Traffic engineers working toward a world without left turns

Traffic engineers are working to design traditional left turns out of existence, calling them the “weakest links in the system.”

“Left turns are a big issue with intersections,” said Brian Walsh, State Traffic Design and Operations Engineer for the Washington State Department of Transportation. “Of the 7 million crashes that occur on the system nationally, 2.4 million of those crashes occur at intersections. What do we take care of at intersections? Mostly left turns.”

While they can’t get rid of them, UPS is already in the business of bypassing them. In fact, the shipping giant spends a billion dollars a year on software that helps to avoid left turns.

“It’s based on algorithms and technology, but it boils down to two words: network optimization,” said Susan Rosenberg, Spokeswoman at UPS headquarters in Atlanta. “UPS does minimize left turns for efficient routing. You’re coming to hard stop, you’ve got to make a left turn, you’re going to be idling at your stop that’s burning fuel. That’s taking time. You’ve got more of a safety impediment than if you were just taking a right turn.”

But, if left turns simply didn’t exist, UPS wouldn’t have to spend so much time and money trying to avoid them. That’s why traffic engineers are working on new intersection and interchange designs that will solve the problem.

One of the newest solutions – that is used mainly at interchanges with high traffic volume – is something called a diverging diamond interchange. There are only six of them in use nationally.

“It’s kind of counter initiative, but on the cross streets – the street that passes over the freeway for an example – they flip everybody right prior to the interchange onto the left side of the roadway,” Walsh said. “So you’re actually driving on essentially the wrong side of the roadway like the English would.”

The design allows a driver to merge onto the interstate without having to stop at a signal or take an unprotected left turn. WSOT is considering building such an interchange at I-90 and Barker Road in Eastern Washington.

While the system can’t be redesigned overnight, Walsh said they try to find an easier way to make left turns every time they build, or rebuild, an intersection.

“Trying to handle left turns a better way is why we’re coming up with, nationally, a lot more innovative type of designs, he said. “The diverging diamond interchange, the indirect left turns type of intersection, they’re all there to try to handle left turns safe as well as bit more efficiently.”

Simulation of a Diverging Diamond operation: