We ran into a similar problem at my last job. I mapped crash locations in our game to the map in which they took place. When viewed in aggregate it ended up turning into more of a map of where people tend to congregate in the game then where problem spots actually exist.
In some ways, the map functions as that denominator. That's still pretty interesting. Also, it's pretty clear looking at the correlation between injuries and auto occupants how much more dangerous it is to get in a car than on your feet. :)
The first pass actually had a nice grid pattern on the map that was where ambient audio emitters happened to overlap. Something about the transition between two ambients emitters was falling over.