I don't know what the bumper cars were like where you grew up, but a crash at 45mph, or even 20mph, is not quite the same thing and considerably less enjoyable. :-)
I think collisions below 40 mph are quite survivable if they're not two-way head on/overlap and the cars are below a certain weight and designed to bounce against each other.
They just need to make every autonomous car on the road 1500-2000lb and install rubber bumpers. The neural net will figure it out itself after a few million real world collisions and daily commutes will gradually become less jarring and eventful.