Here’s Why Automatic High Beams Keep Getting Confused By Road Signs

Advanced Driver Assistance Systems (ADAS) have proliferated across the new vehicle market, and many of these systems have seen mixed success. Some, like automatic emergency braking and blind spot monitoring, are great ideas, but most of these systems are fallible. That includes high beams that automatically dim when the headlights of an oncoming car are detected. While this headlight-switching technology can make driving easier, it can also make driving more dangerous if the lights dim when there’s no oncoming traffic to be concerned with – a malfunction that can be triggered by the sudden appearance of reflective road signs.

First, let’s learn a little bit about how modern automatic high-beam systems work. You know those cameras you see at the tops of modern car windshields? Among many functions, at least one of those cameras is used to detect light from oncoming vehicles and vehicles ahead, then beam that information through the car’s computer network and cycle the high beams. In theory, this should allow for more precise detection of vehicles within the camera’s framing and depth of field. In practice, a camera system has limitations, namely that light detection isn’t the same thing as headlight detection.

To understand what’s going on here, we must first know a little bit about image sensors. Over the past few decades, two types of image censors CCD and CMOS have seen a degree of popularity, and Tokyo Electron’s Nanotec Museum has done a great job summarizing the differences and similarities.

A charge-coupled device (CCD) image sensor has an array of capacitors, each carrying an electric charge corresponding to the light intensity of a pixel. A control circuit causes each capacitor to transfer its contents to its neighbor, and the last capacitor in the array dumps its charge into a charge amplifier. The bucket-brigade style of data transfer is characteristic of CCD sensors.

In contrast, a complementary metal oxide semiconductor (CMOS) image sensor has a photodiode and a CMOS transistor switch for each pixel, allowing the pixel signals to be amplified individually. By operating the matrix of switches, the pixel signals can be accessed directly and sequentially, and at a much higher speed than a CCD sensor. Having an amplifier for each pixel also gives another advantage: it reduces the noise that occurs when reading the electrical signals converted from captured light.

The key here is that an image sensor captures light, and light is light no matter if it’s a direct source or reflected. While modern automatic high-beam systems are usually fairly good at detecting direct light sources like headlights within their cameras’ framing, reflected light can be confused for headlights, which is annoying when our roadsides are full of reflective surfaces.

Some 52 years ago, 3M engineer Tim Hoopman started to have a crack at prismatic retroreflective sheeting that could be laminated to road signs for increased reflectiveness at night. According to 3M, it almost worked, except this initial prototype wasn’t effective from every angle. However, after 12 years of research and development, the chemical firm was finally granted a patent on a film that worked from every angle. Today, laminated sheeting like this is used on a majority of traffic signs in North America, and this is where the problem with automatic high beams starts.

Remember how I said that light is light? It turns out that the high-beam light put out by a car’s headlights, then reflected by road signs to that car’s automatic high-beam cameras may cause the automatic high beams to turn off. At night on low-traffic, well-lit sections of interstate highways, this is more of an annoyance than anything. However, on rural roads without much lighting and with no barriers to gate off crossing wildlife, high beams cutting out mid-drive can be dangerous.

These drop-outs due to road signs have been noted by owners of multiple makes and models of vehicles, like the 5thgenrams forum poster in the screenshot above who wrote that “any road signs plays havoc with the high beams” and posted a video of the situation to YouTube. Watch the clip for yourself, and you’ll be able to see high beams cutting off shortly after approaching road signs on multiple occasions within a 98-second period.

Mind you, Ram owners aren’t the only drivers to be irritated by the performance of their automatic high beams. Polestar owners on Polestar Forum, Honda Ridgeline owners, and I’ve noticed this issue myself in multiple press cars across various makes and models. To a camera, incoming light is incoming light regardless of whether it’s reflected or not, and it’s up to the embedded systems in cars to decipher road signs from headlights. Many do a poor job.

Speaking of inconsistency, many Tesla owners have complained about repeated cycling of automatic high beams, and it’s a larger problem in some Tesla vehicles than others due to variances in override controls. In most cars, you turn on the high beams by pushing the left steering column stalk toward the dashboard, where it then holds in place. However, Tesla’s been on a minimalism streak, and with that has come the elimination of steering column stalks. In the new Model 3, along with the current Model S, Model X, and the Cybertruck, getting the high beams to stay on requires holding a button for a few seconds. As per Tesla:

Press and hold to turn on high beam headlights – touchscreen displays a brief timer and you must hold for the duration of the timer to latch the high beam headlights to the on position. When headlights are on, press the button a second time to turn them off.

When you’re in motion, every second counts, and the quicker you can activate a function, the quicker you can get back to focusing entirely on the road. Keep in mind that at 60 mph, a vehicle travels 88 feet per second, so by even shifting your attention away from driving for four seconds, you’ll have traveled more the length of a football field from goal line to goal line.

It’s worth noting that automatic high beams can be useful safety aids. Humans often aren’t great at judging appropriate times to use their high beams, and the visibility difference between low-beam headlights and high beams can be staggering. In the words of the Insurance Institute for Highway Safety:

Researchers from IIHS and the University of Michigan Transportation Research Institute found that drivers in and around Ann Arbor, Mich., didn’t use their high beams enough (Reagan et al., 2017). Only 18% of drivers who were isolated enough to make use of their high beams did so.

Using high beams when conditions allow makes it easier to spot obstacles in time to avoid them. For example, a driver of a 2023 Subaru Solterra should be able to spot an obstacle in the left lane 202 feet ahead while using low beams and 533 feet ahead while using high beams.

High-beam assist could improve the high-beam use rate if drivers are simply forgetting to turn on their high beams, are unsure whether oncoming vehicles are far enough away to do so safely, or underestimate the effect of high beams on visibility.

Automatic high beams have improved vastly over the past few years, but they’re still sometimes no match for the human eye. While this feature does offer real safety benefits, technology isn’t infallible, and if something is operating in an obviously incorrect way, it’s often best to use your own judgment and override the system.

(Photo credits: Acura, openclipart, 5thgenrams, Tesla Motors Club, Honda)

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