SEATTLE, Wash. - A 20-mile backup on westbound Interstate 90 is slowing down travelers' return to the Puget Sound region from Memorial Day excursions Monday. Washington State Department of Transportation Spokesperson Summer Derrey says the backup goes from about milepost 86 in the Roslyn area to milepost 105 near Ellensburg. Derrey says drivers should expect a two-hour delay if they are heading westbound. The department doesn't expect the backup to clear until Monday night. Derrey says the heavy traffic is solely caused by travelers. There aren't any reports of collisions or closures along that stretch of I-90.