Vehicle seat belts or harnessed restraints that are initially snug allow immediate restraint of the occupant, which maximizes the time of restraint and minimizes the level of loading required to stop the occupant. Other supplemental protection systems, such as padding or airbags, can absorb impact energy between the occupant and the vehicle interior. If a belt or harness webbing is loose, the occupant will travel farther before restraint can begin, increasing the level of force needed to stop the occupant in a shorter time period. Advanced seat belt designs balance between loading the occupant and controlling contact with vehicle interior components.
To optimally reduce the risk of injury, the remaining loads must be distributed as widely as possible over the body’s strongest components. For adults who face forward, these parts include the shoulders and pelvis. For children, especially infants, distributing the restraint loads over larger and sometimes different body areas is necessary. Multiple straps and rearward-facing orientation help take care of these needs.
The primary goal of any occupant protection system is to keep the central nervous system from being injured. Broken bones will mend and soft tissue will heal, but damage to the brain and spinal cord is typically irreversible. In the design of restraint systems, tradeoffs may be necessary that compromise on protection for the extremities or ribs to ensure protection of the brain and spinal cord.
Proper belt placement and good fit are important for effective seat belt restraint when using either the vehicle seat belt or a child restraint harness. Serious restraint-induced injuries can occur when the belts are misplaced over body areas having no protective bony structure. Such misplacement of a lap belt can also occur during a crash if the belt is loose or, with small children, is not held in place low on the pelvis by a crotch strap or other positioning device, such as booster belt guides. A lap belt that is placed or rides up above the pelvis can intrude into the soft abdomen and rupture or lacerate internal organs (Rouhana 1993, Rutledge et al. 1991). Moreover, in the absence of a shoulder restraint, a lap belt routed above the pelvis will compress the soft tissue and organs of the abdomen and load the spine, possibly causing separation or fracture of the lumbar vertebrae in a severe crash (Johnson and Falci 1990, King 1993). Misusing a lap-and-shoulder belt by placing the shoulder belt behind the back removes torso restraint and allows the same problems seen with lap belts only; placing the shoulder belt under the arm provides minimal torso restraint and can increase direct loading to the abdomen and chest compared with a properly positioned belt (McGrath et al. 2010, Louman-Gardiner et al. 2008).
Despite the potential for belt-induced injuries, belt-based restraint systems have significant advantages over supplementary airbag systems. They offer protection in a variety of crash directions, including rollovers, and throughout the course of multiple impacts. Moreover, the force on the occupant is proportional to the mass of that occupant. For example, an adult weighing 80 kg will experience a much greater restraint load into the belts on their chest and pelvis than a child weighing only 20 kg. Even though the child’s bony structure and connective tissue may be weaker than the adult’s, the child’s mass is so much less that the injury potential from contact with belts or other restraint surfaces is also less. In contrast, first-generation frontal airbags produced the same amount of deployment force and resistance to deflation regardless of occupant size, while more recent advanced airbag systems vary deployment force based on the weight of the occupant.
Child restraint designs vary with the size of the child, the direction the child faces, the type of internal restraining system, and the method of installation. All child restraints, however, work on the principle of coupling the child as tightly as possible to the vehicle because it maximizes the time restraint can be applied and minimizes the highest level of force required to stop the occupant. In North America, the child restraint has been traditionally attached to the vehicle with the existing seatbelts. An option available in the US since 2002 is the LATCH system, which stands for Lower Anchors and Tethers for CHildren. After installing the child restraint to the vehicle, the child is then secured in the child restraint with a separate harness. This results in two links between the vehicle and the occupant. It is therefore critical that the seatbelt or LATCH strap be tight and the harness be snug to allow the child to ride down the crash with the vehicle. The tether component of the LATCH system is used with forward-facing harnessed restraints to add an additional point of securement with either seatbelt or lower anchor as the main attachment.