'Superstreets' trade left-turns for better traffic flow, safety

Revved up on the results of the largest-ever real-world study of so-called "superstreets," a group of researchers from North Carolina State University say the unconventional intersection design not only cuts travel time, but makes roads safer too.

Whenever two surface streets meet in a conventional intersection, drivers can choose to turn left, right or continue straight through. As anyone who's ever sat in the left-turn lane -- first waiting for the traffic lights to change, then hoping for a break in oncoming traffic -- that often means a lot of delays.

So-called "superstreets" aim to reduce the time drivers spend waiting ‘to hang a Larry' by effectively eliminating left-hand turns.

Instead, motorists wishing to turn left onto a "superstreet" from a side street are first required to turn right and then make a u-turn around a broad median. Similarly, drivers already cruising the "superstreet" who want to turn left use the u-turn lanes as well.

Although it may seem as though negotiating the additional traffic lights involved and driving the extra distance to the u-turn would slow drivers down, researchers say it actually speeds things up and reduces the number of traffic accidents.

According to the North Carolina study, drivers using "superstreets" experience a 20 per cent overall reduction in travel time compared to similar intersections that use conventional traffic designs. There are an average of 46 per cent fewer collisions too, with the number involving personal injuries slashed by 63 per cent.

After conducting the largest-ever real world study of "superstreets," Joseph Hummer -- a professor of civil, construction and environmental engineering at North Carolina State University and one of the researchers behind the study – believes that combination of benefits makes the intersection known officially in the U.S. as a restricted-crossing u-turn unique.

"The bottom line is, it provides improved safety and more efficient operations," Hummer told CTV.ca. "Many designs or innovations in the traffic business do one or the other, but a "superstreet" does both and that makes it a pretty special thing."

Although they're not a cure-all for traffic-snarled roads, Hummer says the "superstreet" design is perfect for the divided four- or six-lane roads with lots of small side streets and driveways that are common throughout rural and suburban North America.

The reason they work so well, Hummer explained, is because they eliminate many of the dangers that lurk in conventional intersections where roads converge from four directions.

"There are 32 conflict points in the middle of that intersection. That is, places where vehicle streams cross each other, merge with each other or diverge from each other," he said, noting that most of them involve left-turning vehicles.

"That's too many places where, if somebody makes a mistake, a collision happens or drivers simply have to wait for somebody else to get out of the way before they can go through."

By contrast, Hummer says the simplest intersection -- a roundabout -- has just 8 points of conflict. A "superstreet" has 14, but they're largely separated.

That means drivers only have to worry about one thing at a time, and Hummer says it makes the intersection easier to navigate than one might imagine.

"The driver doesn't have to understand the whole plan view of the intersection, just, 'What's the next thing I have to do' and 'What's the next thing after that' and then they're through it."

Although the concept has been around for at least 20 years, drivers rarely encounter the unique configuration. According to Dr. Hummer, besides the two dozen examples in North Carolina, there are just a handful scattered between Maryland, Michigan, Minnesota and Texas.

But he hopes that will change as more people responsible for planning and building roadways warm up to the concept. To that end, the North Carolina Department of Transportation-funded study will be presented at the Transportation Research Board Annual Meeting in Washington, D.C. later this month.

"We're really trying to educate our fellow engineers that this design exists and that, in its niche, it can be beneficial," Hummer said, explaining that many have been waiting to see how the first "superstreets" fare in real-world conditions.

"So I hope that we're able to get the word out that the early results point to very favourable safety benefits and efficiency benefits."

Those benefits, Hummer suggests, should outweigh the extra expense.

"The big cost is in the (traffic signal) controller box," Hummer said, estimating that "superstreets" cost approximately double what a conventional intersection improvement would, "Because of the additional signal controllers and then also because the "superstreet" relies on the wider median to allow the u-turns."

Whether or not the idea catches on, Hummer has some straightforward advice for any drivers interested in getting to their destination quicker and safer.

"It's simple: Don't make left turns. They're the bane of our existence," he said, joking that "some freight trucking companies instruct their drivers to make three rights instead of a left.

"If your highway agency is not providing you with superstreets ... or some other way to avoid left turns you should try to avoid them on your own because they're unsafe and lead to a lot of delay."

Just don't heed his advice too closely, as Hummer says, "If people stopped making left turns tomorrow, we'd be out of business -- they are the reason for most of the congestion and safety problems out there."