A large number of studies and trials have been carried out in the UK and internationally to assess the effectiveness of 20mph speed limits. An online search for studies that find 20mph limits to be ineffective (in terms of reducing speed, collisions and casualties) produces very little. In fact, just one study dominates the results, which is presumably why it is often cited by opponents of 20mph limits.

The title of the study is “Investigating the impact of a 20 miles per hour speed limit intervention on road traffic collisions, casualties, speed and volume in Belfast, UK: 3 year follow-up outcomes of a natural experiment”, published in the Journal of Epidemiology and Community Health.

One of its findings was that ‘restricting speed limits to 20mph in town and city centres doesn’t seem to reduce road traffic collisions, casualties, or driver speed’. However, the research team, led by Professor Ruth Hunter from the Centre for Public Health at Queen’s University Belfast, are keen to point out that if used as a headline, that sentence misrepresents the findings.

The researchers explained that the Belfast study focused on results from a small-scale implementation of 20mph speed limits across approximately 70 streets in Belfast city centre. The study showed that the average vehicle speed before the implementation of the 20mph speed limit was 17mph. Therefore the new speed limit made little difference to vehicle speeds, and researchers did not observe a statistically significant reduction in speed or subsequent significant reductions in collisions and casualties.

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