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Chapter 2 - The Dynamics of a Car Crash

Increasing the speed of your vehicle will make it more difficult for you to drive safely. Doing so reduces your ability to safely drive around curves or roadway obstacles. It also increases the distance you need to stop and the distance you travel while reacting to a hazard on the road. But what happens when you get in a collision?

Speed and Force of Impact

According to the National Safety Council, your chance of dying in a crash doubles for every 10 mph that you travel above 50 mph.18 This is because of the increase in kinetic energy as your vehicle gains speed. Kinetic energy is the energy that an object, such as your vehicle, has when it moves. As you increase your speed, the amount of kinetic energy also increases - exponentially. For example, when you double your speed the amount of kinetic energy quadruples. Thus, if you were traveling at 30 mph and then decided to accelerate to 60 mph, the amount of energy in a crash would be four times greater!

Speed and Force of Impact

This energy can be safely dissipated when you stop normally, as the brakes absorb most of these forces, known at this point as gravity forces or g-forces. But in a crash, these g-forces are concentrated on the entire vehicle, which can lead to tragic results for the occupants.19

An increase in speed or drastic change in direction increases the g-forces on you and the vehicle, such as during a hard turn or from hard braking after traveling at a high rate of speed. You experience 1 g, or your body weight, during normal everyday activities such as sitting down, walking or driving in one direction at a constant speed. If you have ever been on a roller coaster, you probably remember feeling heavy during the ride. The typical roller coaster exerts 2 to 3 g's, with many rides reaching at least 4 g's. At 4 g's, you feel four times heavier.20

To understand the power of these forces, consider the limits that the federal government places on the amount of energy that occupants may be subject to in more than 3 milliseconds in a crash at 30 mph: 60 g's. If that doesn't seem to be achievable given the numbers above, consider that a vehicle that stops in 4 feet from a speed of 60 mph exerts a force of 30 g's.21

Let's imagine that there is a young male weighing 150 pounds sitting in that vehicle. Multiply his weight by the amount of g's (30), and you would get 4,500 pounds. This means that he would be pushed forward with 4,500 pounds of force! If that man is not properly secured by a seat belt, he could slam into the dashboard, if not through the windshield.

Speed and Force of Impact

Newton's first law of motion, or the law of inertia, says that an object will remains at rest or in motion in the same state unless acted upon by some outside force.22 If you've ever driven with an open can of soda, you probably know that it is likely to spill whenever you begin to move or stop unless it is covered. The soda moves because of outside forces. It is still at rest when you start moving, and it is still in motion when you are stopping. If you don't cover your soda, the law of inertia will create a mess!

The law of inertia explains why the young man in the example above would continue moving forward if unsecured even though the vehicle has stopped. In a crash, occupants who neglected to wear their seatbelts, as well as any unsecured objects, will be free to move ahead with considerable force into anything unfortunate enough to be in their path.

The impact of these forces can be seen in the number of fatalities that were the result of speeding on the road. Excessive speed was involved in 21.1% of the fatal crashes in Michigan in 2014 and was a factor in 25% of all fatal crashes nationwide in 2014.23,24 The National Highway Traffic Safety Administration (NHTSA) estimates that speeding costs the United States $40.4 billion dollars annually.25

The Concept of the Second (and Third) Collision

A motor vehicle crash consists of more than just the vehicle crashing into another object. It also involves two other collisions. As a result, when we look at a motor vehicle crash, we are actually looking at three total collisions.

The First Collision: the Vehicular Collision

The first collision occurs when the vehicle collides with another object. In a severe crash with another vehicle or other large object, the car comes to an abrupt stop.

The Second Collision: the Human Collision

At the moment of impact, occupants are still traveling at the vehicle's original speed. Anyone not secured will crash into a part of the vehicle's interior, other objects in the car, or other occupants.

The Internal Collision

The Third Collision: the Internal Collision

After a person's body stops, internal organs still move forward. During this stage, these organs may collide into other internal organs or the skeletal system.

Both the second and third collisions can cause serious injury or death. Therefore your best protection against these additional collisions is to fasten your seat belt. When properly fastened, a seat belt distributes the g-forces over the larger, stronger parts of the body that can better absorb these forces. They stretch a bit to allow your body to slow down to reduce the g-forces, thus protecting you.26

A crash can take place in a matter of seconds. The Georgia Paramedics Against Drunk Driving determined the following would happen to a driver if a car traveling at 55 mph were to crash into a fixed object, in the first second of impact:

First tenth of a second: The front bumper and grill collapse.

Second tenth of a second: The hood scrunches up, rising and striking the windshield. The rear wheels, still spinning, lift from the ground. The fenders begin wrapping themselves around the object. The car's frame stops, but the driver and the rest of the car is still going 55 miles an hour. Instinct causes the driver to stiffen his legs against the crash, and they snap at the knee joint.

The Internal Collision

Third tenth of a second: The steering wheel starts to disintegrate, with the steering column pointing straight at the driver's chest.

Fourth tenth of a second: The front two feet of the vehicle is wrecked, while the rear end still moves at 35 miles per hour. The driver's body is still traveling at 55 miles per hour.

Fifth tenth of a second: The steering column punctures the driver's chest, and blood rushes into his lungs.

Sixth tenth of a second: The driver's shoes, despite being tightly laced, are ripped off his feet. The brake pedal breaks off. The car frame buckles in the middle. The driver's head smashes into the windshield. The rear wheels, still spinning, fall back to earth.

Seventh tenth of a second: Hinges rip loose, doors fly open and the seats break free, striking the driver from behind. At this time, the driver is already dead.27

The above assumes these two things: the vehicle has no air bags (or they failed to activate), and the driver did not buckle up. If that driver had been secured with his seat belt, he would not be hurtling toward the steering column at dangerously high speeds and thus he would remain safe. It is no accident that seat belts are also called "safety" belts!

Vehicle Design

The design of a vehicle is important in ensuring the safety of its occupants. You may not know it, but there are more safety features in a vehicle than just seat belts and air bags. Several vehicle components such as brakes, door locks, windshields, and even the roof must all be designed to meet Federal Motor Vehicle Safety Standards (FMVSS). According to a recent NHTSA report, these features mandated by the FMVSS save more than 20,000 lives on American roads annually.28

Door components, including the locks, are designed to keep occupants inside the vehicle so that they do not get thrown out in a crash. A collision not normally considered severe can jar a door open if it is not locked. If you happened to unfasten your seat belt just before that crash, you would be thrown out of the vehicle onto the pavement and possibly into traffic. In a rollover, you would be crushed. A locked door keeps you safe, not only from people out to hurt you but also in the event of a crash.

People riding in heavier vehicles, such as SUVs, minivans and light trucks, are in greater danger from rollovers for two reasons. First, since SUVs, minivans and light trucks in general have a higher center of gravity, they are more likely to tip over than passenger vehicles. Second, most passenger vehicles have roofs meeting NHTSA roof strength standards, but SUVs, minivans and light trucks weighing in excess of 6,000 pounds are exempt from these standards. As a result, roofs of many heavier vehicles are designed to support only the weight of regular passenger cars. Due to the higher center of gravity and often poorly reinforced roofs of heavier vehicles29, many people have died from rollovers in SUVs, minivans and light trucks.

Vehicle Design

Windshields keep you protected inside your vehicle. Seat belts keep you from being thrown around inside the vehicle, as well as from being ejected. Frontal air bags keep you from hitting the dashboard, and side air bags keep you protected in side collisions. Head restraints keep your head from snapping back. These are just a few of the many design features in your vehicle that help to protect you in a crash.

Perhaps the most important design feature contributing to safety in crashes, other than the seat belt, is the crumple zone. First introduced by BMW in the 1950s, it is normally found in the front and rear and is designed to absorb energy from a crash. The crumple zone works by collapsing from impact, which increases the vehicle's stopping distance, thus cushioning the passenger compartment and absorbing crash energy. The result is that occupants are subjected to substantially less force, increasing their chance of survival. The drawback is that the vehicle often becomes a total loss, but that is better than the loss of a life.30

Crash Incompatibility

The crumple zone offers protection against similarly sized vehicles. But we also share the road with large trucks and buses, and the protection offered by the crumple zone is minimal in a high-speed collision with one of these large vehicles. These large vehicles generate greater forces at the same speed because their size and weight are much greater than those of a car. Thus, if you were to collide with a big rig at 35 mph, you will suffer a great deal. The same can be said about SUVs, though to a lesser extent.

Due to their growing popularity, more and more SUVs are on the road. In 2003, SUVs accounted for 20% of all registered passenger vehicles between one and four years old in the United States, compared to 7% in 1993. That's a substantial increase in only 10 years! In 2003, cars accounted for only 54% of all passenger vehicles, down from 68% in 1993.31

Crash Incompatibility

In a regular passenger car, the chance of death from a frontal crash with an SUV is slightly higher than if the crash was with another car. That can be attributed to the greater size and weight of the SUV as compared to the car. However, in a side collision with an SUV, the chance of dying is about double that of dying in a frontal crash with an SUV. This is because the SUV is designed for off-road use: thus it is higher off the ground and has a stiffer front end.32

This is not to say that you should only keep your eyes out for larger vehicles. You are still better off being hit by a larger vehicle traveling at low speeds than, say, a speeding exotic sports car.

Direction of Impact

There are several types of collisions, but damage to a vehicle (as well as to occupants) often is determined by the direction of impact. The following explains the different types of collisions:

  • Rear-Enders - The most common type of crash is the rear-ender, where one vehicle strikes another from behind. They are most frequently the result of dangerous driving behavior, such as speeding, tailgating, braking too late, or inattention, but can also result from driver negligence and poor brake maintenance. This type of collision usually results in less damage than other types of collisions because the vehicle being hit also moves forward in the crash, cushioning the blow somewhat. Whiplash injuries may result from rear-enders.
  • Head-On Collisions - A head-on crash occurs when two vehicles crash face to face. These are typically the most deadly types of collisions involving two vehicles, particularly if occupants are not wearing seat belts. The vehicles involved stop instantly, and unsecured occupants are thrown forward into the dashboard or windshield, and sometimes ejected from the vehicle.
  • Side/Angle Collisions - At intersections, the most severe type of collision occurs when one vehicle crashes head-on into the side of another vehicle at a right angle. This often results when drivers run red lights or make turns at an intersection without checking for cross traffic. Because the sides of a vehicle are less reinforced than the front or rear, a side collision can be more dangerous to occupants than a head-on collision at the same speed.
  • Direction of Impact
  • Sideswipe Collisions - A sideswipe often occurs when a driver fails to signal, drives too close to the center line, changes lanes without looking, or fails to pay attention while attempting to pass or change lanes. It can occur between two vehicles traveling in either the same or opposite directions. These are normally not serious, although the vehicles will still be damaged.
  • Rollovers - A rollover is one of the most violent types of collisions, as well as one of the most deadly. It occurs when a vehicle overturns. A rollover can be caused by turning too sharply while traveling at high speeds or by hitting objects such as curbs with the wheels. This type of collision has gained extra attention in recent years because SUVs tend to roll over due to their high center of gravity, but it can occur with any type of vehicle. Rollovers are mainly single-vehicle collisions, but can involve other vehicles as well. In a rollover, occupants who do not have their seat belts fastened can suffer head injuries that are often fatal. Poorly reinforced roofs can kill even secured occupants.33

As stated above, turning your vehicle too sharply or hitting a low-lying object to the side may cause your vehicle to overturn. The best way to avoid a rollover is to control your speed and avoid sudden, sharp turns of the steering wheel.

Although there are different types of crashes, speed and vehicle size are major factors influencing the severity of a collision. Your best protection from a crash is being alert and paying attention to the road. Always wear your seat belt to increase your chances of survival.

  • 18National Safety Council Fact Sheets: Speeding, (April 5, 2005) Retrieved from www.nsc.org/nsm/speeding.htm
  • 19Virginia Commonwealth University Traffic Safety Training Center. An Introduction to Kinetic Energy, G Force and Speed Change. Retrieved from www.vcu.edu/cppweb/tstc/kinetic.html
  • 20McCarthy, J. Roller coaster forces get stronger, faster, Special Section: Rollercoasters. Florida Today. Retrieved from www.floridatoday.com/apps/pbcs.dll/article?AID=/99999999/NEWS01/511270302/-1/news06
  • 21Virginia Commonwealth University Traffic Safety Training Center.An Introduction to Kinetic Energy, G Force and Speed Change. Retrieved from www.vcu.edu/cppweb/tstc/kinetic.html
  • 22Newton's Laws of Motion. Engineering Wiki. Retrieved from engineering.wikicities.com/wiki/Newton's_laws_of_motion
  • 23Michigan Department of State Police. (2014). "2014 Michigan Traffic Crash Facts - Statewide" Retrieved from http://publications.michigantrafficcrashfacts.org/2014/MTCFVol1.pdf
  • 24National Highway Traffic Safety Administration (NHTSA). (2016). 2014 Traffic Safety Facts. DOT HS 811 392. Retrieved from http://www-nrd.nhtsa.dot.gov/Pubs/812263.pdf
  • 25National Highway Traffic Safety Administration (NHTSA). (2016). 2014 Traffic Safety Facts. DOT HS 811 392. Retrieved from http://www-nrd.nhtsa.dot.gov/Pubs/812263.pdf
  • 26NHTSA. (1992). National Safety Belt Coalition - Why Seat Belts? Excerpts from "Sudden Impact." Retrieved from www.nsc.org/traf/sbc/sbcwhy.htm
  • 27"The first second of a crash." Retrieved from www.sayno.com/crash.html
  • 28Kahane, C. (December 2004). Cost per life saved by the federal motor vehicles safety standards. DOT HS 809 835
  • 29The Detroit News. September 6, 2005. Revised roof strength rules make sense if rightly applied.
  • 30Wikipedia. Crumple Zone. Retrieved from en.wikipedia.org/wiki/Crumple_zone
  • 31Insurance Institute for Highway Safety Status Report. Vol. 40, No. 5. (April 28, 2005). Vehicle Incompatibility in Crashes. Retrieved from www.iihs.org/sr/pdfs/sr4005.pdf
  • 32Insurance Institute for Highway Safety Status Report. Vol. 40, No. 5. (April 28, 2005). Vehicle Incompatibility in Crashes. Retrieved from www.iihs.org/sr/pdfs/sr4005.pdf
  • 33New Jersey Department of Transportation. Crash Type Definitions. Retrieved from www.state.nj.us/transportation/refdata/accident/policecrashtypedef.shtm

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