Motorway speed limits are cropping up across Europe but are they a worthwhile climate solution?

Authorities across Europe are increasingly looking at introducing lower speed limits on motorways in a bid to reduce carbon emissions despite some pushback on whether it’s a worthwhile intervention.

A 100km/h speed limit during the day was already introduced on Dutch motorways in 2019 and some Austrian regions have implemented the same limit on their motorways.

This could be extended further as transport scientists from across Austria announced their support at the start of February for a 100km/h speed limit on motorways as a low-hanging fruit when it comes to lessening the environmental impact of the transport sector.

This follows similar calls from German scientists and activists following the release of a study in January by the country’s environmental agency which found that a 120km/h speed limit could reduce road traffic emissions by 2.9%.

Road transport accounted for a fifth of the total EU greenhouse gas emissions in 2019, and over three-quarters of the overall transport emissions.

‘You need less fuel’

Understanding emission reductions starts with understanding the way an engine functions, Barbara Laa, a transport researcher at the Technical University of Vienna, told Euronews.

“At lower speeds, you need less fuel because you have less air resistance,”  she said. As the relationship between speed and fuel consumption isn’t linear – if you double your speed you will more than double your fuel consumption – higher speeds always lead to more emissions. 

The International Energy Agency has previously come out in support of speed limits, saying they can be used to reduce oil demand.

Laa was quick to add that not only would a speed limit reduce carbon emissions, it would also reduce nitrogen, noise, and particulate matter pollution and make travel safer.

The Austrian government’s decision to impose a 100km/h speed limit in certain regions – bringing it down from the countrywide 130km/h – was an attempt to clean the air, Laa said. 

As using electric vehicles doesn’t result in emissions, they’re exempt from the lower speed limit. But Laa doesn’t think this is a good idea.

“From an energy perspective, you have the same problem for electric vehicles – that you need much more energy the faster you drive,” she said. “If we use less energy, then it’s easier to decarbonise.”

‘Less congestion’

While the reduced energy consumption of individual vehicles might be clear, how a speed limit might play out on a systemic level is up for debate, according to Dr. Giulio Mattioli, a transport scientist at the Technical University Dortmund. He told Euronews that it’s not a question of whether a speed limit reduces emissions, but rather by how much.

“If you reduce [the speed limit], you will not just have slower speeds for many vehicles, you will also have a more regular flow, less congestion and then that might actually increase the average speed,” he said. On the other hand, the increased travel time associated with a lower speed limit might encourage people to travel by train instead, particularly over longer distances. As such, it’s difficult to predict the full impact of a reduced speed limit.

There has been some backlash toward speed limit proposals. In October 2022, the transport minister of Germany, Volker Wissing, said that it wouldn’t be possible to implement a speed limit in the country as there weren’t enough signs. Others have claimed that the emissions reductions are small, and therefore not worth infringing on peoples’ freedoms to drive fast.

2.9% of Germany’s road traffic emissions might seem like a low number, but Mattioli said it depends on how you look at it. Those that try to discredit the idea simply have to find “a denominator that is large enough” to make the “carbon emissions reductions seem irrelevant.”

Based on calculations he’s seen, emission reductions from a speed limit are “something like 30-50% of the gap that we currently have between Germany’s emissions and its climate targets for this year,” he added.

Speed limits won’t suffice

The 100km/h speed limit implemented in the Netherlands in 2019 was an attempt to reduce nitrogen pollution to meet legal requirements for new construction projects but measuring the exact impact of the policy has been hard due to the COVID-19 pandemic, Bert van Wee, a professor in transport policy at Delft University of Technology, told Euronews. 

“The effect of the pandemic was much stronger,” he said when referring to impacts on travel behaviour. 

Proper implementation of a speed limit also requires good communication, he added. “Make clear to people that they save money because their vehicles will consume less fuel – or electricity in case of electric vehicles.”

Tailoring the message to the country, city, or community in which the speed limit would be implemented is also important. Some will prefer climate messaging, whereas others might prefer safety messaging.

“We cannot think that we will solve the transport emissions problem purely through speed limits, as we will not solve it purely through electric vehicles, or we will not solve it purely through better public transport,” Mattioli argued. “We will need to do all of those things at once and even some more in order to achieve emission reductions.”

in fact, Laa concluded, “if we don’t put in lower speed limits, then we need to be more restrictive with other measures because we still need to come to zero [emissions].”