In road-transport terminology, a lane departure warning system (LDWS) is a mechanism designed to warn the 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 four types of systems:

Another system is the emergency lane keeping (ELK). The emergency lane keeping applies correction to a vehicle which drifts beyond a solid lane marking.

One of the main causes of single vehicle crashes and frontal crashes is lane departure. The goal of the lateral support systems (LSS) is to help to avoid such crashes.

Without those LSS systems, lane departure can be unintentional; the car drifts towards and across the edge of the lane. The car then reach a potentially dangerous situation. This system does not work when the edge of the lane is not marked by a line.

A lane detection system used behind the lane departure warning system uses the principle of Hough transform and Canny edge detector to detect lane lines from realtime camera images fed from the front-end camera of the automobile. A basic flowchart of how a lane detection algorithm works to help lane departure warning is shown in the figures.

Lane warning/keeping systems are based on:

In Europe, the lane departure warning system should be compatible with a visible lane marking identification standard such as commission regulation EU-351/2012.