Normally those wanting a race are the worst drivers going, I tend to let them carry on.
One of the funniest occasions was a lad in his saxo vts, pull up at a set of lights and is revving away. Lights change and he roars off to victory as I pull away in a normal fashion (this is right in the city centre).
I happened to say to my mate 'hope we find the tit buried in a lampost round the corner'. Well, it wasnt a lampost, but was a brand new merc and the saxo was almost inside it, with a very angry business man tearing strips of saxo boy, who is now on the verge of crying.
Was rude not to beep and wave as we passed. It was classic.