My ideas for adding trailing lights to night scene.
I started with a picture of stationary car.
Second layer: Motion blur horizontally, then add layer mask to
reveal only a little of the trailing edges of the car to simulate movement
Third layer: paint light streaks, gausian blur, set to screen blending mode
and fade with gradient mask. The streaks can be painted
straigt by using the shift key with the brush, or wavy with free-hand brush.

original image from Imageafter.com