Momentum is an unintuitive thing. See, for example http://www.brake.org.uk/events/15-facts-a-resources/facts/12...:

A vehicle travelling at 20mph (32km/h) would stop in time to avoid a child running out three car-lengths in front. The same vehicle travelling at 25mph (40km/h) would not be able to stop in time, and would hit the child at 18mph (29km/h). This is roughly the same impact as a child falling from an upstairs window. The diagram below illustrates the impact at various speeds. The greater the impact speed, the greater the chance of death. A pedestrian hit at 30mph has a very significant one in five chance of being killed. This rises significantly to a one in three chance if they are hit at 35mph

Or the charts here:

https://one.nhtsa.gov/nhtsa/Safety1nNum3ers/august2015/S1N_S...