Lane departure warning and prevention systems use cameras to track the vehicle's position within the lane, alerting the driver if the vehicle is in danger of inadvertently straying across lane markings. Some systems use haptic warnings, such as steering wheel or seat vibration, while others use audible and/or visual warnings. Some systems cause the vehicle to actively resist moving out of the lane or help direct the vehicle back into the lane through light braking or minor steering adjustments.

More information at http://www.iihs.org/research/qanda/crash_avoidance.aspx