The Stale Green Light at a Snow-Dusted Intersection
You are westbound on a four-lane arterial in Calgary at 55 km/h. The light ahead has been green since you first saw it about 180 m back. The road surface is dry, but the intersection pavement is shadowed and carries a thin film of packed snow. A pickup truck is following you roughly 1.5 seconds behind. As you close to about 60 m, the light is still green and you have no way to know how long it has been that way. A pedestrian is standing on the northwest corner facing the crosswalk but not stepping off the curb. Your lane is clear beyond the intersection.
1. Should you maintain speed, brake, or prepare to stop as you approach? Explain your reasoning using the information available.
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A green light you have watched for a long time is stale - it could change at any moment, and you cannot predict the change point. The correct action is to ease off the accelerator and cover the brake, reducing speed so you can stop comfortably if the light turns yellow.
Key factors:
- Stopping distance: At 55 km/h you need roughly 40–50 m to stop on dry pavement, more on the shadowed, snow-packed intersection. At 60 m out, committing to a full-speed pass leaves you no margin if the light changes.
- Follower: The truck at 1.5 s is close but not tailgating dangerously; a gradual, signaled slowdown (not a panic brake) gives it time to react.
- Pedestrian: Someone at the curb may step off; a slower approach gives you reaction time and keeps you ready to yield if they enter the crosswalk on your green.
- Traction change: The shadowed snow film means braking performance drops right where you'd need it most.
Maintaining speed commits you to running a light you can't predict. Braking hard is unnecessary and risky with a close follower. The judgement call is a controlled, early reduction in speed - not a decision to stop, but a decision to preserve the option to stop.
2. The light turns yellow when you are 45 m from the stop line at 50 km/h. What do you do, and what makes this the safe choice?
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At 50 km/h you travel about 14 m per second. From 45 m out you'd need roughly 3+ seconds just to reach the line, plus braking distance of about 35–45 m on this surface. That math says you cannot clear the intersection before red and cannot stop comfortably either - you are in the dilemma zone.
Resolution: brake firmly but progressively to stop before the line if traction allows, because:
- 45 m is within a normal stopping distance on dry pavement, and the snow film is thin enough that a firm, straight-line stop with ABS is feasible.
- Proceeding means entering on a yellow that will almost certainly go red while you're in the intersection - a red-light violation and a collision risk with cross traffic that gets a green.
- The pedestrian at the corner may get a walk signal the instant your light goes red.
If, after committing to the brake, you find you cannot stop before the line, the fallback is to continue through without accelerating further, staying in your lane, rather than swerving or slamming to a halt in the intersection. The safe choice is the one that keeps you predictable and out of the conflict zone.