The Washington State Department of Transportation will repair potholes and broken pavement on Highway 18 Saturday, which requires blocking the busy corridor between South King County and Snoqualmie Pass.

That detour might reverberate across the region, while another round of Interstate 5 expansion-joint replacements will constrict southbound traffic to two lanes through downtown and Sodo in Seattle.

Both directions of Highway 18 will close between the Interstate 90 junction and Issaquah-Hobart Road from 6 a.m. to 8 p.m. Saturday, except limited local access.

This area tends to attract some of the state’s worst traffic snarls on summer weekends and holidays. Saturday’s closure may cause drivers to fill secondary roads such as Highway 900 from Issaquah to Renton, or Issaquah-Hobart Road Southeast bypassing the work zone.

At the same time, workers will catch up on Highway 18 pavement, guardrail, drainage and brush-clearing repairs, WSDOT will survey and drill to prepare for a $190 million interchange replacement at I-90 to be completed by 2025, and for a $665 million, five-mile Highway 18 widening from 2025-29. The latter project will be funded by statewide vehicle and driver fee increases the Legislature passed this year.

Meanwhile in Seattle, I-5 lane reductions are scheduled southbound from 7:30 p.m. Friday until 5 a.m. Monday, to replace with stronger materials steel-covered expansion joints built in the 1960s. Southbound traffic will merge right and flow through the collector-distributor lanes downtown, while I-90 ramps into southbound I-5 will be closed most hours.

WSDOT says it will “piggyback” other roadwork onto its Sodo lane reductions in hopes of hindering fewer drivers than usual:

I-5 express lanes will follow an abnormal schedule: southbound from 8:30 p.m. Friday until 6 a.m. Saturday; then northbound from 6:15 a.m. until closing time at 11 p.m. Saturday; then northbound Sunday from 8 a.m. to 11 p.m.

Major events will draw thousands of people to Seattle, including the Capitol Hill Block Party on Friday, Saturday and Sunday, where Sound Transit light-rail trains arrive near the epicenter of the festival every 10 minutes.

The Seattle Mariners, on a 14-game winning streak, will attract as many as 40,000 fans when they host the Houston Astros at 7:10 p.m. Friday, 1:10 p.m. Saturday and 1:10 p.m. Sunday at T-Mobile Park in Sodo. Sounders FC plays the Colorado Rapids at 7 p.m. Saturday at neighboring Lumen Field. WSDOT says it will open I-90 and Edgar Martinez Drive onramps to southbound I-5 for two hours following the ballgames.

Drivers who balk at I-5 can try the Highway 99 tunnel. Seattle Center events, including a Friday-night concert by Machine Gun Kelly and the three-day Pokémon GO Fest, might generate extra traffic there.

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