I like to also enable motion blur on the overlay layer to give the actor a healthy amount of motion blur when the car hits them. For this, enable the motion blur switch on the layer as well as your composition.
If the motion blur is too strong, you can tweak it by adjusting the shutter angle of the composition. You can find the shutter angle option under the 'Advanced' tab in the composition settings. I like to set mine to 90 degrees. If you're not sure how this works, I have a Shutter Angle Tutorial
that explains what it is and why it is useful
And with that, your car hit will be complete! Have fun with this effect!