[maxerickson]

The braking distance isn't linear with speed. It goes with the kinetic energy of the vehicle which is proportional to speed squared.

A vehicle traveling at 30 mph has about 40% more kinetic energy than the same vehicle traveling at 25 mph.

It will also close more of the available braking distance during the reaction period (only 20%, but that's a lot).

So the collision speed for the higher initial speed will be greater than the difference in the initial speeds, not lesser.

That said, doing other things to improve pedestrian safety are likely to be more effective than sweating the posted speed limit.