Why Your Car Might Have Trouble Triggering Traffic Lights In The Snow

There was a time when most traffic lights worked by relying on timers. This isn't ideal, as anyone who's sat muttering at a stoplight in the middle of the night without any other cars on the road can testify. Nowadays, many signals use sensors — such as inductive loops – that detect the presence of vehicles and adjust the lights accordingly. 

Inductive loop sensors use a coil of wire, usually embedded in the road near the stop sign. When a car rolls over that loop, the metal in the vehicle disrupts the magnetic field in the coil, signaling to the traffic controller that someone is waiting. These systems don't rely on weight, as is commonly believed. Instead, they rely on the presence of metals in a vehicle, which is why motorcycles and cycles can sometimes struggle to activate them if they're not positioned correctly.

So, why does it appear that your car has trouble triggering traffic lights in the snow? Surely snow can't block a magnetic signal. After all, magnets work perfectly well under water, so a little snow shouldn't leave you stuck at a stoplight. Well, thankfully, these traffic lights aren't defying the laws of physics, which would be both weird and far more newsworthy. The truth, it appears, is far simpler. The snow hides the road markings that tell drivers exactly where to stop. Sit too far back, a little off-center, or too far forward, and you can miss the most sensitive part of the zone entirely. 

A 2019 paper published in ScienceDirect suggests that inclement weather — including snow, ice, and fog — contributes to roughly 544 million vehicle-hours of delays each year in the U.S., accounting for about 23% of all non-recurring highway delays. While it's a push to suggest that these delays can all be attributed to traffic lights failing to detect waiting vehicles, the cumulative effects of any traffic-light-related issues will likely add to drivers' frustration. Of course, some lights — like the most infuriating traffic light in U.S. history — manage this feat in any weather, but snow can make it worse.

This is why the advice from Jason Saliba, a traffic light engineer, is worth paying attention to. Speaking to KY3 News, Saliba explained that snowy conditions themselves aren't usually the reason traffic lights fail to respond. Instead, and as noted above, the problem can be as simple as where the driver has stopped their vehicle. 

The solution is just as straightforward. As Saliba explains, "If you don't think that your car is getting detected, then you can try repositioning yourself closer to the center of the lane." He also suggested that you need to pull closer to the stop bar, noting that they're usually "about 10 feet behind the edge of pavement for the intersecting street."