Here I have set up a timed exposure which has caught light trails from the headlights and taillights of passing cars, but the light from the vehicles themselves was too small to catch (the car on the right is parked, so shows up on the image). The trick is to not under-expose and miss the detail or over-expose and lose the detail in a blaze of light. I also needed a good depth of field, to ensure that all parts of the image remained in focus.

Camera settings: 10s at f/22, ISO 100.