I actually have a picture like this of the 401 (the busiest highway here). I did it WITHOUT a tripod and with only 6 seconds of exposure. It worked out nicely, but it isn't nearly as clear as the one above.
The point is you can do it just hand-holding your camera at the railing of an overpass (I encourage you to try it out and see for yourself), but you will get MUCH MUCH BETTER results with a tripod and a REALLY REALLY long exposure.
Oh yeah, make sure there is no wind. (This last piece is my personal bit of experience)