How exactly does lane departure warning work?

    Using cameras that cost only tens of dollars, lane departure warning (LDW) systems can help drivers avoid many potential traffic accidents. Cameras and processing software determine the distance between the vehicle and road markings (such as lane lines), and issue a warning when the vehicle is about to cross the line (of course, when the vehicle's turn signal is not turned on). Lane departure warning has become an indispensable function of the active safety system of automobiles. After several years of development, lane departure warning has gradually upgraded to lane keeping assist. In addition to the warning function, the system can also automatically correct the deviation when the car "presses the line". Even the most advanced lane keeping assist system can keep the vehicle in the center of the lane in real time. This result is achieved by the system through continuous fine-tuning of the steering wheel.

    Lane departure warning is part of the so-called "automotive safety cycle": adaptive cruise control keeps the car behind you at a distance; lane departure warning/lane keeping assist watches for vehicles to the side; blind spot detection detects if a car is approaching in the adjacent lane; and rear parking sonar and rearview cameras (some cars have cameras all around the car) monitor what's behind the car. To some extent, this is already "autonomous driving."

How LDW works: Windshield camera tracks lane markings

    In the most common lane departure warning system, the camera installed behind the windshield captures the image of the road ahead. The system analyzes the solid and dotted lane markings on the road from the digital image. As a driver, you should always keep the vehicle between the solid and dotted lines. However, if the driver negligently operates the vehicle and makes the vehicle approach or touch the lane line, the system will issue a visual and auditory dual warning and vibrate the steering wheel or seat. However, if the turn signal is turned on, the system will not issue any warning.

    Lane keeping assist will actively correct the direction of the vehicle based on the warning. Some systems will brake the front wheels on one side to return the vehicle to its original state; while other systems achieve the same goal by turning the steering wheel. In both cases, the driver can actively control the steering wheel to offset the system's actions. What? You are worried that the driver's strength is not enough to offset the system's automatic steering force? That's unless the car is remotely controlled by hackers.

    Occasionally, lane departure warning systems include an array of laser or infrared sensors. Even more rarely, a camera is mounted behind the vehicle to detect lane markings behind it, as in the Nissan Altima. It feels like detecting lane markings behind you isn't as fast as detecting lane markings ahead, especially in curves. Automakers say that's not the case, and that lane departure warnings are primarily used in straight lanes.

Multipurpose cameras do more than detect lane markings

    The camera mounted behind the windshield can have a variety of uses, including: lane departure warning, lane keeping assist, forward collision warning, windshield wiper control, adaptive cruise control, sign recognition.

    Among them, the principle of windshield wiper control is that when the camera finds that the collected image of the road conditions ahead is very blurry, the system will think that there is rain on the windshield. If the wiper is already in the on state, its action frequency will increase.

    As for adaptive cruise control, Subaru's Eyesight system uses two stereo cameras one foot apart on either side of the rearview mirror. It's accurate enough to replace radar as the information collector for adaptive cruise control. Although its detection range is not as good as similar radar-based systems, it is almost the same when driving on the highway.

    Only a few high-end models are equipped with the sign recognition function, among which the new Mercedes-Benz S-Class has this function. By adding an optical character recognition algorithm to the camera system, the system can detect speed limit signs, temporary construction speed limits, accident signs or fog warning signs. At present, many navigation systems will mark the speed limit information of the corresponding road sections, and if the car has a communication system, the temporary speed limit information can also be sent to the in-car system.

Sound warning or vibration warning

    The lane departure warning system has a warning light on the dashboard, accompanied by a lane warning image. When the vehicle is about to leave the lane, one side of the lane in the image will flash or change color. The flashing warning light that the driver usually sees in the vehicle's sideview mirror is a warning issued by the blind spot detection system, not the lane departure warning.

    Of course, the lane departure warning system has further warning methods. In Asia, almost all cars equipped with lane departure warnings will issue a warning sound. In Germany, Ford and Lincoln brand cars use steering wheel vibration to issue warnings. General Motors' new cars use unilateral seat vibration to remind drivers which lane they are too close to.

    Among the various warning methods, many people think that sound warning is the least scientific. This is because not only the driver but also the passengers will be affected by the warning, and sometimes they will even be startled by the warning sound. If the sound warning is triggered multiple times, it is equivalent to conveying a message to the passengers that the driver's driving skills are poor. The vibration warning can only be felt by the driver himself.

    At least so far, no car has a setting that turns off the audible warning but leaves the visual warning on the dashboard. Every car with a lane departure warning system has only an "on" or "off" option. In some cars, the departure warning system is turned on by default when the car is started. All the driver has to do is remember the position of the switch button for the system.

Lane departure warning doesn't work in all situations

    One thing we need to remember about active safety systems is that they are not foolproof. Lane departure warnings are less effective in rainy and snowy conditions, and may even be automatically turned off and send a prompt message to the driver if visibility is poor. Obviously, in rainy and snowy weather, snow covers lane markings and the system has no way to distinguish lanes. At the entrance and exit of the highway, the lane markings may suddenly deviate due to the presence of forks, and the system will temporarily lose its effect.

    Blind-spot detection systems can sometimes mistake tunnel walls for cars in adjacent lanes.

    Alternatively, when lane markings on the road fade over time, the system's functionality will be significantly reduced.

    Should your next car have lane departure warning?

    Lane departure warning is second only to blind spot detection in improving safety. However, due to its high cost, it is not realistic to make it a fixed configuration for new cars. Relatively speaking, the US NHTSA is more likely to approve rearview cameras as standard equipment for new cars first.

    Regarding whether the lane departure warning needs to be equipped, if the driver drives on highways most of the time, then the system is very necessary; if the driver only drives on urban roads, there is no need to equip the system.

Can lane departure warning really reduce accident rates?

    The purpose of lane departure warning is to reduce the accident rate, but has it really done so? The Insurance Information for Highway Safety, funded by the U.S. insurance industry, said that lane departure warning can reduce the occurrence of 7,500 traffic accidents each year. However, in 2012, the Highway Loss Data Institute pointed out that lane departure warning systems will slightly increase the rate of traffic accidents. Among them, in 2012, the accident rate of Buick brand cars in the U.S. market increased slightly year-on-year after being equipped with lane departure warning and blind spot monitoring systems.

    The Insurance Institute for Highway Safety (IIHS) states that “97% of recorded accidents have nothing to do with lane departure assist.”

    The reason for this result may be that when drivers think that they have a safety system as a "backbone", they drive more casually. But please remember that no matter how advanced the safety system is, it can only avoid accidents as much as possible, but cannot achieve a zero accident rate.