The section of Denny Way between Stewart Street and Minor Avenue will be down to just one lane each way, for five weeks.
Seattle City Light is building its first new substation in 30 years to power a growing city, and needs to tear up part of Denny Way.
"That's what this project is delivering, the utility lines to serve the building itself and those utility lines cut across the street," said Scott Thomsen of City Light.
The Denny Way stretch was shifted down to one lane on Monday.
What to expect:
The big impact will be during the weekday afternoon commute, for drivers heading to southbound I-5 or Capitol Hill.
Alternate routes to the freeway like Mercer Street are already jammed during the commute, and Mark Bandy, SDOT's Director of Transportation Operations, said they will probably get worse mid-day.
Bandy also expects backups on northbound and southbound streets as people bail out of traffic on Denny.
He suggests drivers sticking with Denny to southbound I-5 in the afternoon budget 15 extra minutes for the trip.
What SDOT engineers are doing to help relieve congestion:
SDOT engineers will tweak signal timing to try and move cars through the work zone, but Bandy says they can only do so much.
"It's not going to make congestion go away by any stretch of the imagination," Bandy said.
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Get ready for 5 weeks of especially bad traffic: Denny Way down to one lane
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