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At a recent family wedding, the topic of speed limits came up at our table. It got heated, to put it mildly – my brother-in-law ended up storming off. Before he did, he kept talking about the 85th percentile (which we all Googled) and said roads are safer when everybody is going faster. So what’s the answer? Do higher speed limits make roads more dangerous? – Kim, Vancouver

The debate over speed limits reminds engineering professor Gordon Lovegrove of another heated debate.

“Why not link it to the gun debate? ‘It’s not the guns that kill, it’s the people who use them,’” said Lovegrove, associate professor in the faculty of engineering at UBC. “A vehicle is a 3,000-pound weapon – so it’s even worse than the gun debate.”

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Lovegrove is one of the authors of an October study that showed the number of fatal crashes more than doubled on British Columbia highways where speed limits were raised by 10 km/h in 2014.

Proponents for higher speed limits often call for limits based on the 85th percentile – the speed that 85 per cent of traffic routinely travels at, or slower than, on a particular road.

B.C. government

But this month, the province announced it was lowering speed limits – which had gone up to as high as 120 km/h, the highest in Canada, on some roads – on 15 of the 33 sections where it had been raised.

This followed a provincial safety evaluation that showed serious crashes had increased 11.2 per cent overall where the speed limit had been raised.

Ian Tootill, a motorist and founder of lobby group SENSE BC (Safety by Education Not Speed Enforcement) said the speed limit rollbacks are “pandering to special interests.”

“We want a methodology that based on engineering – dispassionately used without their meddling,” Tootill said.

The intent behind the 2014 increase was to decrease the differential speed – the difference between the fastest and slowest cars on the road – and improve safety, Lovegrove said.

“But when it came out showing a 100-fold increase in fatal crashes, we recommended the Minister lower it,” he said.

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Higher speed limits lead to more crashes

In the United States, the “vast majority” of research has shown that whenever you increase speed limits, serious crashes increase, said Chuck Farmer, vice president for research and statistical services with the Insurance Institute for Highway Safety (IIHS).

“We’ve looked at fatal crashes, in particular, three times in 25 years and the evidence is pretty consistent,” Farmer said. “If you raise the maximum speed limit by 5 miles [8 kilometres] an hour, you see a four per cent increase in fatalities – on higher speed roads, 65-70 mph (104-112 km/h), we see an eight per cent increase."

In the United States, the 'vast majority' of research has shown that whenever you increase speed limits, serious crashes increase, said Chuck Farmer, vice president for research and statistical services with the Insurance Institute for Highway Safety (IIHS).

mikeinlondon/istock

In the 20 years between 1993 and 2013, speed limit increases in 41 states caused an estimated 33,000 extra deaths, Farmer said.

“You don’t need to be a physics major to understand that the faster you’re going when you hit something, the more damage there will be,” Farmer said. “Crashes are rare, but most of us make mistakes eventually – and you don’t want to be going fast when that happens.”

The majority of crashes – 96 per cent – are caused by driver error, Lovegrove said.

“When you increase speed, you reduce the time and space and opportunity for the driver to recover from that mistake,” Lovegrove said.

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Proponents for higher speed limits often call for limits based on the 85th percentile – the speed that 85 per cent of traffic routinely travels at, or slower than, on a particular road.

The idea? If limits are set at the speeds most people are actually driving now, fewer people will be tempted to break them.

“The 85th percentile is a general indicator that has been used as a decision aid,” Lovegrove said. “But we also look at the differential speed and the design speed for the highway.”

For instance, the Coquihalla – which has variable speed limits that can be as high as 120 km/h (the highest in Canada) – was designed for 110 km/h, Lovegrove said.

“It’s not like the Autobahn, which was designed for 140 or 150. On a straight stretch on a dry, sunny day, the Coquihalla is awesome and going the 85th percentile might be appropriate,” he said. “But we’ve got curves, blind spots, changing weather conditions and changes in climate.”

And studies have shown that 85th percentile speeds tend to increase after speed limits get raised, Farmer said.

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“It’s not a fixed point,” Farmer said. “There are drivers who will always want to go as fast as they can.”

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