Why You Should Always Pull Up to the White Line at Intersections

The traffic light at my side street almost always turns green within 10 seconds of my arrival, setting a happy tone for the rest of the drive. But on occasion, someone stops too far back or pulls too far over the line, where the traffic light sensor can’t detect them. When that happens, I sometimes have to wait for minutes until someone else stops at the other side of the intersection and finally triggers the light.

Apparently, my frustration is a common one. Threads on reddit and other sites have many people lamenting about unaware drivers, while others confess they never realized traffic light sensors existed.

“Topics like traffic light sensors often trend because they directly impact everyday drivers, especially as myths about their functionality circulate,” says Virginia Lingham, senior vice president at the infrastructure engineering firm WSP. “Additionally, advancements in intelligent transportation systems and the integration of connected vehicles are driving public curiosity about how these systems work.”

If you’re curious about how traffic light sensors work, or just want to keep the flow of green lights and good vibes rolling, here’s what to know.

What Do Traffic Light Sensors Do?

Traffic light sensors detect when a vehicle stops at a red light and inform the traffic light control system that a car is waiting for the light to turn green. This optimizes the amount of time lights can stay green for busier lanes and reduces the amount of time we have to wait at red lights.

Some traffic light sensors combine data from multiple sensors, says Xavier Sabastian, an auto expert at Way.com. This helps:

• Improve traffic flow
• Reduce idling cars, saving fuel and emissions
• Reduce accidents
• Adapt to changes in traffic during peak and off-peak hours
• Address immediate bottleneck issues by live updating the timing of signals

“Certain sensors also provide automatic priority to emergency vehicles passing through the intersection,” says Sabastian.

Types of Traffic Light Sensors

The three most common types of sensors are induction loop, camera and radar. Each has its pros and cons, and where you find them will depend on the type of pavement, regional climate and financial resources of the city or state.

Inductive Loop Sensors

Inductive loop sensors are embedded in or under the pavement and use changes in magnetic fields to detect vehicles. Their coiled wire is loaded with an electrical charge, which changes voltage when a large piece of metal gets within range.

While these are highly reliable, they need to be replaced every time the road is repaved, which has led to the rise of other detection types, like cameras and radar, says a Denver metro traffic engineer with the Colorado Department of Transportation (CDOT).

Video Camera Detection

Video camera detectors point toward oncoming traffic, and some have 360-degree views. The latest generation can also recognize bicycles.

“These are very useful for tracking movement patterns of vehicles in multiple lanes,” says Sabastian. “Studies reveal that video detection systems can reduce delays in traffic by up to 20% in congestion.”

However, they do have some shortcomings says CDOT. “The primary concern is that the lens can get dirty or blinded, just like our eyes,” they say. “Sunrise and sunset can also glare out a camera for several minutes, leading to erratic behavior for a short period of time.”

Radar Detection

Radar detection works similarly to cameras but uses radio waves to detect approaching vehicles. That can give it an edge over video cameras since it doesn’t work within the visible spectrum of light, says CDOT.

Radar detection is also effective for detecting faster-moving vehicles, says Sabastian, so these are commonly found at high-speed intersections and freeway ramps.

Infrared Sensors

Infrared traffic sensors detect approaching vehicles by using their heat signatures. “These are good under low-visibility conditions like fog or rain and mostly used in areas with extreme weather conditions,” says Sabastian.

Other Sensors

Microwave sensors are efficient in detecting both stationary and moving vehicles and are mostly found in suburbs with fair to moderate traffic, says Sabastian. Other emerging tools include video analytics and LiDar, says Lingham.

Do All Traffic Lights Have Sensors?

No, not all traffic lights have sensors. Areas with complex road networks, like cities, often optimize traffic flow using fixed timers. Also, older lights in rural and other areas also still use timers.

“The majority of traffic signals outside major urban grid systems operate with some form of vehicle detection,” says Lingham. “However, each state and local agency has its own operating procedures and preferences, and these systems can vary widely by region.”

What Happens If You Don’t Drive Up to the White Line at a Traffic Light With Sensors?

The sensors won’t detect your vehicle, so the light won’t change from red to green. Especially with loop sensor technology, “if you’re too far back or over the white line, it’ll just act like there’s nobody there,” says Sabastian. “This may result in longer wait times, as the system assumes there is no traffic in your lane. For intersections with high traffic volumes, such an oversight can disrupt the entire flow of vehicles.”

What Is the Proper Distance to Stop From Traffic Light Sensors?

Stop near the white line, aka the stop bar or limit line. A good rule of thumb is to align the view over your vehicle’s hood with the line since detection zones typically start at the stop bar and include a car length or back (about 40 feet).

Can Traffic Light Sensors Fail?

Yes, sometimes traffic light sensors can fail, but many have a fallback mechanism that recognizes when a direction has not been triggered for a significant length of time and then assumes that means the sensor is broken. It will then behave as if there is always a car present, until it can be repaired, says CDOT.

“Every driver out late at night has been stopped at a red light when no conflicting vehicle is present, and it may feel unnecessary; it is likely this important safety feature in action,” says CDOT.

Will Flashing My High Beams Make the Light Turn Green More Quickly?

No, flashing your headlights will not impact traffic light sensors because they don’t detect headlights, says Deege Carse, director at Trackershop.

“There’s also the misconception that the heavier your vehicle, the more you’re able to trigger a green light,” he says. “In reality, the weight of your car can’t do anything to trigger a green light. With inductive loop technology, it’s the metal or iron in your car that triggers this and lets the system know that you are waiting for light to turn green.”

About the Experts

• The Colorado Department of Transportation spokesperson is a traffic engineer for the Denver metro-area.
• Virginia Lingham, is a Professional Engineer (PE), senior vice president and national ITS lead with WSP, a global engineering firm specializing in transportation, infrastructure and other sectors.
• Xavier Sabastian is an author and auto/insurance expert with the Way.com auto app.
• Deege Carse is director at Trackershop, a supplier of GPS trackers and vehicle security systems.

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