2008 Audi A4 / S4 — Owner's Manual
braking guard is active at speeds above approximately 30 km/h and operates within the system limitations ⇒ page 140 even when the adaptive cruise control is deactivated. The radar sensor is fitted at the front of the vehicle ⇒ page 130, fig. 137. The system requires reflecting surfaces in order to be able to measure the distance. If the ACC is unable to measure the distance it will not respond.
Once the system has measured the distance, it can assess a dangerous situation where a vehicle in front brakes suddenly or you are driving fast and approach a vehicle traveling at a much slower speed. The system warns you as soon as it anticipates that a potential collision with the vehicle in front can only be avoided by hard braking or by taking evasive action. The system has a preset response time so that it can warn you in time. In certain situations, the system response time will be reduced dynamically, e.g. when you are accelerating onto a motorway.
Braking, lateral acceleration, and steering angle are other factors which can shorten the system response time. The warning will be deactivated if you press the accelerator pedal right down in order to overtake.