Hi!
This comes from famous Book Fahrenheit 451 from Ray Bradbury. The fragment is located in Part Three of the story.
Montag, the firefighter, wasn't actually hit by a car. This is a description used by Ray Bradbury to convey the feelings inside Montag's mind when he was injected by the Mechanical Hound with an anesthetic.
Ray Bradbury's description of Montag's knee as being hit by a car really meant to describe the numbness and pain he felt when injected with the anesthetic by the Hound.