Portsmouth City Council’s pioneering 20mph scheme has successfully reduced average speeds and road casualty numbers, latest interim results have concluded.

The city was the first local authority in England to implement a signed-only, area-wide 20mph scheme, which covers most of its residential roads – or 94% of the council’s road length.

The most signifi cant reductions in average speed occurred on the sites with higher average speeds before the scheme was introduced. Speed reduced by an average 6.3mph at sites where the average was 24mph or more before the scheme was introduced. The overall average speed reduction was 1.3mph.

The number of recorded road casualties fell by 22% from 183 to 142 per year in the two years after the scheme was implemented, compared with the three years before.

However, the Department for Transport-commissioned report found that 20mph zones – including traffic claming measures – were more effective than signed-only schemes for casualty and speed reduction.

Transport for London and Hull City Council’s 20mph zones resulted in a 9mph and 10.5mph speed reduction respectively.

The Atkins report cautions other councils against replicating Portsmouth’s scheme on an areawide scale without complementary traffic-calming measures.

In Portsmouth, the scheme has been successful due to its dense population and narrow residential streets, with cars parked on both sides. ‘As such, the provision of the 20mph speed limit on such roads only formalised an existing practice and helped reduce incidences of aggressive driving.’

Portsmouth is considering implementing 20mph zones on roads where average speeds are still in excess of 24mph.

Simon Moon, Portsmouth’s head of transport and street management said: ‘This latest interim report is very encouraging, in particular, the number of recorded casualties falling by 22% and a significant reduction in speed in sites monitored with higher average speeds.’

He said the scheme was about ‘changing the culture of driving in the city, which is a long-term job’.