5 Things That Make Traffic Signals Change

Driving through an intersection with traffic lights

What makes traffic light signals change?





Traffic signals are Intelligent Transportation Systems (ITS) designed to allow vehicles to safely cross paths while maintaining an efficient flow of traffic. To do this, traffic signals react to the presence of the vehicles and pedestrians they are guiding.

Although you probably don’t notice it, each mode of transportation interacts with traffic signals in a unique way. Here are the five main traffic light triggers.

1. Passenger vehicles

Ministry of Transportation and Infrastructure intersection traffic signals are “actuated,” which means a vehicle sends a call-out for its light to turn green when it drives over a “loop” in the road. There are various vehicle detection systems out there; the ministry uses a wire embedded into the pavement and connected to the traffic signal controller.

When a vehicle drives over the loop, the traffic controller detector senses the change in electromagnetic field caused by the introduction of metal (from the vehicle) over the loop. This starts a countdown for the light to turn green.

Most traffic lights on major highways use a combination of actuated and “fixed” traffic signals. This means the traffic signals facing highway traffic will rest (or, remain fixed) on green until the side street signals are activated by a vehicle over the loop. This helps keep highway traffic flowing.

2. Emergency vehicles

Have you ever noticed those small white and blue lights perched on a traffic signal beam? They may not mean a lot to you, but they do to emergency vehicles.

Signal Pre-emption device to hear vehicles approaching

Many traffic signals are equipped with an emergency vehicle pre-emption device, which allows emergency vehicles to activate a green signal in the direction they are travelling. The most common ministry pre-emption device is triggered by the sound of the emergency vehicle’s siren. That’s when the white and blue lights come into play.

Since multiple emergency vehicles may approach an intersection from different directions at the same time, one direction is given priority. The white light indicates pre-emption granted in that direction of travel, while the blue light indicates the intersection is being controlled by an emergency vehicle approaching from another direction.

3. Pedestrians

Most intersections include pedestrian “walk” signals that indicate when it is safe to cross the road. Pedestrians push a button, which sends a signal to the traffic controller calling for a green light in their direction along with the pedestrian walk symbol.

While it may take time for the green to activate at an intersection, traffic signals at enhanced pedestrian crossings (like the one pictured here) react immediately by activating flashing lights next to the roadside and/or overhead pedestrian signs. This alerts oncoming traffic to slow down and yield to crossing pedestrians.

Controlled crosswalk with lights

4. Buses

We all have daily schedules to follow; but on the road, it’s tough to find a stricter driving schedule than a public transit bus driver’s. Transit users rely on buses reaching their stops on time.

A graphic showcasing bus priority lanes and traffic lights
Credit: Translink’s The Buzzer

Bus lanes are not the only things that help buses maintain a consistent schedule – buses have their very own traffic signal, too. Some traffic lights include a rectangular white light at the very top that allows buses to proceed through the intersection ahead of other vehicles. Remember those actuated loops for passenger vehicles we mentioned earlier? Buses also have them; only difference is they cover more ground so that smaller vehicles cannot trigger the signal.

5. Trains

Ever wonder how railway crossings on roadways are activated? As a train approaches a crossing, it sets off a sensor built into the track, which prompts the rail crossing lights to flash. When an intersection is nearby (60 metres or less from the tracks, to be exact), a green light activates for vehicles closest to the tracks, allowing these vehicles to safely clear out of the vicinity of the rail crossing. The rail crossing lights flash for a minimum 24 seconds before the railway gate slowly lowers. The gate is completely closed five seconds before the train arrives.


So, there you have it: five ways traffic signals turn from green, yellow, red, and back to green. They’re pretty smart. Next time you’re waiting at a red light, don’t fret, it knows you’re there. You’ll be back on your way soon.

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Page 1 of 43 comments on “5 Things That Make Traffic Signals Change”

Leave a Reply to tranbceditor Cancel reply

  1. Thanks for your reply. My husband and I disagree on this subject so I still need further clarification. When the pedestrian lights are activated the through traffic is stopped by the red light. My husband turns left into the intersection at this point and waits before the crossing. When the pedestrians have crossed he waits for the light to turn green before he proceeds … this means the light is green for the through traffic. I think he should proceed through the red light when the pedestrians are safely across. Can you say which is correct.


    • Hi again Yyvonne,

      A driver should move through the intersection when it is safe to do so. This means they should not sit in the intersection if they are clear to move as they could hold up traffic moving on the new green light. Hope that this helps. It’s not everyday we can help settle a husband and wife disagreement! đŸ˜‰

  2. If turning left from a side road at a t junction, can I proceed into the intersection when the red pedestrian light is activated, then move through the red light when the pedestrian has crossed, or do I need to wait for the light to turn green. There is no light on the side road.

  3. A friend and I recently took a trip to Vancouver from Oregon. She was the driver and I, the passenger. At a major intersection, there were numerous red lights but when she began (with the rest of the traffic), it looked to me to still be red. Are the lights configured to only be seen accurately head-on from the driver side of the car or was I perhaps seeing a different light for a right-hand turn lane. I am perplexed.

    • Hi Shannon,

      We put “louvers” around our signal heads so they are primarily visible to the traffic in the lanes they are directing. For our left turn only signals, which are likewise protected by louvers, we use an illuminated red circle. In the U.S., there typically is a red arrow for this situation.

      From your position in the passenger seat, you may have been seeing a red left turn only signal, while the driver was seeing the green signal directly ahead.

  4. I only moved to BC in 2007, so I was a bit surprised to meet all the loop-activated traffic signals in use everywhere out here. I guess they don’t hold up so well in the ultra-cold of an Alberta winter with plows scraping the road and all.
    Anyway, I do have a beef with the loop-activated signals – they should fit into a “green wave” type system. Far too often, I get up to speed from a light on a highway, and a vehicle on a side road triggers the next light causing me to have to brake and stop. This usually happens because there is no traffic on the main road keeping the loops there activated. Additional loops should be placed further from the light on the main road as well to handle the approaching high-speed traffic.

    Instead, I think that the traffic on the side road should have to wait until a short time before the light cycles to allow that traffic to proceed. In the long run, I think that a small number of cars idling on the side road would produce less greenhouse gas emissions that those that have just gotten up to speed, and been forced to stop, and then accelerate again.

    • HI Brian,
      Thanks for your suggestions. We actually do employ some of these techniques on the traffic signals on Vancouver Island, however throughout the rest of the province, we don’t use them much. The reason is, when travellers move from town to town in BC, there’s not much development between them so there is seldom the need for employing signals outside of city limits. Use of the “green wave” depends a lot on signal spacing, the number of signal phases and the traffic composition and the travel speed. Within municipal boundaries most of the signals would belong to the municipality and the operation of those signals falls upon the local government to operate their signals as they deem fit.

      For those signals that do belong to the Ministry within city boundaries, some will be coordinated while others will not. Even when coordinated, perfect coordination is difficult to achieve as the best coordinated systems will have (and this type of set-up is rare):
      • All intersections have the exact same traffic volumes on each of the legs of the intersections
      • Spacing between signals is the same (200 – 400 m)
      • Lane configurations are the same (all have the same amount of through lanes, left turn lanes, and right turn lanes)
      • Signal phasing is all the same (same arrows, etc.) on all the legs
      • The coordinated street is a one-way street

      Hope you found this useful.