At 20 MPH, 5% of pedestrians involved in a car-pedestrian collision will die.
At 30 MPH, it is 37-45%.
At 40 MPH it is 83-85%.
If you are thinking of traffic as simple car vs car, then speed limit should be one thing--probably a lot higher than they are now. Because people in cars are pretty impervious to injury.
But as soon as you realize that **this street is going through a neighborhood** everything changes. Honestly we should not allow cars (or anything else) to go faster than 15-20 MPH in neighborhoods and places where there are many people.
Fast driving drives away people and endangers them. That is the opposite of what we want in our communities.
The most effective strategy for eliminating traffic fatalities has relied heavily on this approach--reduce speed, and fatalities reduce at least proportionately.