Why do night photos of highways use yellow and red lines to represent moving traffic?

That doesn’t mean traffic, they are all left behind by real car lights.

That is when shooting night scenes generally requires a long exposure. During the exposure period, the car's headlights (yellow) and taillights (red) leave virtual images on the film or photosensitive CMOS.

For example, if you expose a light spot for a long time and then shake your hands vigorously, the final image will be a ball or a strip of light.