Emissions of CO2 are proportional to fuel consumption, which varies with speed. The optimum speed for minimal emissions with engines of present design is about 44mph. Slower speeds would reduce CO2 emissions from vehicles on the inter-urban network. By discouraging longer journeys they would help to restrain traffic growth and improve the competitiveness of public transport for longer journeys, further aiding CO2 reduction.

In towns, slower speeds would make walking and cycling safer, allowing much greater use of these non-polluting modes. They would also encourage smoother driving styles and reduce stop start traffic conditions, both major determinants of fuel consumption and emissions in urban areas. 10-15% of CO2 emissions are a result of driving style.

Appropriate speed management to curb CO2 emissions from traffic would reduce the demand for excessively powerful vehicles and lead to vehicles better adapted to the lower speed conditions under which they are most often driven.

In the long-term slower speeds would lead to less dispersed patterns of development, and a much less resource intensive transport system, with accompanying reductions in CO2 emissions.

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