Adding trails

[Guest]

Hi, a while back I shot some clips of a busy city street at night and now that I've sped it up in FCP3, all I want to do is add trails to it...it's hard to explain. But if you've ever seen a picture of a street at night where the film was exposed for a little while and there were just a bunch of red and white trails (the cars' lights), that's the effect I'm looking for.

Here's a frame grab so you can get the idea.

http://www.angelfire.com/va3/gabetester/street.jpg

Any ideas?