Vehicle Features That Help In Crashes
As mentioned above, seat belts are designed to stop vehicle occupants from continuing at the same speed the vehicle was traveling just before the impact. The seat belt allows an occupant to be connected to the whole vehicle, which allows for the momentum of a person’s body to be slowed down at the same speed as the car.
You may think that being attached to the car in a crash is a bad thing, but this is not the case in most crashes. Seatbelts allow an occupant to take advantage of the vehicle’s energy-absorbing design (discussed below).
The secondary job of a seatbelt is to spread the stopping force of the collision across the sturdier parts of a person’s body in order to minimize injury. Most modern vehicles are equipped with pre-tensioners, which pull inward on the belt in the event of a crash. If the passenger is seated properly in the vehicle, the pre-tensioner forces the person’s body into the best possible crash position.
It should be remembered that, while a seatbelt can restrain a person’s torso effectively (seat belts reduce your chance of dying in an auto accident by at least 45%), it cannot restrain the head. Furthermore, crashes at very high speeds effectively nullify the protections of a seatbelt.
Front and side airbags are designed to help slow and stop vehicle occupants from continuing forward after a crash and being ejected from the vehicle. They also prevent drivers from striking the steering wheel and windshield, and protect other occupants from striking hard surfaces like dashboards and windows.
An airbag can also prevent the occupant’s head from swinging through the full range of motion and damaging the neck.
Both seatbelts and airbags spread out the time that the force of deceleration (from a crash) is applied to a passenger. Deceleration in itself does not cause injury; it is the sudden deceleration over a very short period of time that does. Airbags and other restraining technology minimize injury in all but the highest speed crashes by lengthening the deceleration time.
Crumple zones on cars allow the vehicle to absorb some of the initial impact and decrease the force applied to the vehicle occupants.
The crumple zones are located toward the front and rear of a car and, as their name implies, are designed to collapse easily in the event of a collision. Thus, instead of a car coming to an abrupt stop, along with the passengers within the car, the crumple zone absorbs some of the impact force by flattening.
Crumple zones allow the car to decelerate more slowly and spread the energy of the car to other structural components of the car.
A car’s passenger compartment is built much more sturdily and does not collapse around the occupants. Instead, it redirects energy away from the driver and passengers and reduces injury.
For a stark depiction of how a crumple zone of a modern car works, see this video of a crash test between a modern car and an older car without a crumple zone.
Is Bigger A Vehicle Better Or Safer For An Accident?
Generally speaking, you are more likely to survive a car crash if you’re in a bigger vehicle than the one you’re in a collision with.
If you hit another vehicle traveling at the same opposite angle and speed, the larger, heavier vehicle will have a greater momentum force and, therefore, will cause more damage to the smaller, lighter vehicle. The larger, heavier vehicle may push the smaller, lighter vehicle backward and even crush the smaller vehicle. This is commonly seen in accidents between tractor-trailers and passenger vehicles, where the truck driver walks away unscathed while the passengers in the other vehicle suffer serious injuries or are killed.
According to the National Highway Traffic Safety Administration (NHTSA), an occupant of a passenger vehicle, (such as a sedan), is 3.3 times more likely to be killed in a head-on collision with a “light truck,” (which includes SUV’s, pickups, and vans). So, at least in head-on crashes, the type of vehicle plays a more significant role for occupant safety than does the vehicle’s crash test rating.
However, driving an SUV is not your only option to stay safer on the road. Studies performed by the Insurance Institute for Highway Safety (IIHS) have demonstrated that any additional size and weight of the car you’re driving (versus the one you collide with) can favorably influence the protection of the occupants in the event of a crash.
The IIHS studies also revealed that, in general, occupants in smaller cars are the least protected when there is a crash. The tradeoff for price [JS1] and fuel economy can, in fact, be very significant.
However, the percentage of fatalities in rollover crashes, where there may have been no collision with another car, was highest for SUV’s, at 43%, followed by pickup trucks, (40%), vans, (24%), and passenger cars at 20%.
There are, of course, many factors to consider when selecting a car, such as the number and variety of safety features now offered on modern vehicles. You should also select a car that has performed well in dynamic crash tests, that is, tests that involve more than a single, frontal impact. Rollover testing is an example. For more information on specific vehicles, check the IIHS’s ratings page here.
Can You Survive a 70 mph Crash?
Theoretically, yes, but it very much depends on the type of collision and the safety features of the vehicle.
According to research, the highest speed at which you are likely to survive a head on collision without serious injury is 43 mph, assuming the proper use of safety belts in a well-designed car with crash structures like crumple zones and airbags, (discussed above).
However, the odds of surviving a head-on crash drop exponentially at speeds above the 43-mph mark.
It is essential to note here that the IIHS conducts their frontal crashes, (head on, moderately overlapping and small overlapping), at 40 mph, not at higher speeds. Car manufacturers simply do not design their vehicles to protect occupants at speeds above this, because the kinetic forces involved are just too great.
So even though the car you’re driving, or considering, has a “5-star” or “Good” frontal crash test rating, this only applies if you’re traveling 40 mph at the time of a frontal collision with a car of similar (or less) weight as yours.
The forces in a collision are quadrupled when the speeds are doubled, because kinetic energy increases with the square of velocity, according to Newton’s formula. This means, practically speaking, a crash at 80 mph carries four times the energy as a crash at 40 mph.
So, if your car has a “good” frontal crash score, and you collide with another vehicle of equal or lesser weight head on at 40 mph, your odds of survival are close to 100%.
But a 70-mph crash involves 306% more kinetic energy than a 40-mph crash. In crash studies, when a car is in a collision at 300% of the forces it was designed to handle, the odds of survival drop to just 25%.
Therefore, in a 70-mph head on collision with four occupants in your car, odds are that only one person in the car will survive the crash. Are you willing to take a chance like that?
As we have seen, there are a number of common crash scenarios that can easily be avoided by reducing speed, eliminating distractions and leaving extra space between you and other cars. We’ve also seen that even though we may think we are excellent drivers, we could all use a (safe) dose of humility when it comes to operating our vehicles.
Vehicle collisions are all too common, and we hope that by understanding some of the physics involved in a car crash, you will take precautions such as always using your seatbelt and researching safety ratings when making a vehicle purchase. But learning the physics involved should also instill more driving caution in general, given the size, weight and speed capabilities of the vehicles we use every day.