what shutter speeds do people use? Going to test this out tomorrow....
It depends on a) how busy the road is (you need a slower shutter speed to capture more car headlights if it's a quiet road) and b) how dark the street is (ie how much of the street will be sufficiently lit).
This shot below, was a 5 second shutter speed and even on Oxford Street I could have done with more traffic (for once).
Oxford Circus at Night by
AlistairBeavis, on Flickr
This one was 8 seconds:
All Souls at Night by
AlistairBeavis, on Flickr
This was taken at Dawn so just as the sun was coming up but still got decent trails in a 2 second exposure due to the amount of traffic.
fence trails by
AlistairBeavis, on Flickr
Other points:
Both were taken in Shutter Priority (S-mode on my Sony, "Tv" mode on Canons I think), so that you pick shutter speed and the camera picks the relevant aperture. You could put your camera into full manual mode and pick your own aperture though.
Use a low ISO too (these were both ISO 100) because if you put it too high you'll get lots of grainy noise and could over-expose the shot with such a long shutter speed.
You'll also need to focus manually before you press the shutter.
If you don't have a remote shutter release, put the camera in self-timer mode, (2 sec delay is enough) so you don't wobble the camera when pressing the shutter.
Note: Take lots of shots and be prepared for 95% of them to be rubbish. You might get cars stopping in front of you, or not enough cars or the wrong settings.
(I took loads on Oxford Street and got plenty of wierd looks from people too)