Speed limits – A review of the evidence

Speed limits are an important dimension of road safety management, but driving at speeds in excess of the posted speed limit is common in Britain, although this has been reducing over the last decade. Higher average (but not necessarily maximum) speeds mean shorter journey times, which in turn give economic and social benefits from lower costs and greater accessibility. However, excessive speeding can result in unwanted and costly environmental damage and increased deaths and injuries to motorists and other road users.

This paper assesses the different sides of the speed limit argument and looks at how things including safety, fuel consumption and carbon emissions are impacted by speed.

Key conclusions
• The environmental effects of speeding are principally increased noise and emissions of pollutants and greenhouse gases. The main pollutants are carbon monoxide; hydrocarbons; nitrous oxides; and particulates. The emission rates of these pollutants are highest in slow-moving traffic and are at their lowest between 25 mph and 55 mph. Above 55 mph they rise steadily with increasing speeds, as do CO2(carbon dioxide) emissions.

• Noise from road traffic can be a significant nuisance. At speeds up to 25 mph engine noise will usually dominate and this can be made worse with frequent acceleration and braking. Above this speed tyre noise becomes increasingly important.

• Speed is a contributory factor in 27% of deaths from road accidents in Britain, and, with stopping distances trebling between 30 mph and 60 mph, it is estimated that a 5% increase in speed can result in a 10% increase in accidents and a 20% increase in fatalities – depending on the type of road. Similarly the risk of a pedestrian being killed if hit by a car increases from 10% at 30 mph to 70% at 50 mph.