What do those flashing left-turn arrows mean, anyway?

Alicia Stice
The Coloradoan
Flashing yellow arrows can help prevent crashes, even if drivers don't understand them.

Traffic engineers know what you're thinking. And they know that sometimes you're wrong about the decisions you make behind the wheel.

That's why cities across the country have started installing flashing yellow arrows above left-turn lanes.

According to research from the U.S. Department of Transportation, these blinking signals can actually get drivers to make safer choices behind the wheel — even if some people find the signals confusing.

Researchers studying driver behavior found that when people saw a round, green light above their turn lane light up, many would reflexively hit the gas instead of yielding to oncoming traffic. 

When people blow through left turns without yielding, they can end up in dangerous car crashes. Crashes that involve left turns, called approach-turn crashes, made up 8 percent of Fort Collins' total crashes and 14 percent of the city's severe injury or fatality crashes from 2012 to 2016.

Researchers have found drivers are more cautious and likely to wait for oncoming cars to get out of the way when presented with flashing yellow arrows.

"While people didn't always know what it meant, they knew it didn't mean go," Fort Collins traffic engineer Joe Olson said.

When an intersection has a flashing yellow arrow in use, cars turning left can go even if the cars next to them heading straight have a red light, as long as the drivers yield to oncoming traffic.

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Traffic departments across the U.S. are slowly making round, green lights above left-turn lanes a thing of the past. Although cities do not need to immediately replace all signals — you've probably seen some of the old ones around town — they can't install new signals in the old style.

When the city replaces or modifies intersections, it either leaves the left-turn lane with no light or uses the new arrow system.

In Fort Collins, five more intersections are set to get the flashing yellow arrows later this year: 

  • College Avenue and Laurel Street
  • College Avenue and Trilby Road
  • Boardwalk Drive and Harmony Road
  • Laurel Street and Meridian Avenue
  • Laurel Street and Meldrum Street

In addition to helping prevent crashes, the flashing yellow arrows can also give local traffic engineers a tool to change the way cars move through an intersection at different points during the day, traffic engineer Martina Wilkinson said.

"There's places where in the middle of the night, there's very little traffic," she said. "So you can run it as a flashing arrow then, but in peak hours, we want to manage it a bit more, so you can go to the green arrow and (solid) yellow arrow."

For example, the intersection of U.S. Highway 287 and Monroe Drive has protected turns — meaning cars can only turn left when they have a green arrow — during busy daytime hours. But, engineers can shift the signal to a flashing yellow arrow when there are fewer cars, which helps ease congestion at the intersection. 

"The rationale for using them is that research shows it reduces the number of left-turn crashes," Olson said. "But the flexibility allows us to hopefully improve safety and make things more efficient."

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