CBS 17 investigates the science behind the timing of yellow lights


RALEIGH, N.C. (WNCN) — It’s called the “dilemma zone” – that time when a traffic light turns yellow as you are either approaching or going through an intersection.

Because of the way yellow lights change, many drivers insist those lights aren’t timed right.

As it turns out, yellow light timing is based on complicated math formulas used by engineers to keep drivers safe, but now there is evidence that some yellow light timing may need to be recalculated.

We all know the rules of the road. Green means go, but for many drivers yellow means go faster.

That yellow light can also determine if you get a ticket or not – making it the superstar of traffic enforcement.

Driver Bella Hanzel said she doesn’t think yellow lights are timed long enough to go through.

“Absolutely not,” she insisted.

State traffic engineer Kevin Lacy oversees more than 9,000 North Carolina Department of Transportation traffic signals in the state.

“The old adage is — you see yellow — speed up,” he said.

When it comes to yellow lights, driver Seth Black said he always wrestles with the decision.

“Maybe I can make it — maybe I can’t make it or I’ll have to slam on the brakes real fast,” he said.

Scientists call this the “dilemma zone” because people have a hard time judging yellow lights.

Once a light turns yellow in the dilemma zone, drivers have to either make a panic stop or hit the gas and race through the intersection.

When it comes to vehicles, fractions of a second can prevent or cause a crash.

“In the driving world, if you’re doing 45 miles an hour, one-tenth of a second is a little over six and a half feet,” said Lacy.

Traffic engineers say how long a light stays yellow is determined in advance based on speed limit, road conditions, driver judgement and perception/reaction time.

Driver Michael Spruill believes, “if you’re going the right speed, yellow lights are long enough to get through properly.”

But, drivers running lights long after the yellow has faded away is a growing problem.

“We’re not talking about a half second or so,” said Lacy. “We’re talking deep into the red.”

Lacy said NCDOT has video of drivers running the red light after its been red for two, three or even five seconds.

The state tries to account for actions like that, with what they call dynamic red lights, meaning drivers crossing the intersection don’t get a green light right away after the opposing light has gone red.

“You’ll have all red for one to two seconds,” said Lacy. “That’s to clear the intersection.”

But, let’s say you’re in the middle of the intersection and your yellow light suddenly goes red. Are you in line for a ticket?

“If you enter the intersection on yellow, you have the legal right to clear it,” said Lacy.

When it comes to yellow lights, there are two ways you encounter them.

You either go straight through an intersection or you make a left or right turn.

For the last couple of years, an Oregon man has been fighting with the group that sets traffic light standards, known as the Institute of Transportation Engineers.

Mats Järlström claimed that yellow lights in his state were mistimed so that turning drivers didn’t have enough time before the light went red.

In October 2019, ITE said Järlström was right and engineers would study the turning situation at yellow lights.

Consumer investigator Steve Sbraccia asked Lacy if Järlström’s complaint had any validity here.

Lacy said, in North Carolina, we do things a little differently when it comes to turning on yellow lights.

He says our state does follow most of the ITE recommendations, but about 10 years ago North Carolina adopted a different timing calculation for drivers turning giving them a bit more time.

According to federal regulations in the uniform traffic control manual, yellow lights must be no less than three seconds and no more than six seconds.

Lacy said if ITE re-works its formula for turns and it turns out to be better than what the NCDOT currently uses, the state might adopt it.

“Not everything you can calculate down to the Nth degree is practical to apply in the real world,” he said.

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