from 11 p.m. Saturday to 8 a.m. Sunday; and the Highway 527 ramp to northbound I-405, closed from 9 p.m. Saturday to 9 a.m. Sunday.