Slower cars kill you slowly. Dropping the speed from 40 MPH to 20 MPH means it takes twice as many milliseconds to grind you against a planter, mailbox, bus, parked car, etc.
This is wildly untrue--to a greater extent than one might expect. If you're a pedestrian and are hit by a car at an impact speed of 40mph, you have a roughly 50% chance of dying. The odds of death if you're hit at 25mph are roughly 12%.[0]
That's in addition to the fact that slower cars are much less likely to hit you at all, because they have more time to brake and maneuver before impact.
