How does it work? Involuntary lane departure warning (LDW) system

This system, known generically as LDW (Lane Departure Warning System) is able to detect whether the vehicle has crossed a lane-dividing line unintentionally, alerting the driver so that he can correct the path.

This steering assistance system is a new safety feature incorporated by different brands in some of their high and medium range models, allowing it to prevent an important number of accidents related to situations of fatigue, sleepiness or distraction while driving.

 How does it work?

Three different technologies are currently used to detect lane lines.

Infrared sensors: they are mounted at the bottom of the front bumper and among their advantages are simplicity and low system cost. Infrared light is projected onto the road and is reflected by the lane lines; this reflection allows the system to know when the vehicle is inside a lane.

Infrared sensors located at the bottom of the front bumper

Computer vision: A dynamic camera that continuously records and detects road markings is installed behind the windshield. These images are analysed and allow knowing in advance the correct path to be followed by the vehicle. If the path travelled differs from the correct path, the system can react before the lane is left.

Laser scanner: This is another computer vision system implemented with laser technology. Its main drawback is its high cost. 

Any change of lane (starting from a certain speed) that is not accompanied by appropriate signalling by means of the direction indicators shall be considered as involuntary lane departure and the system will inform the driver either by lighting a warning light on the dashboard or by a warning sound or a vibration of the driver's seat.

Torque applied to the steering wheel to notify the driver and correct the path

An evolution of this system allows the driver to indicate (by means of a slight torque applied to the steering wheel) in which direction he should turn the wheels to correct the involuntary deviation suffered by the vehicle. This evolution is applicable to vehicles with electric power steering. In the case of Citroën, this evolution is known as LKAS (Lane Keeping Assistance System)

The system will not be operating under any of the following conditions:

• If the vehicle is travelling at a speed of less than 60 -80 km/h (depending on the vehicle)
• When the system cannot detect the lane dividing lines.
• If there are adverse weather conditions, such as strong rain, snow, direct sunlight or shadows.

The system will not give an alert when any of the following conditions are met in the last 3 seconds:

• The correct turn signal is activated.
• When the driver makes a sharp manoeuvre.
• While the driver is pressing the brake pedal.
• If an intentional lane change is made.

1. Infrared light

2. Control unit

3. Vibrators in the driver's seat

1. Infrared light

2. Control unit

3. Vibrators in the driver's seat

System components:

The main elements of the involuntary lane departure warning are:

Control unit: It is the main electronic element. It processes the information received from the sensors of the system and acts in case alerting the driver is required.

Lane sensors: They are responsible for determining the position of the vehicle relative to the lines painted on the road. As said before, they can be infrared, computer vision or laser scanner.

Elements to warn the driver: Visual, sound or motion devices warn the driver in case of involuntary lane invasion.

Control devices: This includes all those elements that allow the driver to manage the system. Information display, warning lights, activation/deactivation switch.