80% Life

There is a very disturbing ad on the television about road saftey.

According to the ad if you hit someone, while driving, at the speed of 40 miles per hour (approx 65 kilometers per hour) then there is an 80% chance that they will die.

On the other hand if you hit someone at 30 miles per hour (approx 48 kilometers per hour) there is an 80% chance that they will live.

The ad ends with the words 'It is 30mph for a reason!'

This got me thinking that why is it at 30mph. Is 80% survival rate good enough?
Who decided 80% was good enough?
Or was this analysis carried out after the speed limit was set at 30mph?