Over the course of the last month I've been experimenting for the first time with photographs of light trails. I've learned a few things and I've taken a LOT of photographs. Even for me.
After a few tests I settled on a pretty closed aperture (F22 mostly) and a low ISO (125-200) to enable a nice slow shutter speed of 30 secs. Obviously for this long exposure photography style a tripod is essential, I also used the 2 second delay feature of my Canon in order to avoid hand-induced shake at the start of each shot. I happen to have this view of a busy road from the window of my (3rd floor) front room - I recommend finding a location where you can look down onto traffic, so you can get a variety of white, red, and orange lights. I particularly enjoyed getting an ambulance - see the blue flashing lights?! A junction will provide crossing-over trails, which I like, and also the ocasional "ghost" vehicle (when a vehicle sits still for enough seconds for the camera to get a sharp image of it amongst the movement). Using long exposures usually means that the camera will take a long time to process each image once the shutter is closed - so don't expect to be able to take another photo as soon as one is done. With this in mind you may end up becoming more selective about when to press the shutter - for example I sometimes waited for a build up of traffic off shot so that I was guaranteed plently of light trails once the shutter was open.
The only other hints I can add are that this is addictive - so if you're outside doing this type of photography then stay warm, as you'll be there a while! Also obviously BE SAFE. Photographing traffic does not mean being on a road - find somewhere out of harm's way to shoot from. Oh and if you're going to be out for hours at night then make sure no one is worrying - ensure anyone who needs to know your whereabouts is kept informed.