Do lower speed limits on motorways reduce fuel consumption and pollutant emissions?

Based on a simulation, cutting motorway speed limits from 120 to 110 km/h could deliver fuel savings for current technology passenger cars of 12–18 %, assuming smooth driving and 100 % compliance with speed limits. However, relaxing these assumptions to a more realistic setting implies a saving of just 2–3 %.

Significant fuel savings can be achieved by encouraging drivers to maintain a consistent speed and restrict their speed (eco-driving), including through effective enforcement of speed limits.

Cutting speed can also significantly reduce emissions of other pollutants, particularly reducing NO_x and particulate matter (PM) output from diesel vehicles. The safety gains from slower driving are also indisputable.

Transport: a major contributor to greenhouse gas emissions

Transport is the only sector whose greenhouse emissions increased between 1990 and 2008. Transport’s total GHG output rose 25 % in the 32 EEA member countries (this excludes the international maritime and aviation sectors), accounting for 19.5 % of total emissions. CO₂ is the main component of transport greenhouse gas emissions (99 %) and road transport is, in turn, the largest contributor to these emissions (around 94 % in 2008), thus accounting for 18.2% of total emissions.

New vehicles are, on average, more energy efficient than older vehicles, and the improvement will increase as a result of recent EU regulation on cars and CO₂ and the agreement on similar legislation for light commercial vehicles. However, full fleet penetration of new technologies takes almost two decades. Moreover, impacts will also be offset by the likely growth in transport volumes. As such, other measures must also be considered to achieve short-term improvements in GHG emissions and energy consumption.

It is also worth noting in this context that, because CO₂ emissions are directly linked to fuel consumption, measures designed to reduce GHG emissions from transport would also help reduce dependence on oil imports. Targets set recently in EU papers and strategies, such as the Roadmap for a low carbon economy and the recently published White paper on Transport, encourage implementation of such measures.

The idea of using more stringent speed limits to reduce travelling speeds on motorways and thereby cut fuel consumption and transport emissions has received much attention recently. Among all the potential measures available, stricter speed limits could have an immediate effect on fuel consumption and emissions. Scientific evidence and knowledge sharing could help make lower speed limits more politically acceptable by clarifying the environmental consequences, as well as the impacts on safety and mobility.

Current speed limits differ across EU Member States, and the competence to define them generally lies with national governments. Some countries also apply variable speed limits related to traffic and weather conditions. For these reasons it is not possible to simulate the precise effects of a speed limitation across all EU Member States. In addition, the actual fuel consumption benefits of lower speed limits depend on factors such as the type of cars using the motorways, driving patterns, the frequency of speeding, road load patterns and congestion. Estimating the benefits is not straightforward but this note aims to convey the main messages on the relationship between speed and fuel consumption.

Simulating reduced speed limits

Emission models are generally used to assess the impact of speed management measures. COPERT is a robust emission model widely used in Europe, with COPERT 4 being its latest version. Its consumption factors are expressed as a function of the mean travelling speed and have been obtained based on tests of a variety of passenger cars and driving cycles.

For the purposes of this note, EMISIA ([1]) conducted a simulation of three driving cycles in order to simulate the fuel consumption impact of reducing a motorway speed limit from 120 to 110 km/h. The simulation used two medium class vehicles, representative of the typical diesel and gasoline passenger cars used in European countries (1.4 litre Euro 4 emission standard, as presented in the annex).

The three cycles simulate were as follows:

• ARTEMIS 130: a typical driving cycle assuming a speed limit of 120 km/h, which is not fully respected, meaning that some speeding occurs.
• Speed limit 110 km/h: a driving cycle assuming that all drivers fully respect the speed limit and that the vehicles are very smoothly driven at the speed limit. This is an artificial condition but may demonstrate the maximum potential results of introducing a new speed limit.
• ARTEMIS 120: similar assumptions to ARTEMIS 130 and considering that the reduction of the speed limit from 120 km/h to 110 km/h will also decrease cruise speed by 10 km/h. As the ARTEMIS 130 cycle, it is assumed that the speed limit of 110 km/h is not fully respected and some speeding occurs.

The three driving cycles used in the simulation are shown in Figure 1.

Figure 1: speed profile of the driving cycles used in the analysis

Source: EMISIA - ETC/ACM

Results and discussion

The simulation reveals that shifting from the ARTEMIS 130 cycle to fully respecting the speed limit and controlling the speed at 110 km/h would produce a significant drop in fuel consumption — 12 % in the case of a diesel car and 18 % in the case of a gasoline car.

However, shifting from ARTEMIS 130 to the more ‘realistic’ ARTEMIS 120 cycle produces a much smaller reduction of 2–3 %. This is mostly caused by the fact that when a car travels at a lower average speed, the wind resistance decreases and therefore the car requires less energy.

Table 1: Characteristic values for the three driving cycles used

The simulation results shown in Table 1 demonstrate that fuel consumption generally decreases with speed, although the exact benefits are context specific. Figures 2, 3 and 4 likewise illustrate the link between average speed, fuel consumption and pollutant emissions for Euro 4 diesel and gasoline cars with engines of 1.4–2.0 litre capacity.

Figures 3 and 4 show that reducing speed in the above range has a beneficial effect for all pollutants except for CO (in the case of diesel vehicles) and NOx (in the case of gasoline vehicles). The benefits of reducing average speed from 100 km/h to 90 km/h range from 25 % (gasoline CO) to 5% (diesel PM). Crucially, decreasing speed reduces the two pollutants currently most important in Europe: diesel NO_x and PM.

Figure 2: Impact of travelling speed on fuel consumption (Euro 4 diesel and gasoline passenger cars, 1.4–2.0 litre engine capacity)

Note: emissions expressed relative to their values at 100 km/h, for which the value '1' is assigned.

Source: EMISIA - ETC/ACM

The rise in diesel CO and gasoline NO_x emissions at decreasing average speeds is largely due to the operation of after-treatment devices. The diesel oxidation catalyst operates more efficiently at high speed due to the higher temperature, therefore oxidising carbon monoxide more effectively. Diesel vehicles are minor contributors of CO, however, and CO is not a problem for air quality in Europe. As such, this impact of decreasing average speeds would not cause problems.

For gasoline engines, increasing speed up to approximately 115 km/h leads to lower NO_x emissions, although emissions increase again above that speed. Gasoline vehicles emit much less NO_x than diesel vehicles. According to COPERT, a gasoline Euro 4 car emits 19 mg/km NO_x compared to 560 mg/km of a corresponding diesel car at 100 km/h. Therefore, the overall effect on NO_x of reducing the speed on motorways would be positive because diesel NO_x is dominant and clearly drops with decreasing speed.

Figure 3: Impact of travelling speed on various pollutants (Euro 4 diesel passenger cars, 1.4–2.0 litre engine capacity)

Note: emissions expressed relative to their values at 100 km/h, for which the value '1' is assigned.

Source: EMISIA - ETC/ACM

NO_x denotes ‘nitrogen oxides’; PM denotes ‘particulate matter’; THC denotes ‘total hydrocarbons’; CO denotes ‘carbon monoxide’.

Figure 4: Impact of travelling speed on various pollutants (Euro 4 gasoline passenger cars, 1.4–2.0 litre engine capacity)

Note: emissions expressed relative to their values at 100 km/h, for which the value '1' is assigned.

Source: EMISIA - ETC/ACM

In summary, whereas heavy goods vehicles speed limits in motorways are in line with the optimum speed in terms of energy and CO₂ reductions per vehicle-km (80–90 km/h), decreasing car passenger speed limits in motorways could lead to substantial benefits.

The modelling results also suggest that speed limitations of 80–90 km/h on motorways when entering cities and on city ring roads could significantly reduce both fuel consumption and pollutants emitted, in addition to delivering safety benefits.

On the other hand, energy and emissions benefits from more stringent speed limits on local roads (e.g. from 50 to 30 km/h) are less clear. The key argument for lower speeds on local roads is therefore the desirability of a safer and more tranquil local environment, rather than environmental considerations.

A new balance in setting speed limits?

Setting a speed limit is about balancing three core priorities: mobility, safety and the environment. Factors such as Europe’s dependence on fuel imports, concerns about sustained oil supplies and better environmental understanding are encouraging governments to rethink their speed limit decisions and work to find a new optimum balance.

Central to their decision-making will be public willingness to change behaviour. Encouragingly, a recent public poll (Flash Eurobarometer Report, no. 312, Future of Transport) indicates that about two thirds of EU citizens are 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 (^[2]).

This suggests that there is clear value in providing citizens with a clear understanding of the benefits and costs. After all, a speed limit decrease of 10 km/h in a motorway (from 120 to 110 km/h) would mean an extra travel time of just eight to nine minutes in a 200 km long trip, assuming perfect flow conditions. That is arguably a limited price to pay in exchange for the fuel savings and environmental benefits. At the same time, it seems clear that drivers’ theoretical support for lower limits is insufficient — steps to improve compliance, including tighter enforcement, will be essential to achieve concrete results.

Annex    

Specifications of cars used in the simulations

Current general speed limits in European countries

Source: DG TREN, 2010. Energy and Transport Statistical Pocketbook.

Notes:

UK, IE, CY and MT drive on the left hand side of the road, the other Member States drive on the right hand side (SE since 3.9.1967).

Signs in UK are in miles per hour.

The higher figure shown in the 'outside built-up areas' column generally refers to the speed limit on dual carriageways that are not motorways.

Speed limits:

• Germany: Motorways: No general speed limit, recommended speed limit is 130 km/h (more than half the network has a speed limit of 120 km/h or less).
• France: Dual carriageways 110 km/h. If road is wet: motorways 110 km/h, dual carriageways 90 km/h, other roads outside built-up areas 80 km/h.
• Spain: New motorway speed limit from March 7th 2011
• Italy: 150 km/h on certain 2x3 lane motorways.
• Finland: in winter 100 km/h on motorways, 80 km/h on other roads.
• Poland: Built-up areas: 50 km/h from 05h00 to 23h00, 60 km/h from 23h00 to 05h00.
• Turkey: Speed limit recently changed to 110 km/h on dual carriageways and 120 km/h on motorways