I'd wager that most people aren't spending very much time riding at high speed, and as such the number of crashes will be significantly more at lower speeds (15 mph and below).

In fact I'd almost guarantee it. Seems like every club group ride crash I hear about are people rolling up on each other at the top of a hill or coming to a stop and just falling over.

Racing is a different story, of course, but most riders don't race.