Reducing speed limits on motorways: how good is it for the environment?

The theory is simple enough. Lower speed limits on motorways should reduce fuel consumption and pollutant emissions, particularly for passenger cars. Assuming smooth driving (little acceleration and braking), current technology passenger cars and total compliance with speed limits, it is estimated that cutting the motorway speed limit from 120 to 110 km per hour would reduce fuel consumption by 12 % for diesel cars and 18 % for gasoline cars.

Of course, these assumptions are quite artificial. In reality, a variety of factors are likely to limit the fuel savings, including the energy efficiency levels of the vehicle fleet, driving patterns, speeding and traffic congestion. In a more realistic scenario — including speed limit exceedances and frequent fluctuations in driving speeds — the actual fuel savings would be just 2–3 %.

Transport emissions must be cut

The debate around motorway speed limits is timely, given the urgent need to address global warming and air pollution.

Compared to 1990, the EU has significantly reduced its total greenhouse gas (GHG) emissions, with policy and technology helping break the link between economic growth and GHG emissions in almost all sectors. The only clear exception is transport, where GHG emissions actually rose by 25 % between 1990 and 2008 in the 32 EEA member countries (these numbers exclude the international maritime and aviation sectors).

Transport accounts for nearly 20 % of the European Union's greenhouse gas emissions with carbon dioxide (CO₂) comprising the main component of transport emissions (99 %). Road transport is, in turn, the largest contributor to CO₂ emissions from the transport sector (around 94 % in 2008), thus accounting for more than 18 % of the EU-27's total emissions. As such, Europe must tackle transport emissions if it is to achieve significant reductions in its overall GHG emissions.

Evidently, transport sector's impact on the environment is not limited to GHG emissions. Notwithstanding recent reductions in air pollutant emissions, road transport remains the largest emitter of nitrogen oxides (NO_x) and the second largest contributor of pollutants forming particulate matter (PM). Lowering speed limits (thereby reducing fuel consumption) and cleaner technology, in particular for diesel vehicles, would reduce NO_x and PM emissions and consequently help improve Europe’s air quality.

Cleaner technology is not enough

New vehicles are, on average, more energy-efficient than older vehicles. The recent EU regulation on cars and CO₂ and the agreement on similar legislation for light commercial vehicles will improve this further. Unfortunately, however, full fleet penetration of these new technologies is expected to take almost two decades. Moreover, reductions in GHG emissions are likely to be offset by the expected growth in transport volumes. Other measures must therefore be considered to achieve cut GHG emissions and energy consumption in the short term.

Europeans favour slower journeys — at least in theory

Setting a speed limit requires striking a balance between mobility, safety and the environment. On a 200 km long trip, the reduction of the speed limit from 120 to 110 km/hour would mean an extra travel time of around eight to nine minutes, assuming perfect traffic flow.

Are European drivers ready to accept slightly longer travel times? The answer is probably "yes". According to a recent public poll (Flash Eurobarometer Report, no. 312, Future of Transport), about two thirds of EU citizens were willing to compromise a car's speed in order to reduce emissions.

The reality on the roads, however, appears to be quite contradictory. Around 40–50 % of drivers (up to 80 % depending on the country and type of roads) drive above legal speed limits ([1]). Clearly, drivers' theoretical support for lower limits is not enough. Compliance and tighter enforcement are also essential to achieve concrete results.

More information

• For additional information on simulation models, see the background note.
• Greenhouse gas emissions data viewer

• Towards a resource-efficient transport system — TERM 2009

[1] Speed management OECD-ECMT 2006