Why will every driver go over 20mph/30kph? Are they incapable of maintaining that speed? All school and community zones in my country are 30kph; are we wasting our time with those?
I’m a vision zero proponent, so I don’t care about the number of collisions; I care about the number of fatal collisions first, serious injuries second, minor injuries third. So even if 20 mph maintains, or even increases collisions; so long as it reduces casualties, it’s positive. Bumpers are replaceable; people are not. The AAA document you link even says a 10% reduction in mean speed reduces fatal crashes by ~34% in the executive summary.