Traffic delays were longer than expected on Interstate 90 and Interstate 405 on Sunday night during rolling slowdowns scheduled by the state Department of Transportation.

Seattle City Light crews raised power lines near the I-90/I-405 interchange and drivers experienced delays for nearly an hour in both directions of I-90 and I-405 between midnight and 4 a.m.

Engineers initially anticipated 20- to 40-minute delays, but are now advising drivers to allow an extra hour if traveling between midnight and 4 a.m. the next three nights — Monday, Tuesday and Wednesday.

Sunday night's extended delay was caused by problems with equipment. Delays of up to 50 minutes are expected for the next three nights (July 30-Aug. 2) at the I-405/I-90 interchange.

Each night, the State Patrol will initiate periodic rolling slowdowns in both directions of I-405 and I-90 near the interchange. All ramps leading to the interchange will be closed to prevent traffic from entering the freeway, including I-90 on-ramps eastbound from East Mercer Way on Mercer Island and Bellevue Way in Bellevue; westbound I-90 on-ramps from 150th/156th Avenue Southeast, 148th Ave Southeast, Southeast Eastgate Way and Richards Road/128th Avenue Southeast.

Also closed will be northbound I-405 on-ramps from Southeast 44th Street, 112th Avenue Southeast and Coal Creek Parkway, and southbound on-ramps from Northeast Fourth/Eighth Street, Northeast Sixth and Southeast Eighth Street.

At the start of each slowdown, the on-ramps will be closed at the same time, and will reopen one at a time as slowed traffic is escorted by state troopers.

The transportation department is recommending motorists reschedule trips on I-405 and I-90 during those four-hour periods, or consider other routes. Updated traffic information will be posted online at www.wsdot.wa.gov/traffic/seattle. Traffic information also will be provided on variable freeway message signs, on highway advisory radio and on a toll-free 511 traveler information phone line.

During the work, power lines will be disconnected and transferred to taller poles, or tightened and reconnected to an existing pole southwest of the interchange. But area residents should not lose power.