The “lane-keeping assist system” is a further development of the modern “lane-departure warning system” (LDWS) driver assistance system. Both systems monitor the carriageway markings – usually using camera systems fitted behind the windscreen. This reduces the strain on the car driver, at the same time as providing information about the driving condition of the vehicle. LDWS was a straightforward warning system intended to inform the driver by sensory means (touch, vision, hearing) if the vehicle crossed a carriageway marking, whereas the lane-keeping assist system (LKAS) is able to support the driver in staying within a lane. This means it combines the features of a convenience system with those of a warning system. The system usually assists the driver through electronic assistance with the steering force. The characteristics of this support depend on the distance of the vehicle from the edge of the lane, among other factors. A configuration that emphasizes convenience generates torque forces at either side of the lane center line in order to keep the vehicle in the middle of the lane. A configuration that emphasizes safety, on the other hand, does not intervene until the vehicle is close to the lane markings. In this case, a much stronger steering support torque is applied. This can also function as a tactile warning in the sense of LDWS.
The systems normally have a speed range from 65 to 180 km/h, and are activated by the driver. Following an internal check and location of the lane markings, the system starts providing support. This is indicated in the area of the display instruments. The selected speed range and the minimum possible cornering radius of 230 m means that the system is not particularly suitable for urban driving or driving on country roads. Use of this system is more sensible on motorways and similar fast roads.
The driver remains responsible for controlling the vehicle even after LKAS has been activated, as a result of which the system measures the steering torque applied by the driver. Internal algorithms are used for calculating continuously whether the driver has the steering wheel in his/her hands. If this is not the case, a warning is issued to the driver and the system is automatically deactivated. This ensures that the driver does not surrender his/her responsibility for controlling the vehicle. The system requires about 5 to 15 seconds to perform this check, after which it is capable of reaching a safe decision. LKAS does not just take effect on the vehicle, but also warns the driver.
This means the system does not provide all the steering force, but leaves the driver with responsibility for a small proportion. Furthermore, the steering torque has an upper limit. This means the driver can overrule the system at any time. The system is deactivated if the driver applies the brakes, for example. The LKA system is automatically reactivated at the end of the braking procedure. The same process is activated if the driver announces a lane change by operating the turn indicators.
Many vehicle manufacturers are already offering lane-keeping assist systems today. In conjunction with automatic cruise control, which controls not only the vehicle speed but also its distance from the vehicle in front, this means a high level of automation is already possible today in vehicle control. Although these system do not currently permit automated driving, they do represent the technological basis for it.