The pro speed drivers.
basic physics. Mass and speed > the greater the speed , the harder the impact of any collision.
The chances are, hitting someone at 20MPH there is less chance of killing someone than at 30mph and so on.
Moreover, as some folk are keen to blame pedestrians/kids - yes, adults and kids step out into the road without looking, but if you are not speeding you have more time to react and avoid a collision.