One thing that can cause congestion on any type of road, is diversions to it from other roads in the area.
I remember a situation here in Aberdeen where they closed a bridge over the river for maintenance work, diverting the traffic that would normally use that to another bridge. This caused huge tailbacks of traffic, in those days I walked to and from work, a distance of about 2 miles each way. Now as I just reached the main road after leaving work I passed a car with a distinctive number plate. The same car passed me again about 500 yards from the end of my street, so in the time it took the car probably about half an hour to go two miles.!!