Speed Limits Report 

Continuing our work and close ties with the 2020 Climate Group ECCI commissioned work from the Transport Research Laboratory to provide an independent, authoritative overview of issues relating to speed limits, including associated political and pressure group views.

The aim of the work was to provide independent authoritative evidence which underpins our (more political) discussion about what should be done, if anything, on speed limits. All the choices involve trade-offs (whether social, economic or environmental), so the issue is not "what is the right answer", rather it is: "what is most appropriate for Scotland" given its aims to become a economically vibrant, resilient, low carbon society.

Read on about key insights and next steps.

  • Carbon dioxide emissions: Reducing speeds to 40mph is likely to have a positive impact on vehicle emissions; reducing speeds beyond ~40mph is likely to have a disbenefit;

  • Observed vehicle speeds: Reducing a speed limit alone typically results in a change in average speed of as little as quarter of the change in speed limit; use of speed zones with additional physical measures are effective at reducing vehicle speeds.

  • Road safety: Small changes in mean speeds can be expected to result in much larger changes in crash outcomes. In other words, lowering vehicle speeds reduces severe crashes (resulting in severe injuries/deaths) disproportionately. But context matters. Reduction of speed in urban environments reduces severe crashes more than in rural areas. Keeping traffic flowing at similar speeds is also safer: roads with a small speed differential between the fastest/slowest vehicles are safer than roads with high-speed differential.

  • Air pollutants: Like carbon dioxide emissions, pollutant concentration is minimized at ~40mph; decreasing traffic speed beyond 40mph is likely to increase pollutants

  • Noise: Noise increases with speed and traffic volume. Noise arises from both engine and tyre noise. The ratio of tyre to engine noise is greater for cars than HGVs and tyre noise is more dependent on vehicle speed than engine noise. The greatest benefits of speed reduction on noise are seen on lower speed roads with low proportion of HGVs.

  • Communities and health: Reducing vehicle speeds can reduce social exclusion – by removing barriers to local mobility - and increase “healthy” modes of transport such as walking and cycling. Communities with the greatest levels of deprivation are likely to experience the greatest community and health benefits.

  • Journey times: Changes to journey times are normally the biggest contributor to economic impacts of speed limit changes, but are frequently over-estimated because they assume free-flowing traffic. The relationship between traffic flow and road capacity is crucial. The economic cost of reducing a speed limit typically peaks when flow is approximately two thirds of capacity. Drivers are poor at estimating time gains or losses when travelling at different speeds: savings in travel time by small increases in speed when driving at a high speed are overestimated; savings in travel time by a small increase in speed when driving at a low speed are underestimated. 

(available on request info@edinburghcentre.org) 

Next Steps

We propose to work with the Public Engagement Sub-Group (with input from the Transport Sub-Group) to run an event in 2013 to explore and develop wider public and political engagement with the speed limit issues.