There are any number of ways to tell drivers they can turn left when it's safe, from a solid green light to a blinking red one, but in 2009 a federal traffic guide anointed the flashing yellow arrow as the traffic signal of choice. Soon cities and suburbs across the country adopted it for their intersections (according to Wikipedia, only four states have yet to join the flashing yellow club). Some, like Washington County in Oregon, part of metropolitan Portland, used stimulus money to make a widespread change.
From a traffic standpoint, the flashing yellow arrow has a lot to offer. Drivers overwhelmingly understand what it means — essentially, wait for a gap in conflicting flow before turning — and this flexibility reduces left-turn lines and ultimately congestion. But new research on the flashing yellow has revealed a rather glaring flaw: drivers focus so much on oncoming cars that they fail to notice pedestrians.
"It's more efficient, because we're using gaps in an opposing stream of traffic," says David Hurwitz, a transport engineer at Oregon State University, of the flashing yellow arrow. "But it has safety implications."
To determine just what those implications are, Hurwitz recently collaborated with Christopher Monsere of Portland State University, as well as some engineers from Washington County (via Joseph Rose at The Oregonian). The researchers gathered people into a driving simulator and observed their eye movement and vehicle control during a series of left-turn scenarios based on actual county intersections. When drivers reached a flashing yellow they had to consider two lanes of 45-mph traffic, a bike lane, and pedestrians in the opposing crosswalk before turning.
The researchers discovered that drivers were so concerned with finding gaps in opposing traffic that they often failed to fixate on pedestrians crossing to their left. (A "fixation," says Hurwitz, was defined as looking at a specific point for more than a tenth of a second.) Depending on the pedestrian movement, some 4 to 7 percent of drivers in the simulator failed to notice the walkers at all — percentages that Hurwitz and company call "alarming." As the number of oncoming cars increased, the amount of time spent fixating on pedestrians decreased, too.
"When you see no fixations on anything of importance, that's concerning," he says. "So when we don't fixate on a pedestrian it means we're not actively considering their presence."
Washington County has already implemented a modification. Now, if pedestrians press a walk button before crossing the street, the intersection signal holds the flashing yellow until the next light cycle. The fix isn't perfect — pedestrians who arrive at the crosswalk after the flashing yellow begins must wait longer than before, and of course the system isn't activated if the button isn't pressed — but it's a step in the direction of safety.
Hurwitz hopes that, in time, this type of work will demonstrate the ongoing need to consider pedestrian behavior when designing traffic signals in the first place — a process he calls "predominantly vehicular-centric."
"I think the immediate results of this work would be an increased awareness in messaging to drivers that they need to be searching for alternative modes of transportation when they make this maneuver," he says. "They need to be paying attention to cyclists and pedestrians, in particular, when they're searching for their gaps."