This will be on video footage where one thing is moving in the frame; it is a toy race car. I'd like to have a motion blur trail behind it, kind of like a cross between a Tron effect and an extreme motion blur (hope that makes sense). It's as if the car is moving so fast that it's leaving a substantial trail of image behind.
Pixel Motion Blur would work as well
For the trail I would roto the car an comp that on top of the original. Then You can play with several instances of it with blur applied and glow.
For the Tron like trail use the technique in the ipad light trails from the tv spots- plenty of tutorials on that one around.