Victor gives some of the reasons for Matt's call:
Greater Manchester has the North West’s highest rate of overall pedestrian casualties. Last year 744 people were needlessly killed or seriously injured on Greater Manchester’s Roads and over 7000 people were hurt. Matt is calling for 20mph to become the speed limit on all residential roads, excluding major routes. Lowering urban and residential speed limits to 20 mph has been found to decrease child pedestrian accidents by up to 70%. In Portsmouth the 20mph limit on all residential roads has reduced casualties by 22%.Beyond this argument from safety, there is a strong case that winning back city streets from the dominance of the car is an important part of building stronger and more civilised communities.
There is more evidence and campaign news on 20's Plenty for Us.