Contact Us


Research shows eliminating left turns reduces crash severity

BYU profesor of civil and environmental engineering, Grant Schultz.

BYU traffic engineers Grant Schultz and Mitsuru Saito are on a mission to stop people from making left turns on busy roads. It’s not because they want to make life more difficult; it’s because they want to save it.

As it turns out, left turns are quite dangerous. According to federal data, 53 percent of crossing-path crashes involve left turns (compared to only 5.7 percent involving right turns) and 36 percent of fatal crashes involving a motorcycle results from a vehicle making a left turn in front of the motorcycle. Additional data show left turns can be three times as fatal to pedestrians and bicyclists as right turns.

Those statistics are why most traffic engineers, including those from the Utah Department of Transportation, frown on left turns and often push to eliminate them with innovative solutions, such as diverging diamond interchanges and continuous flow intersections. In their recent UDOT-funded research, the BYU duo has found something much simpler makes a major difference on intermediate roads: medians.

“When you get to a certain level of vehicle traffic, the conflicts are so high you need to replace two-way left-turn lanes with a raised median,” said Schultz, a professor of civil engineering. “Our research finds medians reduce crossing conflicts 32 to 50 percent.”

Those two-way left-turn lanes are the middle, yellow-painted lanes all drivers are familiar with, where cars can turn left in either direction. According to their study, when a street reaches 34,000 to 38,000 vehicles per day (such as University Parkway in Orem), it’s time for a median, and when a street reaches 42,000 to 44,000 vehicles per day (such as 10600 South in Sandy near the freeway), it’s time for something more expensive.

For their analysis, Schultz, Saito and BYU statistics professor Dennis Eggett, along with grad student Kyung Min Kim, used a simulated environment to analyze conflicts on roadways. In a traditional safety impact analysis, researchers must have actual crash data on existing roadway conditions, along with additional data from a few years later measuring the impact of any improvement to the roadway.

The researchers use the simulation to provide safety-based recommendations without having to build something — like a new interchange or a median — to see what will happen.

“Being able to be proactive and to make predictions is much better than waiting for accidents to happen,” Saito said.

Their research, published in the Transportation Research Record of the National Academy of Sciences, confirms that whenever left-turn conflicts can be eliminated, the severity of crashes is significantly reduced. To that end, the team — as well as counterparts at UDOT — are continuing to look for ways to get left-turns out of major roadways.

“Our job as traffic engineers is to try to help minimize conflicts and help people travel safely,” Schultz said. “Medians may seem inconvenient, but we are trying to help the driver, not trying to upset them and make their lives more challenging.”