For normal accidents involving two passenger cars, the occupants are usually protected by the crash zone designed into the cars. The crash zone chassis will absorbed the impact and reduce the danger to the occupants.
However, if passenger car rear ended a lorry or trailer, the contact point with the car is way above the crash zone resulting in fatal crash to the car occupants as shown in the following video. At 2:09, the impact of the the highest rated car upon hitting the lorry resulted in fatal accident. The contact point on the car occurs at the point way above the crash zone i.e. at the weaker point of the car.
Thus, in developed country, in order to avoid fatal accidents, all lorries or trailers must be installed with under-ride guard-bar at the back of the trailers or lorries. The under-ride guard-bar will be in contact with the chassis/crumple zone of the passenger car in the event of a crash and the crumple zone of the passenger car will absorb the crash energy, Thus, the chassis/crumple zone of the vehicles could perform its duty as designed to protect the occupant from fatal accidents.
In order to avoid future accidents involving passenger car and lorries or trailers, the under-ride guard-bar must be installed on all lorries or trailers.