What you could do, is tie some empty cans to their car with about 40ft of fishing string and have the cans sitting under the car. They have to move some distance before the cans start to move.
Or get the biggest pair of granny panties you can find, and zip tie them to the rear of the car. Roll them up under the bumper so that you can't see them when you walk around the car. When they drive down the road. They will unroll and flap in the breeze.
We do this kind of stuff at work.
