14-1 TRANSPORTATION ACCIDENTS
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striking vehicle. At slow speeds, the bumpers will absorb some energy. At higher speeds the structure of the two vehicles will crush and absorb much of the energy. The front vehicle will accelerate forward unless restrained in some way.
Within a few hundredths of a second after impact, the vehicle occupants in the
forward vehicle will press into the seat backs. The seat cushioning and structure will absorb
much of the energy related to the acceleration of the occupants. Consider a person sitting
in the front seat during a rear-end collision. Unless the head is well restrained by the head-
rest, it will move rearward over the top of the seat in a phenomenon called whiplash. There
are two components to the head movement: translation away from the body trunk and rota-
tion about the neck. If the seat back remains rigid and quite vertical, the rotational com-
ponent will likely be much greater than if the seat back structure yields. If the seat back
yields, it absorbs some of the energy and reduces the rotational component. The resultant
force on the head from the two components can be quite sizeable even at fairly low-impact
speeds. Depending on the speed at impact, the body may slide backward up the slope of
the seat and impact on the rear structure of the passenger compartment.
Rollover If there is enough lateral impact and movement, a vehicle may roll over. The
tires may slide or contact another object. If sliding friction or restraining forces are large
enough compared with the forces of lateral motion, the vehicle will roll over. The chance
of rollover increases for vehicles with high centers of gravity. The vehicle structure may
crush during the rollover, causing reduction of the occupant space. Because occupants may
be thrown around, use of restraints will lessen the chance for injury. Some vehicles incor-
porate side impact structural elements to minimize reduction of occupant space. An
increasing number of models include side impact air bags to cushion impacts on
occupants.
The Second Crash
When a vehicle crashes, the occupants continue in motion until they impact on the interior of the passenger compartment. The term second crash refers to this crash of occupants against the interior surfaces of the vehicle after vehicle impact. Very often, the second crash is the primary source of passenger injuries, not the vehicle crash itself.
Vehicles designed to absorb energy and to reduce the transfer of forces to the occu-
pants will reduce the second crash. Passenger restraints and air bags reduce the likelihood
of the second impact occurring. Well-anchored seats help minimize the second crash. A
seat that lets loose can add mass behind the passenger in a frontal crash. Interiors have
many design features to reduce the sources of injury. Padded dashboards that distribute
forces, laminated windshields that prevent glass from being imbedded in tissue, and elim-
ination of protruding knobs and controls all reduce the probability of injuries. Energy-
absorbing, collapsible steering columns significantly reduced the rate of injury to drivers
during the second crash.
Crashworthiness
Crashworthiness refers to the ability of a vehicle to withstand an accident without intrusion of or reduction in the integrity of the passenger compartment. When a crash occurs, not only is it important to minimize the second crash, it is also important to ensure that the passenger compartment stays closed and retains it shape. The idea is to keep occupants inside a protected zone during a crash. Passengers thrown from a vehicle face a much greater risk of injury and death than those who stay inside.