Do lower speed thresholds on motorways reduce fuel consumption and pollutant emissions?
Based on a simulation, cutting motorway speed boundaries from one hundred twenty to one hundred ten km/h could supply fuel savings for current technology passenger cars of 12–18 %, assuming sleek driving and one hundred % compliance with speed thresholds. However, calming 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 boundaries.
Cutting speed can also significantly reduce emissions of other pollutants, particularly reducing NOx 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 enlargened inbetween one thousand nine hundred ninety and 2008. Transport’s total GHG output rose twenty five % in the thirty two EEA member countries (this excludes the international maritime and aviation sectors), accounting for Nineteen.Five % 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 ninety four % in 2008), thus accounting for Legal.2% of total emissions.
Fresh vehicles are, on average, more energy efficient than older vehicles, and the improvement will increase as a result of latest EU regulation on cars and CO2 and the agreement on similar legislation for light commercial vehicles. However, total fleet invasion of fresh 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 boundaries 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 boundaries could have an instant effect on fuel consumption and emissions. Scientific evidence and skill sharing could help make lower speed thresholds more politically acceptable by clarifying the environmental consequences, as well as the impacts on safety and mobility.
Current speed boundaries differ across EU Member States, and the competence to define them generally lies with national governments. Some countries also apply variable speed thresholds 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 boundaries depend on factors such as the type of cars using the motorways, driving patterns, the frequency of speeding, road blast patterns and congestion. Estimating the benefits is not straightforward but this note aims to convey the main messages on the relationship inbetween speed and fuel consumption.
Simulating diminished speed boundaries
Emission models are generally used to assess the influence of speed management measures. COPERT is a sturdy emission model widely used in Europe, with COPERT four being its latest version. Its consumption factors are voiced as a function of the mean travelling speed and have been obtained based on tests of a multitude 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 influence of reducing a motorway speed limit from one hundred twenty to one hundred ten km/h. The simulation used two medium class vehicles, representative of the typical diesel and gasoline passenger cars used in European countries (1.Four litre Euro four emission standard, as introduced in the annex).
The three cycles simulate were as goes after:
- ARTEMIS 130: a typical driving cycle assuming a speed limit of one hundred twenty km/h, which is not fully respected, meaning that some speeding occurs.
- Speed limit one hundred ten km/h: a driving cycle assuming that all drivers fully respect the speed limit and that the vehicles are very sleekly driven at the speed limit. This is an artificial condition but may demonstrate the maximum potential results of introducing a fresh speed limit.
- ARTEMIS 120: similar assumptions to ARTEMIS one hundred thirty and considering that the reduction of the speed limit from one hundred twenty km/h to one hundred ten km/h will also decrease cruise speed by ten km/h. As the ARTEMIS one hundred thirty cycle, it is assumed that the speed limit of one hundred ten 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 exposes that shifting from the ARTEMIS one hundred thirty cycle to fully respecting the speed limit and controlling the speed at one hundred ten km/h would produce a significant drop in fuel consumption — twelve % in the case of a diesel car and eighteen % in the case of a gasoline car.
However, shifting from ARTEMIS one hundred thirty to the more ‘realistic’ ARTEMIS one hundred twenty 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