Ohhhkaaaay Let me try to put it in layman's terms (if I can).

In the past, a photographer would have to set the shutter speed of a camera manually according to the level of light. These days, depending on the coding in the camera film or the sensors in camera, the ASA rating (shutter speed for a given light level) is dictated to the camera. Hence automatic.

So the webcam knows, thanks to its coding, to slow down its shutter speed at night because of low-light. Therefore, the shutter opens for longer -- long enough to capture a car with headlights on coming into the frame and moving a car-length or two along before the shutter closes and saves the image.

The reason that virtually only the light streaks are seen is probably due to the fact that the headlights are so bright that the contrast of the car itself, which is dark and blends more with the background, is not noted strongly.

Webmaster says that the same type of light streaks would occur if you had a super slow shutter speed camera pointed to the night sky. The stars would leave long streaks (as the earth rotates and star positions move in an arc).