How does lane departure warning work?
Lane departure warning alerts you that your car is about to veer out of lane and warns you to get back into lane. That’s the basic idea, but there are several versions of the technology available now, including ones that react and steer away from the lane edge and even proactively keep the car centered. All forms of lane departure warning employ a low-cost camera mounted in the windshield near the rear view mirror that continuously watches the striped and solid lane markings of the road ahead. It is part of the circle of safety, the three most common and useful driver assists: protecting you to the front (adaptive cruise control and forward collision warning), side (lane departure warning), and rear side (blind spot detection).
Technology has driven the cost of the three together down to less than $1,000 on many cars. Thinking about buying a car with some form of lane departure warning? Here’s what you need to know.
The three flavors: warning, assist, self-centering
There are now three major forms of lane departure warning:
Lane departure warning. This is the original. It is a warning only. When you let the car drift near, onto, or over the lane marking, the car alerts you. As the driver, you have to take corrective action by steering the car back to the middle of the lane. It doesn’t work if the road has no lane markings. It may not work, or not as well, if your state waits until the lane markings are faded before repainting. Lane marking dots are sometimes harder to track, especially if their coloring has faded. If it’s raining or snowing, the camera may have trouble, too. By design, the lane departure warning system doesn’t alert you if you have your turn signal on, or (some cars) if you apply the brakes.
Lane keep assist. This helps once you let the car drift too far. The car then steers itself away from the lane marking. The driver has to re-center the car in the lane. It’s also called lane keeping system, lane assist, side assist (Audi), lane departure alert with steering assist (Toyota), or lane departure prevention (LDP is sometimes applied also to lane centering assist).
Lane centering assist. This is the best and newest system, as long as you trust technology. It’s a fully proactive system. Lane centering assist always tries to keep the car centered in the current lane. It works as long as the car senses you have your hands at least lightly on the steering wheel, and as long as curves aren’t too sharp. If you have lane centering assist and adaptive cruise control, you have the beginnings of what some people would call self-driving. At the very least, the combination is enough to save you from inattentiveness if you speed along 5-10 seconds looking at a music playlist or (shame, shame) scanning a full-screen text. “Lane centering assist” is not as well-established as the other two terms.
Lane departure warning options
Pay attention to when the lane departure systems kick in and how they alert you. This is make-or-break stuff when it comes to driver satisfaction.
Minimum speed to engage. Lane departure warning kicks in between 30 mph and 40 mph or the rough metric equivalent, 50-65 kph. It isn’t meant meant for low-speed, stop-and-go driving, partly because the camera couldn’t see enough of the lane markings.
Audible warnings. Some cars sound warning beeps when your car crosses a solid or dotted lane marking, then silences itself once it’s away from the lane edge. The other alert form is …
Haptic feedback warnings. Other cars vibrate the steering wheel or seat cushion. The vibration is unlike any vibration the car makes when it’s on, say rough or gravel-covered road, or when the anti-lock brakes engage. Seat cushion feedback can vibrate the left or right side only, to direct your attention to the offending side of the car.
Broadly speaking, Asian cars are more likely to use audible warnings, European cars are more likely to use haptic feedback, and American cars use some of each (but not both on the same car). Audible versus haptic isn’t really an option. You decide if one or the other is so important you’ll abandon one brand for another.
Visual feedback. This is a secondary alert: Virtually every car shows a visual indicator that the car is drifting out of lane. Typically it’s a a pair of striped lane markings, sometimes with a car in the middle, typically yellow or green lines when LDW is active and all is well, but red or flashing, on the offending side of the lane. In most cars, the instrument panel indicator isn’t big enough to get the driver’s attention in a hurry, thus the need for audible or haptic feedback. Some cars are now putting lane departure warning icons in the head up display, where it’s more likely to noticed.
Early versus late warnings. Some cars let you choose whether the alert and/or lane correction happens before the car reaches the lane marking, on the lane marking, or once the car has gone over. The early alert sounds safest, but it also means more alerts suggesting you haven’t been paying attention.
Adjustable audible warning levels. Some cars let you adjust the loudness of the warning. But: Passengers who don’t like audible warnings will be less annoyed with softer beeps, not unannoyed.
Force to overcome LKA/LCA. With lane keep assist and lane centering assist, the car steers back into lane. You can always overcome the automatic steering by turning the wheel harder than the car does. Some people, not many, will believe the force required is too high. Typically this is not an option; it’s just how each car comes.
LDW automatically turned on at each startup? You can disable lane departure warning. Since it’s a safety item that reduces accidents, some automakers (the majority) revert to LDW-on when you restart the car. Some remember when you turn it off.
Road departure mitigation. Some LDW systems recognizes the edge of the highway as different and more dangerous than just drifting into the next lane. They will tug the car back onto the road if it’s possible. Acura pioneered road departure mitigation.
How it works: windshield camera tracks lane markings
The most common LDW system is a camera mounted high up in the windshield, often as part of the rear view mirror mounting block. It captures a moving view of the road ahead as much as 150 feet ahead. The digitized image is parsed for straight or dashed lines — the lane markings. As the driver, you’re supposed to center the car between the two lines. As the car deviates and approaches or reaches the lane marking, the driver gets a warning: a visual alert plus either an audible tone, a vibration in the steering wheel, or a vibration in the seat. If the turn signal is on, the car assumes the driver is intentionally crossing over the lane, and there’s no alert.
Then there’s lane keep assist. When the car reaches the lane marking, the car nudges itself away from the marker, sort of like bouncing off the walls in Pac-Man. Sometimes the steering change is effected by braking the opposite front wheel and the car pivots back into the lane. The car can also move you back by turning the steering wheel. In either case, the driver can easily overcome the car’s intentions by turning the wheel. It doesn’t require superhuman efforts. If you read a story about a car that fought the driver for control of the wheel, it’s either urban legend and untrue. The driver was so startled he or she presumed the car’s steering was more powerful than it really was, or someone pretty clever has developed an amazing hack and we’re in bigger trouble than we thought. But that hasn’t happened yet.
There are variations. For instance, the automaker uses a rear-facing camera to watch the lane markings behind the car, such as Nissan. That seems counterintuitive and possibly slower in adapting to a curved road ahead. Not so, say automakers, and besides, most often lane departure warning is used on fairly straight roads.
Mazda uses an audible alert but rather than beep, the car speakers play a synthesized rumble strip sound. It’s convincingly lifelike.
Multipurpose camera does more than watch lanes
Once there’s a camera in the windshield facing forward, the camera can serve multiple uses:
- Lane departure warning (LDW). Or lane keep(ing) assist or lane centering assist.
- Forward collision warning (FCW). The system tracks an object getting closer and the closing speed. Even with a single (not stereo) camera, the algorithms compare speed against how quickly the outline of the car grows in the field of view. If the car shape ahead grows larger in a hurry, your car decides it’s getting dangerously close. A red warning light and the word Brake! or Brake Now! flash.
- Pedestrian detection, city braking. The camera and processors detect pedestrians in your path at speeds up to about 20 mph and brakes (safely), ditto for cars ahead you failed to brake for. For city braking, some cars may use radar.
- Windshield wiper control. If the image is detected as blurry, the algorithm suspects that may be rain hitting the windshield. If your wipers are set to intermittent, it adjusts the delay for more frequent swipes. (Most cars have separate rain sensors, since intermittent wipers are on most cars, but not as yet lane departure warning.)
Adaptive cruise control. Subaru Eyesight employs stereo cameras set about a foot apart, on either side of the rear view mirror. They’re accurate enough to substitute for radar in an adaptive cruise control system that paces the car in front of you. I found they don’t have the extreme range of radar-based ACC, but if you’re driving legal highway speeds, it’s a non-issue.
- Sign recognition. Hand the video feed to a pattern and optical character recognition algorithm and the car can tell you the posted speed limit, a temporary construction speed limit, or an accident or fog alert from an overhead sign. This capability is only on a handful of cars. Europe has a head start on the US because its common signage made it easier to recognize a traffic sign from a billboard or a message on the back of the truck ahead. Posted speed limited information is on many navigation systems now (not all automakers show it). If the car has telematics, temporary speed limits could also be sent to the car. So, theoretically, you could set cruise control on highways to the current speed plus, say, 7 mph.
- Traffic light recognition. A color camera in the windshield can tell when the light goes green, just in case you’re not watching. This has potential in combination with a telematics system that sends traffic light information including when the light ahead is due to change phase (that is, green to yellow to red, or red to green). If you can’t make the next light before it goes red, the car may urge you to slow down and save gas.
Lane departure warning vs. blind spot detection
Lane departure warning uses a camera that looks ahead to tell if you’re drifting out of lane. If your turn signal is off, it alerts you.
Blind spot detection has sonar or radar sensors that look back and to the side. It alerts you to cars coming up quickly into your blind spot. The warning on your outside mirror, or on the A-pillar, is for a car in your blind spot. An icon showing two cars side-by-side lights up in the mirror. You get a haptic or audible alert only if your turn signal is on, the opposite of what happens with a lane departure warning. Also, the lighted side mirror icon flashes.
Why LDW doesn’t work 100% of the time
No machine vision system is perfect. Lane departure warning works less capably in rain or snow, and it will shut down, sending an alert to the driver, when visibility is limited. Obviously, it doesn’t work when snow covers the road, or if there are no lane markings at all. On highway exits where the markings veer off, the system has to rely on the single lane marking that remains (and also scan the road ahead for where the right-side marking picks up again). There will be occasional false alerts when the car says you’re drifting across a lane and you’re still properly centered. This is markedly better than five years ago, though.
LDW works less well when lane markings are old, or when the lane markings are raised dots, rather than 20-foot painted stripes. A vigorous program to renew road surface markings before they fade would benefit machine vision systems such as LDW, benefit drivers with so-so eyesight, and benefit all drivers in rainy weather. This is one very small reason why America’s infrastructure is no longer world class.
Should your next car have lane departure warning?
In the scheme of optional driver-assistance aids, lane departure warning ranks near the top in importance with blind spot detection in improving safety. As the cost comes down, it’s possible the Department of Transportation might mandate LDW and BSD on cars within five years. If it chose one, it would be blind spot detection. But with the Trump administration looking for fewer, not more, rules, it may not come to pass in the next four years.
Lane departure warning makes sense if you drive a lot of highway miles. Lane keep assist makes more sense to keep you from drifting across lanes, and lane centering assist is better still.
What if LDW doesn’t reduce accidents?
One bedrock assumption about lane departure warning is that LDW provides peace of mind, reduces accidents, and cuts down on fatalities and serious accidents. But is it true? In 2010, the Insurance Information for Highway Safety, a group supported by the insurance industry, said LDW could prevent 7,500 fatal accidents. Then the insurance industry’s Highway Loss Data Institute in 2012 concluded that lane departure warning systems may be linked to slightly increased accident rates. One theory was that driver assists made drivers overly confident and they drove beyond their abilities.
“Part of the disconnect … has to do with the fact that crashes in which vehicles drift off the road aren’t common, even though they account for a large proportion of fatal crashes,” the IIHS said. “Lane departure warning would be irrelevant to about 97 percent of police reported crashes.”
Since then, more research has shown that cars with driver assists generally have fewer accidents. More interesting was a recent IIHS survey of 184 drivers in Virginia and Maryland that showed two-thirds had lane departure warning disabled. The IIHS didn’t say this, but one possibility may be that drivers don’t like loud, repeated warning beeps. It would be helpful to break out LDW use in cars with haptic (silent) versus audible alerts.
Recommendations: By all means, get LDW
Lane departure warning systems all work pretty well. I’ve never driven a car with LDW that didn’t detect road markings and provide some kind of warning, day or night, except in snow or heavy rain. That’s good. Here are my recommendations based on driving more than 100 cars with driver assists including lane departure warning:
- Look for cars with lane departure warning in a bundle with blind spot detection and adaptive cruise control. Example: The Honda Sensing System has all three and adds just $1,000 to the price of the vehicle. Toyota Safety Sense-P makes lane keep assist (“lane departure alert with steering assist”) and a pre-collision system standard on many models.
- If you can get it, go for lane centering assist, which is better than lane keep assist, which is better than lane departure warning.
- You will probably prefer haptic warnings over beeps. The reason is simple: The passengers won’t hear when you get too close to the lane edge, which often leads to snippy comments on your driving skills.
Having lane departure warning and adaptive cruise control (or at least forward collision alert) may save foolish drivers. Someone who tries to write a text while rolling along will probably be saved from his or her foolishness: Let the car drift out of lane, you’re warned or the car automatically returns to its lane. Let the driver ahead hit the brakes and your car brakes automatically, too. This is not how lane departure warning and adaptive cruise are supposed to be used: saving us from ourselves.
For drivers who want to add lane departure warning electronics to their cars, devices exist, both as phone apps and dash-top devices that sometimes combine a traffic camera, navigation, FCW, and LDW. From the devices I’ve used, it’s uncertain how well their accuracy compares with integrated devices that come with the car. Also, they alert you every time you change lanes intentionally, since they have no way of knowing your turn signal is on.
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