Sight of a speeding car, maybe

BLUR

Think of what happens when something moves so fast that its edges become indistinct, like a spinning fan or a whirring propeller.

When you see a car moving extremely fast, its shape and details can become indistinct—just a quick, fuzzy streak. The word "blur" perfectly captures that visual effect, where motion makes an object hard to focus on. The clue plays on the literal meaning of "sight" (something you see) combined with the idea of speed, hinting at the answer without being too obvious.