Hi,
One thing that I've found helpful is to turn the aperture up around f/11 or so. This creates a really cool star effect on street lights and stuff.
I would also definitely recommend picking up a remote with a trigger lock on it. I have the XTi (400d) and I use the RS-60E3 remote. It was pretty cheap, $30 (U.S.).
I typically start at around 30 second exposures, then adjust from there. For me, every shot requires a little bit of experimentation because of differences in lighting, position of the traffic, etc...
If you're interested, my traffic shots are located in this set on my Flickr account:
http://flickr.com/photos/hypnoeyez99/sets/72157604614661890/