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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:
The first production lane departure warning system in Europe was developed by the United States company Iteris 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:
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 facilitates 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.
Lane Keeping Assist is a feature that in addition to Lane Departure Warning System automatically take steps to ensure the vehicle stays in its lane. Some vehicles combine adaptive cruise control with lane keeping systems to provide additional safety.
While the combination of these features creates a semi-autonomous vehicle, most require the driver to remain in control of the vehicle while it is in use. This is because of the limitations associated with the lane-keeping feature.
A lane keeping assist mechanism can either reactively turn a vehicle back into the lane if it starts to leave or proactively keep the vehicle in the center of the lane. Vehicle companies often use the term "Lane Keep(ing) Assist" to refer to both reactive Lane Keep Assist (LKA) and proactive Lane Centering Assist (LCA) but the terms are beginning to be differentiated. 
Requires driver control while vehicle is in use, but adjusts steering if vehicle detects itself drifting out of lane:
Allows unassisted driving under limited conditions
|2015||Mercedes||C-Class, E-Class, S-Class||autonomous steering, lane keeping, adaptive cruise control, parking, and accident avoidance. Semi-autonomous traffic assistant for speeds up to 37 miles per hour.|
|Tesla||Model S||Part of the autopilot system to be released in 2015. This combines automatic lane change (after signal is applied), adaptive cruise control, and sign recognition to regulate speed and location.|
|Volkswagen||Passat||Part of the driver assistance pack plus in the new VW Passat B8. It contains a traffic jam assist which is active up to 37 miles per hour. This system steers, brakes and accelerates. Another part is the emergency assist which takes the complete control over the vehicle when the driver does not react anymore. The vehicle is brought autonomously to a complete stop without any driver intervention.|
Lane Departure Warning Systems and Lane Keeping Systems rely on visible lane markings. They typically cannot decipher faded, missing, or incorrect lane markings. Markings covered in snow or old lane markings left visible can hinder the ability of the system.