If you've driven during a Canadian summer, you know the feeling all too well: a crowded highway, tense drivers and a closed lane up ahead.

You begin the frustrating, sometimes dangerous process of merging into the adjacent lane and a long, sluggish queue of cars forms as traffic creeps past the construction site.

It doesn't have to be this way, traffic experts say.

Leon James, professor of driving psychology at the University of Hawaii, says the notion that "merging early is better" is responsible for the typical congestion caused by lane reductions on highways and roads.

"A lot of drivers think that if they merge into the open lane as early as possible, that they are helping to keep traffic moving. But all it really does is create an empty lane without any cars in it, and a crawling, grid-locked lane opposite," says James.

"People think it's courteous and makes them feel good without realizing that they are in fact causing a lot of trouble for everybody on the road."

Merging hundreds of metres before a construction zone begins can result in a chain reaction of drivers in the open lane having to slam their brakes, thus generating what researchers call a traffic wave.

Traffic waves can impact driving conditions up to 30 kilometres before the actual merging point on busy highways, says James.

The zipper merge

Researchers studying traffic flows are constantly experimenting with novel ways of decreasing congestion around construction zones, and the debate between early and late merge proponents is a divisive one, says James.

A technique known as the "zipper merge," which requires drivers to stay in the closed lane right up until the construction pylons begin and then take turns merging, has been gaining popularity in municipalities throughout North America.

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(Graphic by Duk Han Lee/CBC)

The Minnesota Department of Transportation introduced the zipper merge in 2002 and has been expanding its use throughout the state every year. Additional signage is added for several miles prior to a major construction site that tells drivers to stay in both lanes and when it is time to merge.

Extra warning and clear instructions a farther distance from the construction site is key to the system, says James. "It gives drivers the time they need to prepare mentally for the changes ahead" rather than being thrown into disarray by a sudden need to merge.

According to their website, it has reduced congestion by up to 40 per cent on Minnesota's busiest freeways and helped to reduce traffic accidents in construction zones.

Canada sees an average of about 40 fatalities and 2,000 injuries due to car crashes in construction zones every year. A number of municipalities throughout the country are hoping the zipper merge will help to increase safety and reduce congestion, says Angela Gardiner, manager of the transportation branch for the city of Saskatoon.

Saskatoon city officials conducted a zipper merge pilot project in early May at a construction site on a stretch of highway that sees 80,000 vehicles a day and received "tremendously positive feedback from the public," says Gardiner.

"Previously, we had most of the people in one lane in a big, long line and you would have the odd person racing up to the front and trying to cut in at the last minute. It's that difference in speeds that has proven to cause traffic jams and be unsafe," says Gardiner.

The city is working with regional police to ensure that drivers who are not allowing other cars to merge at the proper time are ticketed or at least warned that they are legally required to do so.

No statistical data was collected on the original project, but Gardiner says the city is starting a new project at a different construction site that is expected to last weeks or months, giving the city enough time to measure how well the zipper merge is working. 

There will likely be more pilots in other cities throughout Canada and more data with them. Currently the city of Sudbury, Ont. is also experimenting with two-lane merges at an intersection in the city's north end.

New behaviours

The Saskatoon pilot project was accompanied by a public-awareness campaign, which Gardiner says is essential if the zipper merge is to become the new normal.

"We aren't just talking about changing some signs, we are talking about changing driving behaviour," she says.

According to James, early merging and the traffic jams that result from it encourage driver behaviour that significantly increases the risks of accidents and incidents of road rage.

'The signage is just so bad right now. Like signs that just say "Merge" – does that mean merge now, merge later, where to merge, what? It's completely puzzling to the driver and now they merge early and it creates traffic jams.'—Leon James

"When people stuck in the long line of cars see a driver in the other lane try to rush to the front and sneak in, they perceive it as 'that other driver is cheating,' and then they get angry," says James.

"When drivers are emotionally impaired, they can become like vigilantes and begin acting aggressive and hostile, and it's dangerous."

James thinks that before the zipper merge can work effectively, widely accepted misperceptions about highway driving need to change.

"There's a lot of drivers who think that if they drive all the way to the beginning of a construction zone before merging, that no one will let them in," says James.

"But that's just not the case, and never really has been the case. Maybe one or two people will pass you, but of course someone will eventually let you in."

Better signs are the key

Canadian road signs are designed and deployed by provincial transportation departments. Municipalities can choose to use the signs in ways that fit particular traffic and construction situations, but they must adhere to safety standards determined by the province.

At most construction projects in Saskatoon, signs read "Lane Ends Ahead" and "No Passing" to warn people about upcoming obstructions, which was sending the wrong signal to drivers, says Gardiner.

"The signs were essentially giving people the idea that they should all be in one lane and it caused problems."


Proponents of the zipper merge argue it will reduce traffic jams, improve commute times and make Canadian highways safer. (CBC)

To prepare drivers for the zipper merge, the city added signs telling drivers to use both lanes and specifically indicated when they should merge, which were key to successful merging, says Gardiner.

According to James, ambiguous signs and poor driver education have led most people to think that merging early is a better idea.

"The signage is just so bad right now. Like signs that just say 'Merge' – does that mean merge now, merge later, where to merge, what? It's completely puzzling to the driver and now they merge early and it creates traffic jams," says James.

"This whole issue needs leadership from provincial transportation departments to come up with better signage to improve the flow of traffic around things like construction sites."