‘Speed kills’ is a long-running slogan in the UK, aimed at encouraging motorists to think of the consequences of driving recklessly and to slow down.
In recent years, there have been increasing attempts to lower the speed limit, particularly in urban areas, from 30mph to 20mph (48km/h to 32km/h), with the aim of reducing deaths and injuries, particularly among pedestrians. There is no doubt that there is a reduction in impact force and significantly shorter stopping distance when drivers brake at the lower speed.
But is there a definite, provable link between lower speeds and a reduction of injuries? A new study from the Institute of Transport Economics - Norwegian Centre for Transport Research looks at this, as part of a larger report into wider effects of lower speeds.*
The aim of the study was to see how moving from 30mph to 20mph could contribute to a more sustainable urban development, said Aud Tennøy, chief researcher, Sustainable Urban Development and Mobility at Transportøkonomisk Institutt.
A place of greater safety
The study notes that, in recent years, many European cities have introduced a 30 km/h and 20 mph speed limit in large zones or entire cities to achieve goals related to sustainable urban development. These can include not only greater safety, but also reductions in noise and pollution, change in travel behaviour and more vital, pleasant, safe and inclusive urban environments.
30 km/h zones are a common sight in Europe (© Monikabaumbach| Dreamstime.com)
Many Norwegian cities also want to increase the use of 30km/h speed limits and zones. “There are constant discussions about this between the cities and the regional and state authorities,” Tennøy says. “The Norwegian Public Roads Administration is now reviewing the speed limit criteria and their use. There are uncertainties and disagreements in the Norwegian professional community related to the positive and negative effects of increased use of the speed limit of 30 km/h in urban areas.”
The study has gathered evidence from several cities where this lower speed limit has been introduced, with the aim of creating a foundation for “more knowledge-based discussions related to the use of a speed limit of 30 km/h in large zones, entire cities or as a general speed limit in densely-populated areas in Norway”.
As part of the study, “we found that the measure contributed to a reduction in speeds and significant reductions in the number of traffic accidents in all the cities for which we found results, as well as to a number of other factors that contribute to more sustainable urban development”, she adds. The study revealed that “in all cases…reducing the speed limit to 30 km/h in large areas and entire cities has contributed to reduced speeds for car traffic and fewer traffic accidents”. This was particularly noticeable in the incidence of serious collisions and those involving pedestrians.
Reduced noise and nuisance
“Across cases, it emerges that the measure has also contributed to reduced noise and fewer nuisances due to noise, while it is uncertain whether it has resulted in reduced local air pollution. The measure may result in increased driving time for car and bus traffic, but it appears that the consequences of this is less than expected before the implementation of the measure.”
The results show that lower speed limits in larger areas have contributed to making it more pleasant and safer to walk and cycle in streets and areas, and that it may have contributed to the transition from cars to other means of transport and reduced traffic volumes.
“Evidence on traffic accidents is really convincing,” Tennøy told ITS International. “There was a fairly consistent reduction in accidents. In all cases we found a reduction in accidents and injuries/casualties. The reduction in number of accidents is greater than the reductions in speed.”
Two of the most significant studies looking at the effect of lower speed limits on accident rates took place in two UK cities.
“The proportion of drivers who drove at speeds of 24mph or lower went from 21% to 53% - that’s a really big change” Aud Tennøy, Transportøkonomisk Institutt
“Edinburgh and Bristol have the best analysis because they’ve been well studied. In Bristol, there was a 63% reduction in number of fatal accidents, when controlled for background trends, etc,” she said.
“In Edinburgh where the limit was reduced citywide, the number of accident victims were reduced by 43% but there was [also] a reduction in accidents in streets outside the speed-limited zones. In Edinburgh, this amounted to a 17% reduction in accident casualties across the city as a whole, compared to the three years before the introduction of the speed limits, and controlled for the background trend.”
This suggests that when lower speed limits are introduced in certain areas of a city, drivers become accustomed to operating at lower speeds and that this behaviour ‘bleeds across’ into areas where the original, higher speed limits remain in force.
“If you find a reduction in streets where the new limits apply, but also find reductions in streets outside the zones,” Tennøy explains. “If you reduce speeds in large areas across a city, you get a reduction in accidents in other streets. It seems that this is the understanding now.”
In Wales, after one year, collisions fell by 26% and injuries dropped by 28% (© Graemelamb| Dreamstime.com)
Statistical significance
Previously this was understood as indicating that speed and collision reductions had other causes than the speed limit reduction.
Tennøy cautions that it can be difficult to analyse reductions in accidents or injuries because they are not always analysed in the same way. However, she insists: “I would say that in the cases of Edinburgh and Bristol, the statistical analyses have been carried out in ways allowing to say that they are statistical significant.” This was because the studies had been carefully set up and analysed, taking into account certain variables.
One concern raised by some groups was that the new lower speed limits would mean that all journeys by public transport, as well as by private vehicles, would take longer. While there was evidence that public transport journeys did take longer, this had only marginal consequences. “In cities like Edinburgh there was more walking and cycling,” she says. “Residents said they enjoyed the experience more and that the roads felt safer. The public transport company said in the evaluation that the consequences for them were small.”
The lowering of speed limits appears also to have acted as a trigger for people to explore other methods of moving around the city. “With the exception of one study, the trend is unambiguous: the results indicate that reduced speed limits have contributed to the transition from cars to other means of transport,” the study says.
Other aspects of the study measured both noise and air pollution levels under the new, lower speed limits. Noise, and what was perceived as ‘noise nuisance’, was reduced “and this is unambiguous”. The reduction in noise and nuisance was particularly noticeable at night.
In terms of pollution, results were more varied; some urban studies found that pollution levels dropped significantly, some found no significant drop. In no cases, however, did pollution levels rise.
Slowing down in Wales
One of the biggest ‘laboratories’ for testing the reduction of speed limits has been the country of Wales, which is part of the UK. In September 2023, the Welsh government introduced a 20mph limit in all ‘restricted roads’, usually in residential and built-up areas, which accounted for around one-third of all roads.
This followed a trial of the new limit in eight districts, which led to a drop in motorists’ speeds. According to new statistics released in January 2025, the first year of the new limits saw the number of road deaths drop by 10 and the total of deaths and serious injuries by around 100. However, statisticians have cautioned that three years’ statistics will be required in order to draw meaningful conclusions as to the effectiveness of the lower limit.
The new limit aroused considerable anger in some quarters and the Welsh government has since issued revised guidelines, trying to fine-tune those areas where a 20mph limit makes most sense.
Tennøy said that the Welsh project was slightly different, in that it extended beyond individual zones in town or cities. Nevertheless, officials had found that motorists’ speeds reduced overall by 3.6mph, from 28.7mph to 25.1mph, in the affected zones. “The proportion of drivers who drove at speeds of 24mph or lower went from 21% to 53% - that’s a really big change. In terms of accidents, although there is only one year’s statistics to go by, the number of accidents dropped by 26% and the number of injuries dropped by 28%.”
* Introduction of 30km/h speed limit in large zones and entire cities to achieve the goal of sustainable urban development’ (Aud Tennøy & Gry Rustad Pettersen)
