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.
Qualitative surveys indicate that the scheme was generally supported by residents, although most of the respondents would like to see more enforcement of the 20 mph speed limits.
The survey suggests that the introduction of the scheme has made little difference to the majority of respondents in the amount they travelled by their chosen mode. Levels of car travel stayed similar, whilst the level of pedestrian travel, pedal cyclist travel and public transport usage had increased for a small number of respondents.