Unless it's pitch black out and you can only see the lights, things like reflections and contours of the car give you additional clues about its orientation and speed apart from its lights. We're used to using many different subconscious visual cues to place an object in space.

Maybe I'm overshooting on how much of a hazard it would be, but I have a hard to seeing how making a car any amount harder to see is a good thing.