The second impact is often the deadliest
By Senior Master Sgt. Joe Winfield, Air Force Space Command Safety superintendent
/ Published April 15, 2014
PETERSON AFB, Colo. --
Did you know there are three collisions that occur in every crash? While a vehicle crash in its simplest form may be over within seconds, other objects within the vehicle are still in motion. Let's examine this all-too-common scenario: Vehicle hits object, vehicle either comes to a sudden stop (yikes!) or continues until the forward velocity is absorbed. What about people or the objects riding in the vehicle? They are still traveling at the same velocity as the vehicle.
Since we are not bolted to the vehicle, we naturally react to the forces of the crash. The instant the vehicle stops, we keep moving until something reacts with us and in this case, it's anything inside of the vehicle like windows, those oh-so-soft and supple dash components, and possibly each other. But wait, there's more.
We are squishy and we can't change the fact that the forces of a crash affect our internal organs too. As an occupant impacts something inside the cab and slows down, the internal organs are still moving until, you guessed it, acted on by some other object. In this case, your heart, lungs and liver slamming against the inside wall of your rib cage or your sensitive brain impacting your skull.
The impact of the crash is, of course, speed dependent. Let's say a vehicle impacts a wall at 50 mph. An unbelted occupant, and everything inside the car not secured, will continue to travel at 50 mph until something stops it, or redirects it into a different path. For an occupant weighing 160 lbs traveling at a speed of 50 mph, the force generated at the point of impact will be 8,000 pounds; that's four tons in your face! Not too many bones in the body can withstand that kind of punishment. Can you imagine the damage to internal organs and soft tissues from a blow of four tons against a dashboard? Unlike a vehicle that can be repaired or replaced, the human body isn't so easily repaired, and it's still impossible to replace most body parts. Once the accident occurs, nothing can prevent the second collision. It's your body reacting to the law of physics. All we can do is try to reduce the severity of the second collision and fortunately, there are two devices inside your vehicle that are designed for just that purpose; it's your seatbelt and airbags.
During an impact the seatbelt spreads that four tons of force across your hips and torso, the largest parts of your body, and does this over a longer period of time, thus minimizing injury. It does this through the use of pretensioners, which take out the slack during a crash, and load limiters, which allow the seatbelt to give a little if your body hits it with enough force. If your vehicle was built after 2000, you have both of these safety devices. Something that's often overlooked is the fact that the seatbelt keeps you behind the wheel where you can continue to drive and possibly take further evasive action and prevent you from being ejected from the vehicle--the single-most common cause of fatalities during an accident.
Airbags are passive devices that act in concert with seatbelts to lessen the impact to your body. Statistically, fewer injuries and fatalities occur in crashes of vehicles equipped with airbags, (overall, airbags reduce driver deaths by 14 percent and passenger deaths by 11 percent) but the deployment of an airbag alone during a high-speed collision may not be enough to save the life of an unbelted occupant. Airbags slow down the impact forces; not stop them entirely like a seatbelt. Steering wheel or dash mounted airbags only stay fully inflated for a fraction of a second. As soon as you face-plant the airbag, it deflates to slow down (not stop) the velocity of your head rather than having it bounce off the steering wheel or dash. Remember, airbags are designed to complement seatbelts, not replace them and seatbelts only work if you wear them. Without a seatbelt holding you in place, you are more likely to slide under or dive over an airbag in a frontal collision--both with severe consequences.
So, the choice of not wearing a seat belt really isn't a choice at all. In an accident, the forces of physics are so powerful and so determined to wreak havoc on your body, why gamble with such lousy cards? Without a belt, people can look forward to skull fractures, facial lacerations, broken teeth, broken ribs and worst, death. Remember, if the first impact doesn't kill you, the second one just might.