Enforcement

Title: Interim evaluation of the implementation of 20 mph speed limits in Portsmouth

Organisation: Department for Transport
Date uploaded: 21st September 2010
Date published/launched: September 2010

Portsmouth City Council (PCC) is the first local authority in England to implement an extensive area-wide 20 mph speed limit scheme. This document results from an interim evaluation of the impact of the scheme, focusing on early monitored results.

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Portsmouth City Council (PCC) is the first local authority in England to implement an extensive area-wide 20 mph speed limit scheme – that is introducing signed 20 mph limits largely without traffic calming, covering most of its residential roads which previously had a 30 mph speed limit. This is therefore an important scheme which can be compared to more traditional 20 mph zones, which involve extensive traffic calming.

This document results from an interim evaluation of the impact of the scheme, focusing on early monitored results. It reports on monitored changes in traffic speeds, traffic volume and road casualties, comparing data for ‘Before’ and ‘After’ scheme implementation as well as resident perception of impacts through qualitative surveys. The document is intended to provide an early transfer of information to other local highway authorities on the effectiveness of implementing speed limits through use of signs alone and without providing any accompanying traffic calming measures.

The implementation of the 20 mph Speed Limit scheme was carried out using a combination of post-mounted terminal and repeater signs. 20 mph speed limit roundel road markings were also provided at street entry points on the carriageway adjacent to the terminal post-mounted signs. In some cases of limited visibility, they were also provided adjacent to the repeater signs.

For ease of installation the city was divided into six sectors: Central East, Central West, South East, South West, North East and North West. This amounted to 94% of road length (410 km of the 438 km of road length) in PCC.

On most of the roads where the speed limit signs and road markings were installed, the average speeds before installation were less than or equal to 24 mph. The relatively low speeds before the scheme implementation were because of narrow carriageways and on-street parking, which further reduces effective width of the carriageways. 20 mph signs were also provided on roads with average speeds greater than 24 mph in order to avoid inconsistencies in the signed speed limits in Portsmouth. One of the aims of the scheme was to be self-enforcing (avoid the need of extra Police enforcement) and partly to support the low driving speeds, and encourage less aggressive driving behaviour.

Overall there was an increase in the number of sites that demonstrated speeds of 20 mph or less after the implementation of the scheme. Many sites already had low average speeds of 20 mph or less before the scheme was implemented. At the sites monitored with higher average speeds before the scheme was introduced, there were significant reductions in average speeds. For example for the group of sites monitored with average speeds of 24 mph or more before the scheme was introduced, the average speed reduction was 6.3 mph. The average reduction in mean speeds on all roads was 1.3 mph.

There is insufficient data to comment about the effects of the scheme on traffic routes and volumes. The expectation is that because most roads had fairly slow average speeds before the scheme was implemented, that the changes are likely to have been modest.

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. During that period casualty numbers fell nationally – by about 14% in comparable areas.

There are no large apparent disparities between the casualty changes for different groups of road users (for example pedestrians compared to motorists) or between crashes with different causes. The number of deaths and serious injuries rose from 19 to 20 per year. Because the total numbers of deaths and serious injuries and of casualties by road user type and cause are relatively low, few inferences about the scheme’s impacts should be drawn from these figures.

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.

In conclusion, early figures suggest that the implementation of the 20 mph Speed Limit scheme has been associated with reductions in road casualty numbers. The scheme has reduced average speeds and been well-supported during its first two years of operation.

For more information contact:
Department of Transport Research Team

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