New speed limits aren't designed to stop those who disregard the law and speed anyway, they're designed to improve the survival rates for pedestrians hit by the majority who drive sensibly within the law.
Fact of the matter is if you hit someone at 30mph there is a one in five chance you will kill them, whereas it's one in forty if you hit them at 20mph. (
http://news.bbc.co.uk/1/hi/uk/8009364.stm)