While I haven't read the original source material, the labels for the chart are for collision speeds, not posted speed limits for collisions.

It's true that there's a large problem in many places with speed limit compliance, but this is primarily a problem of design - the streets encourage driving at speeds that are higher than the posted limit - and secondarily about enforcement, or lack thereof.

I agree that there are likely fewer accidents occurring at lower speeds as well, given that they are easier to avoid at those speeds. This is also a good argument for 30 km/h max speeds in urban areas.