An Accidental Technique: Improving a Shot

I liked the composition and the foreshortening of perspective produced by a 300mm tele lens. 
But it was a bit 'busy' in the background.

I took several more shots of the same scene as the light levels dropped.
In this shot I noticed that a bus passed during the longish exposure, effectively blotting out some of the people in the background, thus making the background a bit cleaner.
I could have done this in Photoshop, I guess, but it was a lot easier to use a long exposure (and a bus!).