A comment by Pete Stidman of the Boston Cyclists Union, in response to a proposal to build a Chicago Bike Crash Map:

I would advise against self reporting—because what you get is only crashes within whatever bike subculture is reached by the map and by the web. Boston did that before our map and the spread of accidents across the city is very different in each—and could give people the wrong ideas about priority streets and areas. (This is particularly true in communities of color, which were heavily under-reported in Boston’s self-reported crash survey.)

Boston’s bike crash map is generated from (somewhat-) public data on on ambulance runs and EMT reports; Portland has a self-reported bike incident map.