Unguarded trailers are a risk to the motoring public and to vulnerable road users such as pedestrians and cyclists. Truck and trailer side guards have been mandated in Europe since 1989. North America however has not required side guards, and there are no original equipment manufacturers who build side-guarded trailers.
Low speed frontal motor vehicle crashes pose little risk to occupants due to energy absorbing structures and active and passive restraint systems. However, a similar low speed crash into the side of an unguarded semitrailer can be fatal to the occupant(s). This is due to the fact that the passenger vehicle’s main structure may pass entirely beneath the trailer body, and the weaker passenger compartment may then contact the bottom edge of the trailer. When the trailer body intrudes into the passenger compartment, head and upper extremity injury is likely.
European Directive 89/297/EEC has been in effect since 1989. This directive requires side guarding on all trailers and specifies minimum performance standards which effectively protect vulnerable road users (pedestrians, cyclists, and motorcyclists) from passing underneath a trailer. Although European guards do not prevent all side underride crashes, they do prevent some of them.
Even relatively weak side guards may be able to reduce or prevent the side of a trailer from intruding into the passenger compartment of a motor vehicle depending upon the circumstances of the crash. For example, a low angle, low speed intrusion of the a-pillar of a passenger vehicle underneath the side of a semitrailer could be prevented by redirecting the heading of the vehicle several degrees. This type of redirection can be accomplished with a simple, lightweight guard. In fact, my work has illustrated that strong, lightweight materials can even prevent underride at relatively high collision speeds.