Why slower is better

Lower maximum motorway speed helps reduce CO2 emissions

Driving at lower speeds is better for the climate. In a pilot study CE Delft has estimated the potential CO2 savings arising in various scenarios with tighter motorway speed limits. Lowering the speed limit for cars to 80 km/h can reduce transport CO2 emissions on highways by 30% in the longer term.

Short- and long-term CO2 emission cuts as a share of total motorway CO2 emissions by cars in various scenarios

Everywhere 100, 90 or 80 means that all highway speed limits that are higher than 100, 90 or 80 are reduced to 100, 90 or 80. Lower speed limits remain the same.

The maximum long-term CO2 reduction was estimated to be 2.8 Mt for passenger cars and a further 0.2 Mt for vans. In the case of cars, this means a 30% reduction in motorway emissions. This maximum reduction is achieved with a uniform speed limit of 80 km/h, with strict enforcement thereof. Less drastic tightening of speed limits means more modest emission cuts, but depending on the scenario still leads to a 8 to 21% reduction in motorway car emissions.

It is common knowledge that, on average, vehicles burn less fuel per kilometre at lower speeds.

Relationship between vehicle speed (km/h) and CO2 emission (gram/km) at constant speed

Source: TNO data, adapted by CE Delft.

From private car to public transport
Less widely realised is the fact that, because of the longer travel times resulting, lowering motorway speed limits will also lead to less car-kilometres being driven and a certain shift from private car to public transport.

In the long term the CO2 savings resulting from the reduction in car-kilometres will become increasingly pronounced, as structural behavioural change sets in (people moving closer to their workplace, shops relocating closer to consumers, etc.).

Reduced CO2 emissions are just one of the benefits of lowering speed limits. There will also be improvements in terms of air pollution, noise nuisance and possibly congestion and traffic safety, too. Lowering motorway speed limits also has its downside, though. On average, people will be on the road for longer for a given journey and their annual mileage will be lower. From the perspective of economic welfare, both the lower speed and the reduced volume of traffic count as costs. A follow-up study on the social costs and benefits would enable calculation of ‘optimal’ speed limits.

Social costs and benefits of lower motorway speed limits