I have to agree with kezo …. 20 mph speed limits are potentially more dangerous than 30 mph limits, due to a driver’s concentration and eyes being repeatedly on the speedometer rather than on the road and pavements.
We all have an innate feeling for what 30 mph feels like due to years of inbuilt learning and practice. 20mph will take years to in-bed intuitively and will involve much more time spent looking at the car dashboard to avoid fines.