Canon 600d shoots the video of cars coming and going at night, how can it shoot the effect of traffic flow?

You're talking about video. In this case, you should take a tripod, adjust the position and parameters, and then start recording. How long it takes depends on you, just a minute or two, and then use the video editing software on your computer to speed up the playback. The effect is the traffic effect you want, but don't move the camera during video shooting.

If you're talking about photos, that guy said the same thing before. There is no need to increase the exposure time. It's almost ten seconds. Set the shutter priority mode, adjust the position and parameters, and then press the shutter without moving the camera. When the camera shutter is closed, the photos will be photos with traffic effects.