Brandie Weikle · CBC Radio

· Posted: May 23, 2026 4:00 AM EDT | Last Updated: May 23

LISTEN | Was this the shortest yellow light in the world?:

It’s a split-second decision every driver knows well. 

The traffic light turns from green to yellow, and you’re called upon to perform calculus involving distance, time, velocity and the presence of tailgaters. 

It’s a lot of variables. Get it wrong and you could wind up with a red-light camera ticket — get it catastrophically wrong and you could wind up in an accident.

The engineers behind those yellow lights — including the ones that seem painfully short — say a lot of science goes into how traffic lights are programmed. But they also account for the different ways drivers respond at intersections.

Jodie Marcyniuk, acting senior leader for traffic management at the city of Calgary, says her department follows accepted guidance for calculating the timing of amber lights, as well as the period of “all red.” That’s the short time when cars in all four directions face red and no vehicles can go.

Jodie Marcyniuk, acting senior leader for traffic management at the city of Calgary, says every intersection is treated as a unique case. (Submitted by City of Calgary)

These recommendations rely on basic high school physics, she says. 

“It's looking at speed of approaching vehicles. It's looking at deceleration rates. And it's looking at slope of the road,” she says. “And then it also considers driver reaction time.”

That means that every intersection is treated as a unique case, Marcyniuk says, with traffic engineers examining things like the topography and the speed limit on that stretch of road before deciding how long drivers will see that yellow light.

If it’s safe to stop on yellow, you should

But if you’re running late for work, the family potluck or your kid’s gymnastics class, it’s easy to forget the general principle behind yellow lights.

Marcyniuk says if you can stop safely when the light turns yellow, you should.

“If you can't stop in time, if you do need to enter the intersection, you would still be entering on yellow,” she says. “What I don't like to see is those drivers really pushing it, trying to beat that light and entering later than they really should.”

Some yellow lights may seem particularly short, but factors like the topography and speed limit in the area are taken into consideration, Marcyniuk says. (Adrian Wyld/The Canadian Press)

Standing at the busy intersection of Brock and Taunton streets in Whitby, Ont., Travis Stocking says perfecting the timing for these lights will create what he calls traffic “platoons.”

“So that's a group of cars travelling along the road to arrive at the light when it turns green,” said Stocking, senior traffic analyst for Regional Municipality of Durham, a suburban area east of Toronto.

But, of course, platoons change as new vehicles get in front or other factors slow traffic down.

“You can make it look perfect on paper, but in a practical sense, it doesn't always work out the way you might model it,” he told Ideas producer Seán Foley. 

When an intersection is maxed out, for instance, it won't matter what experts do to finesse the traffic signals, he says  — there's just not enough capacity to get every vehicle through. “And that's where you get congestion and delay.”

Unintended consequences

Marcyniuk says traffic engineers are also aware that going too far in one direction with an amber signal can have an undesired impact on driver behaviour.

“We know that if we extend our yellows to be too long, sometimes that will encourage them to enter later. “They realize ‘this is a long yellow light. I'm going to push it.’ Well, then the driver ahead of them … may be coming to a safe stop, and it can result in an increase in rear-end collisions.”

Ron Usher said it was that kind of scenario on his mind in a situation that ultimately led to him getting a red-light camera ticket at a busy downtown Vancouver intersection about 2.5 years ago.

“I've been rear-ended a couple times in Vancouver by drivers kind of not paying attention,” he said. 

Usher says he’s pretty sure a police officer stationed at the intersection wouldn’t have ticketed him for continuing through the intersection on red because he was being tailgated too closely to stop safely. 

But he decided not to contest the ticket.

“It's way less of a price than some significant accident on a busy street.”

WATCH | Data shows photo radar slows drivers down:

‘The indecision zone’

The yellow light is meant to alert drivers that they’re entering what traffic experts refer to as “the indecision zone,” says Stocking.

That’s the point at which drivers should either stop if they can do so without having to really slam the brakes, he says, or continue forward — providing they can clear the intersection at their current speed no later than the end of the all-red period.

But then there’s the human element.

“You can create a set of rules that's based on a particular driver, but the next driver through the intersection might not behave the same way,” Stocking said.

“You're assuming that every driver is following the rules, which most of them don't, and you're assuming that they're all paying attention, which a lot of them aren't. And you're also assuming that they're being considerate of others, which they're also not.” 

Drivers shouldn’t think of a yellow light as a guarantee there’s time to get through the intersection, he says. “It’s guidance. But ultimately it comes down to the driver paying attention, looking at the road ahead of them.”