There's also the well-known point that nine out of ten pedestrians survive being hit by a vehicle at 30mph. At 40 mph nine out of ten are killed. At 35 mph half are killed. So someone driving at just 5mph over the 30mph speed limit is five times more likely to kill someone if they step or run in front of them.
A driver travelling at 30mph also stands more chance of avoiding a pedestrian stepping out than someone going just that bit faster. Of course, if a pedestrian was hit by those people who reckon there's nothing wrong with speeding they would then maintain it wasn't their fault and "I couldn't miss 'em" or words to that effect.
As the signs say, it's 30mph for a reason. Someone driving at 35 or 40 just gets to the next junction, roundabout or set of lights quicker.