There is growing understanding of the true social and environmental costs of a transport system based on ever increasing private car travel. Various measures to reduce traffic volumes and change the mix of traffic modes — shifting the balance away from car journeys and road freight to walking, cycling and public transport — are being introduced.

Meanwhile traffic and its impacts continue inexorably to grow because people are travelling further faster. Since the early 1970s, the overall distance travelled per person per year has increased by 55% but the average time spent travelling is almost the same.

None of our speed limits have an empirical basis, despite the importance of speed to the overall efficiency of our transport system and its range of impacts. Speed directly affects the frequency and severity of crashes, journey times, fuel consumption and emissions, noise, vehicle operating costs, highway capacity and the amount of land needed for roads. Indirectly it affects transport mode choice by introducing danger and by encouraging dispersed development.

Dispersed development has helped to undermine local economic stability and widen social divisions by simultaneously stranding poorer people in decaying urban centres and pricing them out of homes in rural areas.

Despite these important and far-reaching impacts, the role of speed management in balancing the costs and benefits of the transport system is almost completely overlooked.

Environmental impact assessment and optimising procedures such as cost benefit analysis and cost efficiency comparisons are regularly used to make choices and evaluate performance in many areas of development and resource management. Speed management is exceptional in the absence of this approach. In addition to conventional approaches, the problem of setting speed limits lends itself to sustainability appraisal and to the application of accepted sustainability principles — precaution, prevention, polluter-pays, integration and participation.

Sustainable, evidence-based, speed limits would reduce the impacts of road transport while making their distribution more transparent. They are necessary to enable transition to a more sustainable transport system.

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