Safety chain it! A buddy of mine, 14 years ago, had a Ford Ranger on a dolly behind a big Ryder moving truck. He used the straps over the tires like normal. He was going through a slow area, fortunately, when the strap failed. His truck went off the side of the road, through a fence, and in to an orchard, amazingly without hitting a tree before it stopped. He never noticed it, since he could not see or feel it behind the moving van.
A car pulled up beside him, waving like crazy. He waved and kept on going. Then they started honking. Kids in the car hollered "Mister, your car ran off!".