The idea of this night shot along the famous curves of Lombard Street in San Francisco was to use my shutter speed setting creatively to get the desired effect with the car trails. So the process was to first determine the duration that was about right for the tail lights of a car slowly moving down the curve to fill the frame horizontally.
Of course, there are going to be variations depending on the speed of the car, but the best effects were achieved using a shutter speed duration in the 8-15 seconds range. With the shutter speed in place, it was easy to calculate the other two components of the exposure equation (aperture and ISO).
The exposure information for this frame was 12mm, 15 seconds at f/11 and ISO 200, tripod mounted. The exposure works in part because of the ambient light of the background scene, which is bright enough so you can see Coit Tower and the Bay Bridge in the background (as well as nearby houses), but not so bright that it is blown out at the settings that work for the cars.
I shot the image just before Christmas while hanging out with a friend and listening to Christmas carols sung by inebriated cable car passengers going by on Hyde Street.