Corrected entry: The license plate of the time machine is spinning on the road in the middle of the fire tracks - further down the track than where Doc and Marty stand. But the car (and therefore the license plate) was never there. It had already 'jumped'. Otherwise the license plate had to pass through Marty.
Knever
10th Mar 2009
Correction: Having just watched the scene in question, I can confirm that that is incorrect. It may be confusing because they turn their bodies to look at where the car would have been. They then turn back forward to the last place that they saw the car which is what Marty was then focusing on. After it shows the plate spinning, the next shot shows Marty and Doc facing the camera, and also the plate.
Knever