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What is lane departure warning, and how does it work?

BMW 7 Series F01 July 2009 Miramas France

Lane departure warning employs a simple camera that costs a few dollars. It could save you thousands in crash repairs. The camera plus processing software watch how close you are to road surface markings. It alerts you when you’re about to drift across, but only if your turn signal isn’t on. Lane departure warning has emerged as a key tool for driver safety. The technology has evolved over the last few years to lane keep assist where the car automatically corrects course if it reaches the lane markings, and now a higher level of lane keep assist that automatically keeps the car centered on the road. The corrections are subtle and the driver can always override the car and turn the wheel manually.

Lane departure warning is part of the so-called circle of safety: adaptive cruise control pacing you against the car in front, lane departure warning or lane keep assist watching ahead and to the side, blind spot detection watching for cars coming up in adjacent lanes, and rear parking sonar and a camera behind (sometimes on all four sides) watching behind when you’re backing up. Lane departure warning/lane keep assist is so good now, the best systems could keep you centered for miles and miles. It’s really a self-driving car at that point. All of them cut out after a few seconds if they detect no hands on the steering wheel.

LDW roadway rectangles

How it works: windshield camera tracks lane markings

The most common LDW system is a camera mounted high up in the windshield (photo above), often as part of the rear view mirror mounting block. It captures a moving view of the road 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. That’s lane departure warning.

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, or someone pretty clever has developed an amazing hack (hasn’t happened yet) and we’re in bigger trouble than we thought.

Less often, the lane departure warning technology is a set of laser or infrared sensors. Occasionally, the automaker uses a rear-facing camera to watch the lane markings behind the car, as on the Nissan Altima.  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.

Next page: The LDW camera can be used for other things, too


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