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On North American roads, vehicles commonly travel at 100+ km/h.

It is not the speed itself which can cause bodily injury, rather it is the rapid interruption of that speed (i.e. collisions) which can cause injury or even death. In a motor vehicle collision, injury severity is related to both the magnitude of that speed interruption (change-in-speed) and the rate at which the change-in-speed occurs.

The magnitude of the change-in-speed in a collision is related to the travelling speeds of the vehicles involved. The greater the speed, the more potential damage that can occur. As an example, if we all could drive around at 300 km/h, every collision would involve a huge risk of death or terrible injury. That is one reason for speed limits -- to give us a fighting chance of surviving a collision.

The rate of the change-in-speed in a collisions directly related tot he forces applied to the objects involved. This can be illustrated by comparing two people falling off a roof -- one landing on the pavement, the other onto a foam mattress. The compression of the foam mattress reduces the impact forces on the body, by lengthening the time required to come to a stop. As a result, the reduced impact forces produce less severe injuries.

Similarly, seat belts are designed to lower the rate of the change-in-speed of an occupant in a frontal collision. The following example illustrates this point.

In a frontal collision, a vast amount of the impact energy is absorbed by the crush of the engine compartment of the vehicle. The occupant compartment uses the engine compartment as its own "foam mattress." Following the first instant of impact, the occupant compartment continues to travel forward and slows to a stop, as the engine compartment becomes crushed. All of this happens within a fraction of a second.

A universal law of physics is that objects remain either in their rest position or travelling at a constant speed unless acted upon by an external force. For an unrestrained occupant, there is nothing securing the occupant to the vehicle itself. Therefore, as the occupant compartment begins to slow down, the occupant will continue travelling along at the original speed of the vehicle. The occupant will slide forward toward the dashboard and windshield. By the time the occupant's body reaches the dashboard and windshield, those vehicle components have significantly slowed down or stopped altogether. As a result, this unrestrained occupant will strike the dashboard and windshield with basically the full original speed of the vehicle. (Result: High magnitude of change-in-speed, High rate of change-in-speed = Severe impact injuries).

The properly belted occupant, is "fastened" to the occupant compartment of the vehicle. As the occupant compartment slows down, the seat belt slows the occupant's body at the same time. Therefore, the occupant is using the engine compartment as a "foam mattress". (Result: High magnitude of change-in-speed, LOWER rate of change-in-speed = LESS severe injuries from seat belt contact surfaces).

Unrestrained occupants impact the dashboard and windshield. Restrained occupants receive the forces at the contact surfaces of the seat belt, plus a forward whiplash of the neck. Unrestrained occupants in frontal collisions typically will receive severe impact injuries to any or all of the head, face, neck, shoulders, arms, kneecaps, and hip joints. Restrained occupants in frontal collisions typically will receive injuries to the neck, shoulders, collar bone, and ribs.

In conclusion, seat belts work! They lengthen the time over which a person's body is slowed down, which reduces the forces applied to the body, and hence, reduces the severity of injuries.

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