Roadway with lane markings
In road-transport terminology, a lane departure warning system is a mechanism designed to warn a driver when the vehicle begins to move out of its lane (unless a turn signal is on in that direction) on freeways and arterial roads. These systems are designed to minimize accidents by addressing the main causes of collisions: driver error, distractions and drowsiness. In 2009 the U.S. National Highway Traffic Safety Administration (NHTSA) began studying whether to mandate lane departure warning systems and frontal collision warning systems on automobiles.
There are two main types of systems:
- Systems which warn the driver (lane departure warning, LDW) if the vehicle is leaving its lane (visual, audible, and/or vibration warnings)
- Systems which warn the driver and, if no action is taken, automatically take steps to ensure the vehicle stays in its lane (lane keeping system, LKS)
The first production lane departure warning system in Europe was developed by the United States's Iteris company for Mercedes Actros commercial trucks. The system debuted in 2000, and is now available on most trucks sold in Europe.
In 2002, the Iteris system became available on Freightliner Trucks' North American vehicles. In both these systems, the driver is warned of unintentional lane departures by an audible rumble strip sound generated on the side of the vehicle drifting out of the lane. No warnings are generated if, before crossing the lane, an active turn signal is given by the driver.
Lane warning/keeping systems are based on:
- Video sensors in the visual domain (mounted behind the windshield, typically integrated beside the rear mirror)
- Laser sensors (mounted on the front of the vehicle)
- Infrared sensors (mounted either behind the windshield or under the vehicle)
Timeline of available systems
- Nissan Motors began offering a lane-keeping support system on the Cima sold in Japan. In 2004, the first passenger-vehicle system available in North America was jointly developed by Iteris and Valeo for Nissan on the Infiniti FX and (in 2005) the M vehicles. In this system, a camera (mounted in the overhead console above the mirror) monitors the lane markings on a roadway. A warning tone is triggered to alert the driver when the vehicle begins to drift over the markings. In 2007 Infiniti offered a newer version of this feature, which it called the Lane Departure Prevention (LDP) system. This feature utilizes the vehicle stability control system to help assist the driver maintain lane position by applying gentle brake pressure on the appropriate wheels.
- Toyota introduced its Lane Monitoring System on models such as the Cardina and Alphard sold in Japan; this system warns the driver if it appears the vehicle is beginning to drift out of its lane. In 2004, Toyota added a Lane Keeping Assist feature to the Crown Majesta which can apply a small counter-steering force to aid in keeping the vehicle in its lane. In 2006, Lexus introduced a multi-mode Lane Keeping Assist system on the LS 460, which utilizes stereo cameras and more sophisticated object- and pattern-recognition processors. This system can issue an audiovisual warning and also (using the Electric Power Steering or EPS) steer the vehicle to hold its lane. It also applies counter-steering torque to help ensure the driver does not over-correct or "saw" the steering wheel while attempting to return the vehicle to its proper lane. If the radar cruise control system is engaged, the Lane Keep function works to help reduce the driver's steering-input burden by providing steering torque; however, the driver must remain active or the system will deactivate. 
- Honda launched its Lane Keep Assist System (LKAS) on the Inspire. It provides up to 80% of steering torque to keep the car in its lane on the highway. It is also designed to make highway driving less cumbersome, by minimizing the driver's steering input. A camera, mounted at the top of the windshield just above the rear-view mirror, scans the road ahead in a 40-degree radius, picking up the dotted white lines used to divide lane boundaries on the highway. The computer recognizes that the driver is "locked into" a particular lane, monitors how sharp a curve is and uses factors such as yaw and vehicle speed to calculate the steering input required.
- Citroën became the first in Europe to offer LDWS on its 2005 C4 and C5 models, and its C6. This system uses infrared sensors to monitor lane markings on the road surface, and a vibration mechanism in the seat alerts the driver of deviations.
- In 2007, Audi began offering its Audi Lane Assist feature for the first time on the Q7. This system, unlike the Japanese "assist" systems, will not intervene in actual driving; rather, it will vibrate the steering wheel if the vehicle appears to be exiting its lane. The LDW System in Audi is based on a forward-looking video-camera in its visible range, instead of the downward-looking infrared sensors in the Citroën.
- General Motors introduced Lane Departure Warning on its 2008 model-year Cadillac STS, DTS and Buick Lucerne models. The General Motors system warns the driver with an audible tone and a warning indicator on the dashboard. BMW also introduced Lane Departure Warning on the 5 series and 6 series, using a vibrating steering wheel to warn the driver of unintended departures. Volvo introduced the Lane Departure Warning system and the Driver Alert Control on its 2008 model-year S80, the V70 and XC70 executive cars. Volvo's lane departure warning system uses a camera to track road markings and sound an alarm when drivers depart their lane without signaling. The systems used by BMW, Volvo and General Motors are based on core technology from Mobileye.
- Mercedes-Benz began offering a Lane Keeping Assist function on the new E-class. This system warns the driver (with a steering-wheel vibration) if it appears the vehicle is beginning to leave its lane. Another feature will automatically deactivate and reactivate if it ascertains the driver is intentionally leaving his lane (for instance, aggressively cornering). A newer version will use the braking system to assist in maintaining the vehicle's lane.
- Kia Motors offered the 2011 Cadenza premium sedan with an optional Lane Departure Warning System (LDWS) in limited markets. This system uses a flashing dashboard icon and emits an audible warning when a white lane marking is being crossed, and emits a louder audible warning when a yellow-line marking is crossed. This system is canceled when a turn signal is operating, or by pressing a deactivation switch on the dashboard; it works by using an optical sensor on both sides of the car.
Fiat is also launching its Lane Keep Assist feature based on TRW's lane keeping assist system (also known as the Haptic Lane Feedback system). This system integrates the lane-detection camera with TRW's electric power-steering system; when an unintended lane departure is detected (the turn signal is not engaged to indicate the driver's desire to change lanes), the electric power-steering system will introduce a gentle torque that will help guide the driver back toward the center of the lane. Introduced on the Lancia Delta in 2008, this system earned the Italian Automotive Technical Association's Best Automotive Innovation of the Year Award for 2008. Peugeot introduced the same system as Citroën in its new 308.
Lane departure warning systems combine prevention with risk reports in the transportation industry. Viewnyx applies video-based technology to assist fleets in lowering their driving liability costs. By providing safety managers with driver- and fleet-risk assessment reports and tools, it facilitate proactive coaching and training to eliminate high-risk behaviors. The Lookout Solution is used by North American fleets, and there is research on implementing a lane departure warning system via a mobile phone.