Their effectiveness has not always been fully understood. PACTS latest research has shed light on the situation. It confirms that 20mph limits backed by physical measures have substantially greater speed and casualty reduction benefits than those without.
The project, 'Lower urban speed limits in Europe – what does the evidence show?' (LUSTRE), involved a collaboration of road safety experts, including Professor Mohammed Quddus (Imperial College London) and Dr Akis Theofilatos (Loughborough University), Dr Rune Elvik (Institute of Transport Economics, Norway), Professor Lars-Christer Hyden (Lund University, Sweden), Ellen Townsend and Jenny Carson (European Transport Safety Council). It has been made possible with a grant of £60,000 from the Road Safety Trust.
The report opens with the development of speed limit-setting policy and methods in the UK. The history and impact of speed-reducing methods in France, Germany, The Netherlands, Norway, Sweden and Switzerland follows. Findings from a systematic review and classification of UK studies, known as meta-analysis, make up the third part of the report.
Findings are that the extent to which 20mph schemes deliver actual speed and casualty reductions depends on whether they are supported by other measures, such as road humps and/or changes in relative road width.
Schemes without other measures result in modest speed reductions. Compliance with 20mph limits where no physical measures are in place is poor. When a speed limit of 20mph is introduced with physical measures, speed is normally reduced to less than 20mph, provided it was less than about 30mph before the measures were implemented.
Translating this to casualties, in the UK, signed-only schemes result in approximately 11% fewer casualties than before they were introduced. This figure rises to 40% in schemes where physical measures are introduced, as the speed reduction is substantially greater.
The report notes that further studies of the impact of area-wide speed limits will give more insights. Intelligent Speed Assistance (ISA) could, ultimately, be the most effective way of delivering actual reductions in speed and casualties.
See the report