How is a speed limit determined

What is the point of a general speed limit?

Does a speed limit protect the climate? Would there be fewer road fatalities? The discussions about it are bitter - but what are the facts? An overview.

The main problem with a fact check on this question: There is little and for the most part old data, especially from Germany. This opens up a lot of room for prognoses and assumptions and allows proponents and opponents of a speed limit on German autobahns to prepare the arguments for their own position. The most controversial issues are climate protection and security.

1. What does a speed limit do for climate protection?

Three million tons of CO2 savings per year with a general speed limit of 120: This number is also currently mentioned again when politicians argue about the sense and nonsense of the introduction. However, it essentially goes back to a study by the Federal Environment Agency (UBA) that is already 20 years old.

In 1999, the UBA calculated using data from 1992 that in 1996, with a general speed limit of 120, nine percent of emissions or 2.2 million tons of carbon dioxide could have been saved. In the same study, a savings potential of three million tons was forecast for 2005.

Since then, not really new figures have been collected - only these: According to the Federal Statistical Office, CO2 emissions on German roads in 2017 were 115 million tons. That is an increase of six percent compared to 2010. Even though cars have become cleaner on average, at least in terms of consumption. In short, this is due to the fact that more Germans drive cars, that many of them drive thicker cars and that, on average, they drive longer distances (all of this is also documented by the official statisticians from Wiesbaden).

The Öko-Institut - on behalf of the Berlin think tank Agora Verkehrswende - presented in much more detail what and how it calculated. Result: With a speed limit of 120, CO2 savings in road traffic of two to 3.5 million tons, with a speed of 130 1.1 to 1.6 percent.

How great the savings potential actually is - even opponents of a general speed limit cannot deny that driving slower means lower CO2 emissions. Basic knowledge of physics: If the speed is doubled, the air resistance is expected to quadruple, fuel consumption increases, as does carbon dioxide emissions.

However, many opponents of a general speed limit argue that route-related restrictions, as they already exist, would be more effective. A prognosis that is difficult to assess. The only fact is: According to the Federal Ministry of Transport, 30 percent of German motorway kilometers are currently regulated, 70 percent are not.

Overall, traffic in Germany only ranks third in terms of CO2 emissions (behind the energy sector and industry) - but it is the only significant area in which climate pollution is increasing. Therefore, any mitigation would actually help here.

According to the Federal Environment Agency, the introduction of a general speed limit on motorways would be “a short-term, feasible, cost-effective and effective contribution to reducing CO2 emissions and fuel consumption.” The Öko-Institut also emphasizes this cost-benefit effect.

2. What does a speed limit do for road safety?

There are a little more and more up-to-date figures here, but their classification is more disputed than with a view to climate protection.

On the one hand: motorways are the safest roads in terms of kilometers driven. In 2017, according to data from the Federal Highway Research Institute, vehicles covered around a third of their distance on the autobahn. But only one in eight traffic fatalities (12.9 percent) occurred on a motorway. Of the 409 people who died here, 181 were killed in accidents in which motorists exceeded the maximum speed or were driving too fast for the road or weather conditions.

On the other hand, excessive speed is by far the most common cause of accidents on motorways. This is the case in more than every third case; for all roads this only applies to every eighth collision. There are two prominent examples of the fact that a speed limit can have a noticeable effect here. A study for Brandenburg published by the Potsdam Ministry of Infrastructure and Regional Planning in 2007 showed (PDF file) that where speed limits were introduced in the state, the number of accidents, fatalities and injuries fell significantly. However, in the same period where no limit was introduced, the number of accidents also fell. But even if you take this into account, the effect is also clear.

The second example comes from North Rhine-Westphalia. On the A4 between the municipalities of Elsdorf and Merzenich, a speed limit of 130 kilometers per hour was introduced in 2017 after several serious accidents with numerous injuries and a total of nine deaths in the previous three years. Consequences: no more fatal accidents, fewer injuries, around 85 percent of all drivers adhere to the speed limits, the average speed fell on the roughly ten-kilometer section from more than 140 to 120 kilometers per hour.

The ADAC counters this by saying that overall sections of the route without a speed limit are no more conspicuous in speed accidents than those with a limit. One could conclude from this that there is not necessarily an urgent need for action in terms of accident avoidance - but not that a speed limit on motorways would not do anything for road safety. Wherever speed is already limited today, averting danger is usually the most important reason.

At the same time, there are many international studies that generally show that slower driving improves safety and faster driving increases the potential for danger.

3. What does a speed limit do for the flow of traffic?

This question is almost the most difficult to answer because the methodological problems possibly weigh the heaviest here: Who measures what, where and when, and compares it with what? If you follow the climate and environmentalists, a general speed limit reduces the speed differences. On the one hand, this would minimize the risk of congestion and thus help the climate. And at the same time it would reduce the risk of accidents and thus traffic safety (this argument is also made by the police union, which advocates a speed limit).

The main causes of traffic jams on German autobahns are high traffic volumes and construction sites; The Institute for Transport makes these two factors responsible for a good two thirds of all traffic congestion. Nevertheless, a number of traffic experts and traffic jam researchers estimate that the large differences in speed also have a negative effect on the flow of traffic. A speed limit could ensure more steady speeds with fewer braking maneuvers and lane changes on busy roads and thus reduce the risk of traffic jams, proponents argue.

The ADAC counters this - and is supported by the well-known traffic jam researcher Michael Schreckenberg: Most traffic jam-intensive routes already have a speed limit. A flexible, situation-appropriate speed regulation is more effective.

A look at other countries ...

... actually only has statistical value. He hardly provides any information about which changes in the law could bring about what. The speed limits there have been in place for many years, and the volume of traffic and road situations vary widely. A brief overview is sufficient:

Max. Allowed speedIn townOut of townOn highways
Denmark50 km / h80/90 km / h130 km / h
France50 km / h80/90 km / h110/130 km / h
Italy50 km / h90 km / h110/130 km / h
Netherlands50 km / h80 km / h130 km / h
Austria50 km / h100 km / h130 km / h
Poland50 km / h100 km / h140 km / h
Sweden40-60 km / h70-90 km / h110-120 km / h
Switzerland50 km / h80/100 km / h120 km / h

Lots of data, arguments and considerations show that a general speed limit on German autobahns would have an impact on CO2 emissions and accident statistics - the only question is how big they would be. That can hardly be answered reliably. One thing is certain, however: it would be a comparatively cheap measure and nothing to be done would have no effect at all.

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