A key concept to understand in protection from collision injury is the idea of early “deceleration”. Deceleration is placed in quotes here because, technically, there is no such word. There is only acceleration. When we speed up we have positive acceleration and when we slow down we have negative acceleration. However, we continue to use deceleration here because it is commonly understood.
When a motor vehicle collision occurs we are often influenced by the visible damage to the vehicles. We often believe that visible damage equates to collision severity and injury. While that is partly true, what is missing from this belief is that the damage to the vehicle only describes the severity of the collision experienced by the body of the vehicle and that is not the same as the severity of the collision experienced by the vehicle occupant. The body of a vehicle occupant makes contact with the interior of a vehicle shortly after the start of the impact experienced by the vehicle. When an occupant is unrestrained the occupant begins to move, with respect to the vehicle interior, at about 60 milli-seconds after the start of the vehicle impact. Unfortunately, in many serious collisions, that 60 milli-second delay can be huge and could be the difference between life and death. Since a serious collision can be completed in about 100 milli-seconds the vehicle can slow down greatly before the occupant’s body finally reaches the vehicle interior when contact is made. Remember, that an unrestrained occupant does not begin to slow down until contact is made with the vehicle interior. It is the difference in speed between the vehicle interior and the body of the striking occupant which determines the severity of occupant injury. So we can reduce the severity of injury if we can begin the deceleration of the occupant’s body as early as possible.
There are occupant protection systems in modern vehicles that cause early occupant deceleration. One is a seat-belt. But seat-belt webbings cannot have slack in them or their benefit can be reduced. If we want early deceleration we need to make sure that the lap belt is reasonably snug and positioned low on the pelvis and not high onto the abdomen. The torso webbing can have a little more slack just so that proper positioning can occur during ride-down, Modern seat-belt systems are equipped with pre-tensioners that cause the webbing to be pulled about 4 inches on it is tighter against the occupant’s body. However pre-tensioners cannot stop abdominal injuries the lap belt is positioned too high above the pelvic.
Airbags are also devices designed to achieve early deceleration. Airbags explode so that the bag inflation reaches closer to where the occupant’s chest and head are located. But airbags could be dangerous if the occupant is positioned too close to where the bag deploys, so that injury occurs when the occupant is struck by the expanding airbag.
Other devices such as deploying knee bolsters achieve a similar effect to the lower legs and knees. The combination of these various air bags and seat-belt restraints allows for the occupant to begin decelerating much earlier than if they did not exist and this improves occupant protection.
So remember this important concept of early deceleration. It applies to so many other areas of transportation where forces need to be lessened and controlled.