80mph speed limit 'would increase deaths by 20%'
Statistical model used by Department for Transport predicts 25 extra motorway deaths a year if plans to increase limit go ahead
Government proposals to increase the motorway speed limit to 80mph would be expected to cause a 20%-plus increase in deaths, according to the statistical model the Department for Transport will use to assess the implications of the change.
Road safety experts from Europe and the US are warning the DfT, which begins a consultation on the idea early next year, that while motorways remain significantly safer than other road types they are also especially susceptible to speed limit changes.
The government's announcement in October that it would consider a 10mph increase drew condemnation from road safety campaigners. But the then transport secretary, Philip Hammond, said the change would bring economic benefits through quicker journey times and that safety worries were largely mitigated by improvements in vehicle technology since 1965, when the 70mph maximum was set.
Using the most widely accepted statistical model, drawn up by a Norwegian academic using data from 100 studies in more than a dozen countries, an increase in average traffic speeds of just 3mph a typical change for a 10mph rise would be expected to cause more than 25 extra deaths a year on motorways and more than 100 serious injuries.
Even this is likely to be a conservative estimate, said Rune Elvik, from Norway's Institute of Transport Economics, who devised the so-called power mod! el. "Many drivers would probably say that they are more alert and more prepared for things if they drive faster, and actually there is a little bit of support for that point of view," he said. "But it's by no means sufficient to compensate for the effect of the speed. They are not able to override or repeal the laws of physics."
The power law is not just a matter of opinion, said Richard Allsop, a professor of transport at University College London who heads an EU-wide road safety scheme run by the Brussels-based European Transport Safety Council. "The key basis for the power law is not sophisticated statistical modelling. It's simple but careful before-and-after comparisons in situations where speed has changed for some reason, usually because the limit has changed," he said. "The kind of evidence on which the power law is based is directly relevant to the situation we may be facing."
A DfT spokeswoman said the department had not yet made its own predictions on probable casualty changes if the speed limit did rise, but confirmed that it would be basing these on the power model.
There are caveats to any estimates: heavy goods traffic is not affected, and congestion further limits speeds. Advocates of the change cite existing flouting of the 70mph limit, saying an 80mph maximum would make little practical difference. But campaigners in the US, which has seen dozens of state-wide changes since the federal 55mph maximum was abolished in the late 1980s, say data shows drivers eventually revert to old habits.
"We tracked speeds on interstates for quite some period of time," said Adrian Lund, president of the Virginia-based Insurance Institute for Highway Safety. "You can see that each time the speed limit went up, speeds went up as well. Drivers were still breaking the law it took a little while till they broke the la! w as muc h as they used to, but people gradually tested the tolerance zone."
The overall result is a spike in deaths and injuries, he said: "Each time those limits have gone up we've seen an increase in deaths on roads. The evidence is quite clear that when the speed limit increases on roads like an interstate highway system, or your motorways, it's going to have an effect."
The impact can be even more severe because higher motorway speed limits can spill on to connecting roads, he added: "People tend to adapt to the speed they're travelling. When you have someone exit an interstate highway, for example, we see that they go faster than other people on the roadway. Even if you think you've slowed down, you often haven't slowed down as much as you perceive."
The roads minister, Mike Penning, said safety improvements had caused deaths on British roads to fall more than 75% since the introduction of the 70mph limit. He said: "The department is carrying out detailed work to assess the potential economic, safety and environmental impacts of increasing the national speed limit on motorways to 80mph. We will publish this work and consult fully on our proposals early next year."
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