Shouldn’t speed limits be designed for the ‘85th percentile’ of drivers?

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In your story on speeding and safety, you quoted experts saying that crashes go up as speed limits increase. But what about the 85th percentile? I’ve been reading about it online. Doesn’t it mean that speed limits should be set at the speed driven by 85 per cent of drivers on a road? It makes sense to me because speed limits were set decades ago, back when cars couldn’t go as fast as they can today. Cars are safer now, so shouldn’t speed limits should reflect how fast most of us drive? – Bob, Ottawa

The 85th percentile is an old tool for setting speeds on new roads, experts say – but it doesn’t mean all limits should be decided by the fast crowd.

“It’s got a history that goes back 50-plus years,” said Jeff Lindley, chief technical officer for the Institute of Transportation Engineers (ITE). “Over the years, I would say there has been an overreliance on [the 85th percentile] as a way to set and adjust speed limits in a way that wasn’t intended.”

Also known as the prevailing speed, the 85th percentile speed is the speed “at or below which 85 per cent of vehicles will travel.”

“So 85 per cent are travelling at that speed or less, and 15 per cent are going faster,” Lindley said.

It was one way to set limits on brand new roads – but it was never meant as a safety idea.

“When you’re ready to set a speed limit on a new facility, one way to do it is to let folks drive, do a speed study and set the speed limit somewhere around the 85th percentile speed,” Lindley said.”But design guidance and speed limit guidance has always cautioned that the 85th percentile speed is one piece of information you should consider, but not the only piece.”

To set speed limits, experts must also look at many factors, including the number and kinds of crashes on the road, curves and lines of sight on the road, and whether the road is shared with pedestrians and cyclists, Lindley said.

Experts also need to consider speed the road was designed to handle.

In Ontario for instance, most provincial highways have a design speed of 120 km/h, Ontario’s Ministry of Transportation (MTO) said.

It’s slower in urban areas because there hasn’t been room to expand narrow lanes and shoulders.

Generally, the speed limit is set 20 km/h below the design speed, the MTO said in an e-mail.

Take it to the limit?

On Ontario’s Highway 401, the speed limit hasn’t always been 100 km/h.

When its first section opened in 1947, it initially had three different speed limits – 50 mph (80 km/h), 55 mph (88 km/h) or 60 mph (97 km/h).

Those were replaced by a single 80 km/h speed limit until 1968, when it was raised to 113 km/h. In 1976, it was lowered to 100 km/h.

Since 2019, Ontario has been testing a 110 km/h speed limit on some sections as part of a two-year pilot.

Alberta, Saskatchewan, New Brunswick and Nova Scotia have 110 km/h speed limits on some highways. British Columbia has a 120 km/h limit on the Coquihalla.

There’s an argument to raise speed limits because, well, a lot of us drive over them.

But relying on the 85th percentile alone to set speed limits is “outdated,” because the majority of drivers often drive faster than the road’s design speed, said Tarek Sayed, professor of transportation engineering at the University of British Columbia (UBC).

“It assumes that 85 per cent of people will be able to choose a safe speed for conditions,” Sayed said.

British Columbia did a speed study in 2013 that showed that the 85th percentile speed was at least 10 km/h higher than the speed limit. So, the province increased the speed limits by 10 km/h on 1,300 km of highways.

After two years, fatal and serious crashes went up by over 11 per cent.

“They had to change nearly everything back,” Sayed said. “High speeds increase the chance of collision, but it will also increase the severity – it’s physics, it doesn’t take an engineer to tell you that.”

Everywhere speed limits have been raised, serious and fatal crashes have increased, Sayed said.

Plus, part of the problem of using the 85th percentile to set speed limits is that it tends to go up after speed limits are increased.

In the United States, whether the speed limit is set at 75 mph (120 km/h), 80 mph (129 km/h) or 85 mph (137 km/h), “there is still a percentage of the population that will view that as a floor and go faster,” ITE’s Lindley said.

Cars are safer now than they were even a decade ago, and that’s why fatalities have mostly been declining since 1999. But those safety benefits disappear at higher speeds.

“I think what people are missing is that if a collision happens and you’re driving at 150 km/h, your chances of survival are quite low.” UBC’s Sayed said. “We keep saying we should drive faster and vehicles are improving, but, again, nobody should die or get injured on our roads.”

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