Why change to 20mph when drivers ignore the current limit?

Small reductions in speed greatly increase survival rates

Evidence from many studies confirms that a 20mph speed limit does reduce average speeds.
An extensive before-and-after study by Napier University of vehicle speeds in villages found that 85% of drivers dropped their speed by 3 mph. That may not sound much, but even small reductions in average speed greatly improve survival rates for pedestrians and vehicle occupants (see “The effect of speed panel”).