This is how you do this
 

This is how you do this... If you want to show the person being hit then video a side shot of the car hiting the person at super slow motion... meaning the car drives... OK rolls into the person. The person then acts as if the car really hit them and flies slow motion onto the cars hood or onto the ground... then you cut to another angle... for the slow motion footage just speed it up in post.... that's it... looks great and you even get to use the same actor... WHY ... because thay are acting like a car hit them...

I just tried this myself for the first time. Worked out OK. Pull the camera back and zoom in to compress the depth. Have the car drive up slowly (like 5mph) but steadily. The actor goes down behind the car as if he is hit but he is really a safe distance from the car. He goes down pretty slowly. Then speed it up in post. Use context and sound effects to your advantage as others have posted. Keep the action with the car as brief as possible and linger on the buildup and aftermath to sell the shot.

nb: Do not add the scream if there are no bystanders present.


;-)