How much would the scheme reduce deaths and injuries?

In Portsmouth, they introduced 20mph limits across the city and recorded data for 3 years before and 2 years afterwards. This showed a 22% reduction in injury collisions. However other similar areas of the UK also saw reductions of 14% without introducing limits so it is reasonable to conclude an 8% reduction due to the 20mph limits. Using Department for Transport figures for the costs of injury collisions, an 8% reduction in Worthing’s injury collisions would save £2 million in total costs (incl. human suffering) every year.

In London, 20mph zones have seen much higher reductions in injuries: 46% reduction in deaths and serious injuries (49% for children). In the Netherlands they estimate 30kph (18mph) residential roads to achieve a 70% reduction in casualties. Obviously the results are very dependent on how well observed the new speed limits are.

However research has shown even a 1mph reduction in average speeds results in about a 5% reduction in collisions.