That study is along a single road in a single city, and only looks at the absolute number of collisions. As it says, one of the main benefits of lower speeds is

That study is along a single road in a single city, and only looks at the absolute number of collisions. As it says, one of the main benefits of lower speeds is that it dramatically decreases kinetic energy - but the study didn't take into account how dangerous those collisions that happened actually were. Number of injuries and fatalities seems like a much better measure of "safety" to me than just number of collisions.