What we should be aiming for is a civilised use of vehicles, with a slower, safer and more even flow of traffic. The psychological effect of not expecting to be able to travel at any real speed would be enormous. More eye contact and interaction generally would help humanise our relationships as drivers and pedestrians. Drivers would be much more willing to keep the traffic flow moving by giving way to each other. Most signal-controlled pedestrian crossings could revert to ordinary zebra crossings. Lower speeds reduce accidents and the damage to the human body when they do occur.
Children could walk or cycle to school without suffering the casualty rates which have confined them to cars in recent years. By reducing the differential between journeys by car on the one hand and by bus and bike on the other, people would be encouraged to use their cars less. Smoothing traffic flow with less braking and accelerating would reduce pollution levels.
The 20mph speed limit would not be cost-free - there is no such thing as a free lunch - but curbing the impact of traffic has an important part to play alongside curbing the numbers of vehicles on our roads.