Evaluating the citywide Edinburgh 20mph speed limit intervention effects on traffic speed and volume: a pre-post observational evaluation

Glenna Nightingale*, Andrew James Williams, Ruth Hunter, James Woodcock, Kieran Turner, Claire Cleland, Graham Baker, Michael Kelly, Andy Cope, Frank Kee, Karen Milton, Charlie Foster, Ruth Jepson, Paul Kelly

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

9 Citations (Scopus)
4 Downloads (Pure)


Objectives: Traffic speed is important to public health as it is a major contributory factor to collision risk and casualty severity. 20mph (32km/h) speed limit interventions are an increasingly common approach to address this transport and health challenge, but a more developed evidence base is needed to understand their effects. This study describes the changes in traffic speed and traffic volume in the City of Edinburgh, pre- and 12 months post-implementation of phased city-wide 20mph speed limits from 2016-2018.
Methods: The City of Edinburgh Council collected speed and volume data across one full week (24 hours a day) pre- and post-20mph speed limits for 66 streets. The pre- and post-speed limit intervention data were compared using measures of central tendency, dispersion, and basic t-tests. The changes were assessed at different aggregations and evaluated for statistical significance (alpha = 0.05). A mixed effects model was used to model speed reduction, in the presence of key variables such as baseline traffic speed and time of day.
Results: City-wide, a statistically significant reduction in mean speed of 1.34mph (95% CI 0.95 to 1.72) was observed at 12 months post-implementation, representing a 5.7% reduction. Reductions in speed were observed throughout the day and across the week, and larger reductions in speed were observed on roads with higher initial speeds. Mean 7-day volume of traffic was found to be lower by 86 vehicles (95% CI: -112 to 286) representing a reduction of 2.4% across the city of Edinburgh (p=0.39) but with the direction of effect uncertain.
Conclusions: The implementation of the city-wide 20mph speed limit intervention was associated with meaningful reductions in traffic speeds but not volume. The reduction observed in road traffic speed may act as a mechanism to lessen the frequency and severity of collisions and casualties, increase road safety, and improve liveability.
Original languageEnglish
Article numbere0261383
Number of pages14
JournalPLoS ONE
Issue number12
Publication statusPublished - 31 Dec 2021


  • 20mph speed limits
  • Public health
  • Transport
  • Evaluation


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