The 20mph campaign/s and arguments...

Children in particular are poor at detecting approaching speeds:

Scientists at the Department of Psychology, Royal Holloway, University of London, have discovered that primary-age schoolchildren cannot accurately see, or judge, the speed of vehicles travelling above 25mph. In fact, six- to 11-year-olds might sometimes not be able to tell that a vehicle is approaching owing to a trick of the mind that also affects adults, although it is far more pronounced in children.
As Professor John Wann, a 55-year-old driver, cyclist and motorcyclist who led the research, explains: “It’s not a matter of children not paying attention but a problem related to low-level visual detection mechanisms. Even when children are paying very close attention they may fail to detect a fast-approaching vehicle.”

“It seems clear-cut,” says Prof Wann. “Driving in excess of 20mph in a residential or school area not only increases the potential severity of any impact with a pedestrian, but also increases the risk that a child will injudiciously cross in front of your car.”

http://www.telegraph.co.uk/motoring/road-safety/8644983/Why-20mph-limits-help-save-lives.html

When you think about it, it's basic human biology. Humans were created (or designed) to withstand impact speeds that equal the impact of running as fast as possible into a tree or rock. Above 20mph the risks increase tremendously. 20mph zones save lives, the evidence is indisputable. To demonstrate this. I can drive into you at 20mph, then 35mph, you can tell us which hurts more.