Two years after implementation of 20mph speed limits on 94% of the Portsmouth street network, results have shown that speeds fell, especially on roads where speeds were already high. Casualties have declined and attitudes of people to 20mph and both walking and cycling have improved.
Since speeds were already low on most of the streets in the scheme (Portsmouth has narrow residential streets with lots of car parking), the overall reduction in speeds was low - around 1.3mph. However, on the streets where average speeds were greater than 24mph a 6.3mph reduction occurred.
Comparing the 3 years before the scheme was implemented and the 2 years afterwards, the number of recorded road casualties has fallen by 22% from 183 per year to 142 per year, faster than the fall in casualties in comparable areas elsewhere in the country.