Prepare to Stop When Lights are Flashing

Flashing Yellow Advance Stop Warning SignKate asks “When your light is green at the intersection ahead but above you the advanced yellow overhead warning lights have begun to flash, should you be preparing to stop for that green light?”

Prepare to Stop

The Learn to Drive Smart manual deals with this by presenting a picture of the signal and identifying it with “Signal lights ahead – prepare to stop when lights are flashing” on page 33. There is no indication given about the state of the traffic lights at the intersection ahead.

Driver's Perception of Yellow Lights

A yellow light tends to be perceived by drivers as a cautionary indication that they may either pay attention to or ignore depending on their experience and the road conditions at the time. This is generally true when the yellow light is flashing.

Flashing Yellow Light

The Motor Vehicle Act tells us that the driver of a vehicle facing the flashes of yellow light may cause it to enter the intersection and proceed only with caution, but must yield the right of way to pedestrians lawfully in the intersection or an adjacent crosswalk.

Solid Yellow Light

However, a solid yellow light at an intersection tells a driver to stop. There is one exception to this rule and that is when stopping for the yellow light cannot be done safely. Examples of unsafe situations would include being too close to the intersection when the yellow light illuminates or when you are being followed too closely by another vehicle.

Which Light To Obey?

Kate’s question really has two parts, the flashing yellow lights and the green light at the intersection. As you approach the intersection, you face the overhead warning light first and the intersection signal second. You are required to take each signal into account as you approach it, so the green light doesn’t play any part in the equation until after you have passed the yellow flashing lights.

Light Timing

The overhead warning lights are timed such that a driver approaching them may see them illuminate and know that the green light ahead will be yellow when they get to the intersection. The driver will be required to stop and have the time to realize it, prepare and come to a safe and comfortable stop before the stop line.

Not First in Line

The system is a good one if you are not the first driver in line when the flashing yellow lights come on. You know that the driver in front of you should stop, so you prepare to stop too. Follow the routine and you will be at a reduced risk for collision because the first driver is less likely to brake suddenly as they might be with a yellow or red light alone.


When I used an unmarked car for traffic enforcement I would watch the overhead lights come on, slow to a stop and often ticket the driver behind me in the other lane who blew through without stopping. The judicial justice in traffic court was comfortable convicting that driver with only these circumstances.

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In my experience, these devices may be utilized:

  • When the traffic light itself is hidden around the next curve, so as to warn drivers before they come upon the slowing or stationary traffic ahead that is already responding to the traffic light changing from green to amber; and will continue to flash throughout the amber and red phases, to reduce the chance of rear-end collisions.
  • When the traffic light is on a relatively severe gradient (downhill, not uphill, eh?) as the braking distance will be longer due to fundamental physics.
  • On higher speed roads - Hwy 19 naturally comes to mind - as once again, the braking distance will be longer due to different fundamental physics. In this instance, then if you're driving right on the speed limit and the lights start flashing at, say, 80 meters distance ahead - then the actual traffic light at the intersection will go from green to amber when you're 80 meters distance away from the that intersection. Thus preventing last-second decision making on whether to brake or continue as the driver tries to calculate (much too late) his point-of-no-return.

Slightly off-topic, but related; it's kind of horrifying to see how many drivers, approaching an intersection where a flashing green arrow (indicating a protected turn) will actually speed up when it becomes an amber arrow even though they could have stopped easily; this is the kind of behaviour that puts pedestrians awaiting their 'Walk' signal at huge risk.

These signs make me feel warm and fuzzy - as if I can tell the future! Any relevant information on the road is always welcome, and putting them on roads that are posted higher than 50km/h but still have ground level intersections is very useful.

One thing I have to note is the timing of these boards can vary from place to place, with some coinciding with the speed limit and some seemingly expecting traffic to be faster than the posted limit. So in some cases when you are going 50-70 whatever it may be and you pass the sign just as it starts flashing - if you are going the speed limit, the intersection will turn yellow then red just as you approach. And some will flash, but by the time you get there the intersection is still green and passable.

In such cases of improper timing a trend emerges among those drivers that are familiar with the intersection - they speed up above the posted limit to make sure they get through, sometimes riding too close to the bumper of the car ahead. The flashing sign starts to be perceived as a "speed-up you slow-poke" light in those intersections, and thus I think all of such signs need to be timed very strict and uniform to the posted limit or even less - so that when its flashing drivers have a consistent expectation to stop.