A speed limit sign in Canada.RWhite94/iStockPhoto / Getty Images
As I understand it, roads are designed for certain speeds. So why isn’t the speed limit always the same as the design speed? How is the speed limit decided? – Dave, Edmonton
Some roads may be built for speed, but that doesn’t mean drivers can handle it, road safety experts said.
“There’s an argument that if the design speed is 120 kilometres an hour, then why don’t we let people drive at 120?” said Craig Lyon, director of road safety engineering with the Traffic Injury Research Foundation (TIRF), an Ottawa-based non-profit. “All our roads are designed for a certain speed, but there’s a factor of safety built into that. We don’t want people going the absolute maximum speed. … Drivers are not perfect and conditions are not perfect.”
When roads are designed, engineers choose a maximum safe speed – the design speed – based on whether they’re designing a highway or quiet neighbourhood street, whether the terrain is level, hilly or mountainous, and whether the road is curved or straight.
But, especially on older roads, that top speed is not necessarily the maximum safe speed for traffic, said Raheem Dilgir, a Vancouver-based transportation safety consultant and past president of the Canadian Association of Road Safety Professionals, a national non-profit.
“The design speed of a road has traditionally been determined based on the road’s function – and not necessarily based on the safety implications,” Dilgir said in an email. “For example, freeways are designed for higher speeds, while municipal and local roads are set for lower speeds.”
But engineers also look at safety when setting design speeds for new roads. They use the design speed to determine roadway features, including the radius of curves, lane width and shoulder width, said Karim El-Basyouny, a professor of transportation engineering and research chair in urban traffic safety at the University of Alberta.
Still, the design speed isn’t meant to be the speed for everyday drivers, El-Basyouny said. “While the design speed does set a maximum safe speed under ideal conditions, it does not account for all possible factors, such as poor weather, heavy traffic or road surface conditions,” El-Basyouny said in an email. “The design speed usually includes a safety margin, meaning the road could accommodate slightly higher speeds in an emergency.”
How big a difference can there be between a road’s design speed and the speed limit?
When Alberta’s government announced plans to suspend photo radar on ring roads in Edmonton and Calgary last year, Alberta Transportation Minister Devin Dreeshen said in an interview that the roads’ 100-kilometre-an-hour speed limit was too low for the 130-kilometre-an-hour design speed.
When the Ontario government announced this year that it was permanently raising the speed limit to 110 kilometres an hour from 100 on more provincial highways, Ontario Minister of Transportation Prabmeet Sarkaria said in a press release that “most of Ontario’s highways were originally designed to safely accommodate speed limits of 110 [kilometres an hour].”
The need for speed?
While design speeds are set by engineers, speed limits are set by governments.
Traditionally, speed limits weren’t necessarily set with safety in mind, Digir said.
Instead, governments would decide speed limits either by setting them at or just below the design speed – or they would look at the speed that traffic was actually going, Dilgir said.
Often, governments would use the 85th percentile – “the speed that 85 per cent of traffic is travelling less than or equal to,” Dilgir said.
But what if 85 per cent of drivers are driving at unsafe speeds?
“Where safety is valued, the 85th percentile method would only be valid if drivers are travelling at safe speeds,” Dilgir said. “It assumes that drivers know what constitutes a safe speed, which is generally not the case – unfortunately.”
Generally, the 85th percentile is an arbitrary figure and isn’t based on evidence that it makes roads safer, Dilgir said.
“That 85th percentile thing, I’m not even sure where that came from,” TIRF’s Lyon said. “We’ve been getting away from that. On a road where everyone was speeding like crazy, it would be awfully high.”
According to a TIRF report, speeding increases both the number and severity of crashes. Driving just 10 kilometres an hour above the speed limit more than doubles crash risk. Driving 20 kilometres an hour above the limit increases crash risk by up to six times.
When British Columbia’s government increased highway speed limits in 2014 based on research that included the 85th percentile speed, there was an 11-per-cent rise in fatalities and serious injuries. The province rolled back speed limits on two highways.
“A similar trend has been observed in most jurisdictions where speed limits were increased, with few exceptions,” Dilgir said. “It’s all based on physics.”
For the past 15 years or so, regulators have increasingly looked at risk, including roadside hazards and the presence of cyclists or pedestrians, when setting speed limits, Dilgir said.
More recently, regulators have been looking at reducing injuries and deaths when setting speed limits, he said. That includes looking at the crash history of that particular stretch of road.
“Speed limits have in the past been more arbitrarily applied; however, based on the evidence that has become clear, they are now being set with greater consideration of the context and the physics involved in collisions,” Dilgir said.
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