214 CHAPTER 14 TRANSPORTATION
of airbags also has helped to reduce injury and death rates at the same time that the number of vehicles and total miles driven continues to increase.
Vehicle size is an important factor in vehicle injuries and deaths. The frequency of injuries among drivers of small cars involved in crashes is more than twice that for large cars. There is also a tendency for small cars to be involved more frequently than large cars in crashes resulting in damage claims.
Event Sequence
The dynamics of vehicle crashes are quite complicated. There are many factors that con-
tribute to the actual crash sequence, depending on vehicle speed, design, and motion.
However, a simplified discussion explains some of the basic phenomena of vehicle crashes
and impacts.
Frontal Impact When a vehicle runs into a fixed object or another vehicle in a frontal
collision, there is a sequence of events that takes place within a fraction of a second. At
very low speeds, the bumper and its mounting will absorb the energy of impact. The
NHTSA Bumper Standard establishes that a passenger vehicle bumper should absorb the
energy from a 2.5mi/hr impact of the vehicle. For a time, the standard was a 5mi/hr impact,
which some manufacturers still meet. At higher speeds, the structure of the vehicle will
absorb the energy of impact by crushing. Today’s vehicles are designed to absorb a great
deal of energy; older vehicles were not. Because of the absorption of energy by the struc-
ture, the rate of deceleration falls quickly with distance from the point of impact. Figure
14-1 illustrates this concept by plotting G load (the ratio of impact deceleration to the pull
of gravity). At some distance from the point of impact, there is little deformation of the
vehicle structure, and from that point on, the deceleration rate is fairly constant over the
rest of the vehicle length.
When a vehicle strikes another object or barrier, it stops quickly. Objects within or occupants continue to move at the original vehicle speed until they impact on something to slow them down. If passengers are restrained to the vehicle structure or its components, they will stop with the vehicle.
Rear-End Collision In a rear-end collision, one vehicle strikes another vehicle in front
of it. The front vehicle is usually at rest or may be moving at some speed less than the