Typical city accidents occur at junctions and roundabouts. A driver is waiting behind other cars approaching a roundabout. He is concentrating on the traffic on the roundabout and sees a gap. He expects the car in front to move forward and accelerates, only to find that the driver in front has not moved. The impact that follows is typical of city driving: low speed, but with a high risk of a debilitating whiplash injury to the driver of the struck vehicle. While injury severities are usually low, these accidents are very frequent and represent 26% of all crashes.
Low-speed AEB systems use sensors to monitor the road ahead, typically 6-8m. One common technology is a LIDAR (Light Detection and Ranging) sensor, typically mounted at the top of the windscreen, which determines whether or not there is an object in front of the car which presents a risk. If there is, the AEB system will, typically, pre-charge the brakes so that the car will provide its most efficient braking response, should the driver react. If the driver does not respond, the car will automatically apply the brakes to avoid, or in some cases to mitigate, the accident. If, at any point, the driver intervenes to avoid the accident, by hard braking or avoidance steering, the system will disengage.
For its fitment survey, Euro NCAP defines city systems as those which can avoid an impact by autonomous braking at speeds up to 20km/h where 80% of all whiplash injuries occur. These systems look for the reflectivity of a typical vehicle and so are not sensitive to pedestrians or roadside furniture. Since these systems are within the sweep of the wipers they can also operate in poor weather conditions.