driving 50 in a 30 is a 66% increase, not a 10%-25% increase like gp was suggesting. When speeding is more extreme it probably crosses a threshold where it's safer to drive the speed limit than to drive with the traffic. But when the difference is 10%-25% (which is accurate IME in the US) then you only have to experience it a few times to know it's less safe to drive the speed limit.