Variable speed limits point to safer, faster commuting: study

Research from a Schulich School of Engineering master's student shows that changing speed limits on the fly can have a big impact on freeway congestion. 

By Michael Platt
Schulich School of Engineering

 

Imagine a cure for Crowchild, or an antidote for the Deerfoot.

It’s the dream of anyone who regularly commutes on Calgary’s major rush-hour arteries, but a remedy may soon be a reality, according to a Schulich School of Engineering study that weighs potential benefit of variable speed limits (VSL) governed by digital road signs. The study, conducted by master’s student Karan Arora under the supervision of Civil Engineering professor Lina Kattan, comes as Alberta finalizes city charters for Edmonton and Calgary, with the draft legislation allowing the cities to employ variable speed limits for the first time.

“The main benefit of VSL is safety, and where it is implemented, that’s the first change that is noted,” says Arora. “Although constantly changing speed limits may not sound like a safe thing to do, fewer collisions is the key benefit. On Deerfoot Trail, my study shows VSL will be 29 to 31 per cent safer.”

Arora’s research, focused on a 13-kilometre stretch of southbound Deerfoot from Airport Trail to 17th Avenue South, estimates the impact of VSL technology as used in Europe and under testing in U.S. cities like Seattle.

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University of Calgary master’s student Karan Arora. (Photo courtesy Karan Arora)
Caption: 
University of Calgary master’s student Karan Arora. (Photo courtesy Karan Arora)

Digital signs tell motorists the current speed limit for the freeway — the point being to slow drivers down before congestion and queues ahead result in brakes being slammed on. Smoother traffic means fewer mishaps, and the impact will not be lost on any driver who has crawled along in traffic tangled by a fender-bender or worse.

“The speed is updated every minute, with an overhead digital board every one or two kilometres, so drivers are unlikely to miss the changes,” says Arora.

In ideal, crash-free conditions, the study suggests VSL will save Deerfoot drivers up to five minutes in rush hour, and eight or nine minutes during off-peak hours.

But Arora didn’t stop there. By adding another modern traffic tool to the mix — namely, using the hard shoulder as an extra rush-hour lane — he found drivers on that same stretch of Deerfoot would experience a 28 to 38 per cent increase in traffic flow, and 23 to 31 per cent reduction in peak hour travel time, or up to 11 minutes.

“The leftmost shoulder of the freeway can be widened to a full lane which then can be used during peak hours to ease congestion,” explains Arora. “It would provide additional capacity on demand during peak hours or during major congestion like a  collision or snowstorm, without major infrastructure expansion requirements.” In the case of an emergency, where first responders need the hard shoulder, the digital signs can warn motorists to clear the lane. 

Arora says VSL alone isn’t a permanent fix in a city where  traffic is on the increase, though the technology will buy time for Calgary’s major road investments. Add in Hard Shoulder Running, as the extra lane is known, and the research shows Deerfoot would be a viable freeway for another 25 or 30 years.

Professor Kattan says variable speed limits are a solution to keeping the flow of traffic smooth — and motorists ultimately save time, though the posted speed limit is lower.

“VSL slows traffic during congestion to make it move faster,” says Kattan, associate professor at Schulich and Urban Alliance Professor in Transportation Systems Optimization. “This might sound counterintuitive but is explained by the fact that when bottleneck occurs, a freeway may lose up to 30 per cent of its capacity, thus losing capacity when it is most needed.”

As well as saving time, Kattan says VSL is also better for the environment. “A properly designed VSL system restores freeway capacity by disseminating bottlenecks before they even form,” she explains. “By eliminating bottleneck formation and stop-and-go conditions, vehicle emissions are also reduced significantly.”

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