What´s the problem?
Throughout Europe and in all industrialised countries worldwide, there are speed limits on country roads and motorways. Germany is the only exception, with unlimited speed limits on around 70% of all motorway kilometres (Federal Highway Research Institute 2017). Only a few other countries, including Afghanistan, Haiti, Somalia and North Korea, do not have such speed limits. With a speed limit of 120 km/h on motorways and 80 km/h on country roads, up to 5 million tonnes could be saved, according to estimates by Deutsche Umwelthilfe CO₂. No other individual measure in the transport sector holds such a large and cost-effective CO₂ savings potential, even in the short term.
What´s the measure?
The legal implementation of a general speed limit on motorways of - for example - 120 km/h.
How can the implementation look like?
The implementation is very simple. With a corresponding resolution and the passing of a law, a general speed limit on the motorways in Germany could be introduced directly and almost without any effort in time and expense. The same applies to a possible reduction of the existing limit on country roads from 100 km/h to e.g. 80 km/h and in towns and cities from the current 50 km/h to e.g. 40 km/h.
How will this work against climate change?
The CO₂ emissions of passenger cars depend largely on the speed at which they are driven. Especially at higher speeds, such as those driven on motorways, the influence of speed on fuel consumption and thus CO₂ emissions is extensive. The reason for this are simple physical principles. The air resistance of a vehicle increases with the square of the driving speed, and accordingly, fuel consumption increases exponentially with an increase in speed. The following example illustrates this fact: If the speed of an average passenger car increases from 100 km/h to 130 km/h, CO₂ emissions increase by about 10%. A further increase of only 10 km/h from 130 to 140 km/h increases the emissions by another 10%! (Figures: Federal Environment Agency Austria²) The laws of physics apply, of course, regardless of the type of drive of the car. In principle, the electricity consumption of an electric car is the same as the fuel consumption of a conventional car. The electricity consumption of an electric car increases enormously at higher speeds.
On the basis of the latest calculations published by the Federal Environment Agency and taking into account the 25 percent increase in traffic performance in the meantime, German Environmental Aid assumes 5 km/h higher average speeds and an improved compliance rate of savings of up to 5 million tonnes CO₂ per year. In addition to the savings of a speed limit on motorways, there are the CO₂ savings that can only be roughly estimated at present if a speed limit of 80 km/h is introduced on country roads.¹ In addition to these immediate savings, a general speed limit in Germany would have the potential in the medium and long term for vehicles to be designed and built differently in the future (independently of further political steering measures). Since Germany is a car country and German car manufacturers export a large proportion of their vehicles, such a trend reversal initiated by a speed limit could also have effects in other countries. The average motorisation of new cars in Germany is rising steadily. Whereas in 1997 the average engine output was still 100 hp, by 2017 it will already be 152 hp. A speed limit can counteract this trend, since the incentive to build and buy highly motorised vehicles decreases if the possibility of driving at high speeds is removed.¹ Why should someone buy a car with 200 hp if he or she can never try out the performance of the car? A speed limit could therefore help to ensure that lightweight construction and economy finally really become the focus of vehicle development. The efficiency of the drives could also be optimized for the limited cruising speed.
What other positive effects does the measure have?
A general speed limit on motorways would increase road safety. There is a fixed relationship between the speed driven and the frequency of accidents, but logically also between the speed and intensity of a possible accident. These are facts that are repeatedly confirmed in real investigations and are absolutely logical in terms of the basic principles of physics. A higher speed reduces the reaction time that remains for a car driver to initiate measures to avert an accident, which can be braking or taking evasive action, for example. The braking distance of a vehicle also increases enormously at higher speeds. At a speed of 120 km/h, it is 108 metres for emergency braking. At 160 km/h the distance increases to 176 m, and at 200 km/h even to 260 metres.¹ In addition, the intensity and thus the consequences of a possible accident increase considerably at higher speeds. The reason for this is the fact that the kinetic energy of a vehicle is always proportional to the square of the speed at which it is moving. This means that the energy released during a collision, for example at the end of a traffic jam, almost doubles when the speed is increased by 25% (e.g. from 120 to 160 km/h), because it does not increase linearly but quadratically. The number of serious accidents and road deaths could be reduced, not only traffic experts say so, but real investigations confirm it. A study by the Ministry of Infrastructure and State Planning of the State of Brandenburg, for example, showed that the introduction of a speed limit on a good 60 km section of the A 24 motorway could almost halve the number of accidents.¹ On motorways with a speed limit, 75 percent fewer fatal accidents occur per billion kilometres driven than on motorway sections without a speed limit. A speed limit could also reduce the number of seriously injured in accidents by 20%.¹ The International Transport Forum (ITF) also calculates that reducing the average speed on country roads and motorways by just 5 km/h could reduce the frequency of fatal accidents by 18 to 28 per cent.
A speed limit on motorways would lead to better traffic flow and fewer traffic jams. On a motorway without a speed limit, the average speed difference of vehicles is significantly higher than on a motorway with a speed limit. A speed limit would reduce the speed difference and lead to more constant traffic with fewer traffic jams. An improved traffic flow not only increases the capacity of streets⁴, but also reduces fuel consumption and CO₂ emissions in traffic.
Not only the CO₂ emissions, but also the emissions of various air pollutants would be reduced by a speed limit. This is related to the effect of better traffic flow.
A speed limit would also lead to a slight reduction in road noise.⁵
How quickly can the measure be implemented?
The measure can be implemented immediately.
How long does it take for the measure to show positive effects?
The effect on CO₂ emissions and road safety would be immediate. The steering effect in the development of new vehicles away from ever more powerful, heavier and faster cars and towards more efficiency and lightweight construction could also unfold very soon.
Further literature, sources
3 Speed and Crash Risk, International Transport Forum, OECD, 2018, p. 12
4 Scholz; Schmallowsky; Wauer (2007): Auswirkungen eines allgemeinen Tempolimits auf Autobahnen im Land Brandenburg. Schlothauer & Wauer Ingenieurgesellschaft für Straßenwesen; Landesbetrieb Straßenwesen Brandenburg
5 Umweltbundesamt, 1999
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