Rear-Impact Collisions

A rear impact collision is when one vehicle hits another from behind.These types of crashes can range from "fender-benders" all the way to both vehicles being completely totaled. 

• Whiplash injuries are common in rear impacts. While the mechanisms by which the injuries are caused are not fully understood, it is known that seat and head restraint design can influence the risk of injury.  
• Conservation of momentum, elasticity, coefficient of restitution, magnification of acceleration, effect of crush distances, braking effect, and G force as they relate to the potential for injury.

                                              

To further define conservation of momentum, physicists classify a collision as either an elastic collision or an inelastic (plastic) collision. An elastic collision occurs when objects collide without lasting deformation or the generation of heat. An inelastic collision takes place when objects colliding become deformed. In an absolute sense, an elastic collision is not possible in the everyday world, because of the force of gravity and resulting friction. However, in the first milliseconds directly after a rear-end collision, the net momentum of colliding vehicles is essentially the same. 

As in the government's rollover test, the IIHS uses both static measurements and a dynamic evaluation to rate how well a car's seat and head restraints support a dummy in a simulated rear-end crash. 

• To begin, researchers place a dummy the size of an average adult male in the seat with the seat back adjusted at about 25 degrees — a typical seat back angle. They look to see if the head restraint is 3.5 inches or less from the top of the head and less than 4 inches from the back of the head.
•  If the head restraints are adjustable, then measurements in the down position as well as in the most favorable position are taken. As in other IIHS tests, a rating ranging from "Good" to "Poor" is given.
• Restraints that receive a "Good" or "Acceptable" score move on to the dynamic test. In this test, a dummy the size of an average adult male is placed in the vehicle's seat, which is then placed on a sled that simulates a stationary vehicle being struck from the rear at 20 mph by a vehicle of similar weight.
•  Instruments measure the impact on the head, neck, spine and torso, and ratings are assigned based on the results. The static geometric measurements and the dynamic test results are combined for an overall rating. 

Rear impact collusion depend on vehicle crush distance, coefficient of restitution, magnification of acceleration, braking effect on acceleration, and effect of G force. The injury to the neck may result. The inverse relationship between vehicle damage and occupant injury imposes a challenging dilemma for manufacturers of vehicles and government regulatory agencies. Decreasing the costs of body shop expenses for an accident damaged vehicle, while at the same time increasing the probability that the occupants of the vehicle will suffer some injury, is neither cost-effective nor sound public policy.