When Duke is in his office, they have a backdrop of the window, and the highway beyond!! It is so dreadful, as a model treadmill, all the cars go past the window at the same speed! A red station waggon goes past the window, 15-17 times!!
In the first post it clarifies this,
that it was simple special effects!
Just another reason why Hellfighter's is near the bottom on my list of favorite Duke films.