The Bulb Mode
I made this photo using the bulb mode and selectively covering, and uncovering the lens with a lens cap during a long exposure. When there were no vehicles, I would carefully place the lens cap in front of the glass to block the light from reaching the sensor. When groups of cars passed, I would remove the cap, and allow the streaks of headlights and brake lights to be recorded. By doing this I could make it appear as if there were more cars on the road than there actually were. My first attempt was about a three minute exposure, but it was too dark. A five minute exposure proved to be the right combination.