Creating light trails with traffic is usually a pretty interesting project. If you would like to try this, you will need a tripod or something else that is firmly set to the ground (I used a wall one time). Read on to learn more.
First, I needed to adjust my camera's settings. To get as little digital noise as possible, I set the ISO to 100. Also, I knew I wanted a five second exposure. This gave me an aperture of f/10.
If you aren't comfortable shooting on full manual mode, you can also use Shutter Priority. Just set your shutter speed (in this case, five seconds) and your camera will automatically adjust the aperture for you.
In addition to using a tripod, I opted to use my shutter release remote. They are fairly inexpensive and very useful. However, you can also set your camera's two-second timer. The idea is that you don't want to be touching your camera when the shutter fires. The slightest movement can ruin your shot.
Finally, wait for the vehicle(s) to get just into frame before firing the shutter. This can take some practice, so don't get discouraged. A nice thing about digital photography is that you can try many times and not have to worry about wasting film. As always, best of luck and have fun!