Weigh Stations
I didn't see Black Dog but I did drive a tractor trailer of band equipment for Travis Tritt, AC DC, Motley, Metallica, Prince, Chicago, Skynyrd and I live in the county where the mountain scenes for Black Dog were filmed, so I will try to answer.
Depending on parking spaces available, officers on duty, highway traffic, number of trucks sitting in line to be weighed, lunch schedule, local advisories or DOT bulletins or weather conditions, scale officials may choose to weigh, inspect the truck, permits, or paperwork, or ignore the truck.
If the trucks begin to back up onto the highway, the scale may let trucks go and close the scale entrance until traffic clears. They may also open and close the scale to confuse presumed illegal trucks waiting for the scale to close before they proceed.
In some states, the trucks are pre-weighed while in motion and then only the heavy loads and permit loads are stopped for weighing.