Although the Seattle viaduct officially closed at 10 p.m., people stayed on the roadway afterwards, cheering and honking horns.

The Seattle viaduct turned into a traffic party Friday night in the hour after the structure closed for good.

Crews started closing on-ramps at 9:45 p.m. and began closing down the State Route 99 mainline about 10 p.m., but by the time traffic crews began closing the main roadway, cars were parked in the side lanes with hazard lights on, horns were honking, and people were cheering.

Although the southbound on-ramp from Elliott Avenue was blocked off to cars, people began walking onto the viaduct via the on-ramp.

At one point, someone set off fireworks in the vicinity of the viaduct.

David Sowers, deputy program administrator for the viaduct replacement program, said at 10 p.m. he anticipated it would take crews between 30 and 60 minutes to clear the roadway off traffic.

All traffic cleared by 11:40 p.m. with police having to push the final vehicle off the roadway after it appeared to have car trouble.

WATCH: Last car exits Seattle viaduct after closure