Yellow has become the hottest color in Washington County these days.

Everyone, it seems, favors the flashing turn arrow that frees motorists from stoplight purgatory. Proof of that can be seen at a growing number of intersections — and by year’s end, many more.

“They’re popular everywhere,” said Nik Costello, a county traffic engineer. “We certainly get a lot of requests for them, more than any other infrastructure.”

The concept of a flashing signal was so simple that engineers wonder why it took so long for its popularity to take hold. The first one installed on a Washington County highway came in 2011.

The signal allows drivers to turn left after yielding to oncoming traffic and pedestrians in crosswalks. It requires careful judgment, because oncoming vehicles have a green light. But it also prevents frustrating delays that motorists experience at solid red arrows.

The flashing yellow arrow is called a “permitted turn.” That’s different from a “protected turn,” when left-turning motorists see a solid green arrow while oncoming traffic sees red.

Of the 68 signals Washington County owns, 24 now have the flashing yellow signals. Nine more will be installed this summer.

Dakota County, in 2009, described its first two flashing yellow arrows as “the way of the future in left turn operations and it can be expected that Minnesota drivers will be encountering them regularly in time.”

That’s the case in Woodbury, which will add six more flashing yellow arrows at city-owned signals this summer. They come in addition to about two dozen flashing signals already owned “under various jurisdictions” in Woodbury, said City Engineer John Bradford.

“They have been very well received,” said Bradford, who said several motorists have informed city officials that they’re pleased with the change. “The fact that people take a positive step to comment has a lot of meaning.”

Before the flashing yellow arrow arrived, drivers sometimes expressed confusion over whether a green turn arrow meant something different from a solid green light, Bradford said.

“It’s a lot more intuitive [to drivers] when you have a flashing yellow arrow instead of a green bulb,” he said.

Adapting existing traffic signals to include the yellow arrow is expensive, but doing so improves traffic dynamics because “all of those signals talk to each other,” Costello said.

Retrofits can cost between $10,000 and $40,000 an intersection.

“It’s not as simple as going out there and pushing a button,” he said.

Flashing yellow arrows first appeared in Minnesota in 2006 as an experiment. Now that they are approved by the Federal Highway Administration, they have become the new standard for hundreds of controlled intersections across the state.

Researchers at the University of Minnesota are developing a statistical model to help determine whether a flashing yellow arrow would be safe at a given place.

A big advantage of the flashing arrow is its flexibility — it can be changed to a solid red arrow at times of day when oncoming traffic is heavy, Costello said.

“The goal is to keep people safe first and foremost,” he said.