Low-Speed Crash Does Not Mean LOW ENERGY
High Speed, HIGH ENERGY can = Serious Bodily Injury
Low Speed, HIGH ENERGY can = Serious Bodily Injury
The Injury Arbiter: Energy
Patrick Sundby, Accident Engineer
Mark Studin DC, FASBE(C), DAAMLP, DAAPM
The modern age, say the last 100 years, has done more to advance mankind than at any other time in our history. Great strides have been made in technology, health, and transportation. The advancements in transportation have made even the farthest corners of the earth accessible in only a few hours. As a point of reference: From Los Angeles to Washington D.C., the SR 71 Blackbird made the trip in under 70 (yes, seven – zero) minutes… almost 30 years ago.
While this fast-paced pattern of “make, then break” records has become the new norm, there are some ideologies in each industry which persist in urban myths, dogma or just plain falsehoods and must be tirelessly worked to dispel them. With regards to transportation, one of the most egregious misnomers is the slower you go, the less likely you are to be injured and this falsehood is easy to dispel because physics and inherent math verifies the truth.
Before we explore this myth, let's discuss the physics of injury. Why can we go so fast, (2,200 miles per hour in the case of the Blackbird’s trip) and not be injured yet we fall a few feet and break a bone? The answer is “time”. The time it takes to speed up or slowdown coming to a complete stop is what causes injury (death if severe enough). “It's not the fall, it's the sudden stop at the end.”
Consider the advancements in vehicle safety and design. There was a time when vehicles did not have any safety equipment. These vehicles would crash and stop; under Newton’s laws, the occupant would keep going at the same speed the vehicle was traveling at and collide with the interior of the vehicle. In this collision, there was the sudden stop for the car, but not the occupant; the result… Injury.
The safety advancement: Lap belts. You no longer hit the inside of the vehicle in a crash, the seat belt did its job to keep you from the sudden stop but the lap belt put extreme, sometimes paralyzing strain on the occupant’s body. The lumbar spine is not designed to keep the top half of you in place during a crash and there is the space that normally resides between the occupant and the seatbelt at some point. Again, in a very short amount of time, the occupant is forcibly colliding with the seat belt, even if it is an inch. However, different parts of the spine are not designed to support adjacent regions with these forces and the adjacent spinal regions have to attempt to stabilize forces it was not intended to stabilize.
The safety advancement: Shoulder belts. These were designed to restrain your upper body and not stress your lumbar spine and again you couldn’t hit the inside of the vehicle in a crash. The shoulder belt did its job to keep you from the sudden stop and hitting the interior of the car, but this design has some flaws too. First, it can distribute the forces unevenly across your body. Secondly, now that your upper body is restrained, your cervical spine has to keep your head in place during a crash – a job it's not designed to do. In addition, the sudden stop of your body means your organs can crash into your skeleton and each other.
The safety advancements: Airbags & designed failure of the seatbelt. Most vehicles on the road today have a big bag of air waiting to deploy in the event of a collision and for front-end collisions, the sensors are located in the front, while the side airbags have side sensors, and both will only deploy if those respective sensors are activated through significant force and resultant deformity of the vehicle. This bag is coupled with seatbelts which will stretch at a predetermined point, and the goal of both is for you to not be stopped so suddenly, in other words, to attempt to give you more time to come to a complete stop.
The safety advancement: Crumple Zones. Manufacturers discovered they could make a vehicle fail in a way that was predictable. This expected behavior meant the vehicle could take extra time to crush (usually in milliseconds) and further extend the time it takes an occupant to slow down.
Safety Advancement Conclusion: These advancements in safety do nothing if the nature of the collision renders them useless or the collision is not violent enough to “need” them (low speed). Seatbelts and Airbags do nothing for an occupant of a vehicle which is rear-ended. If a vehicle has a collision which is not severe enough to deploy the airbag and stretch the seatbelt then both these features are useless for safety. If the vehicle cannot crumple or does so minimally, then the occupant doesn't get the benefit of the energy absorbing design. The safety advancements in modern vehicles are designed to increase the time it takes for you to slow down, but if they are not available to you (as is the case in low-speed crashes), then you are no better off than being in a vehicle with no safety features at all.
The next logical question, "How fast can you go and still have the collision be low-speed AND suffer an injury?" is a complex one. There are dozens of variables which dictate the outcomes of a crash, and as such, each item is a subject onto itself, however, to list a few vehicle types, vehicle mechanical condition, speed, the angle of collision, etc. The answer will come from the math of the forces involved, along with the vehicle and occupant “known information;” the weight of the car and the occupant, the speed of the vehicle, the braking distance, etc. and then apply it to a sample to illustrate how to understand Low speed can equal high energy.
We are going to focus on the forces transferred from one vehicle to another. Understanding how a vehicle behaves in a low-speed event and how the forces are transferred is the key to the debunking the myth of “low speed = no injury” because it is not about the speed, it is about the energy the occupant is exposed to as a result of the crash.
Let’s make the vehicle simpler, a cart with a seat…
(Credit: GM Owners Manuals)
For the moment, let us assume the collision doesn't use the crumple zones or any built-in, predictable, deformation to the advantage of the occupant. We will also assume there are two carts and striking cart weighs twice what the victim cart weighs (including any occupants) and the striking vehicle has a post-impact speed of 4 miles per hour.
Some basic calculations result in us knowing the change in speed for the red cart to be 12 miles per hour. Further, if the change happened in .1 seconds the red cart occupant would experience 5.5 gs at the lumbar spine.
To fully understand how that is derived, and the injury potential, we need to calculate the “G-Forces” the occupant’s body is exposed to in this “low-speed” model. It’s all in the math (physics) that is often confusing, but the final numbers bear significant injury potential.
Now, in the real world, the vehicle will always absorb some of the energy, the transfer is never 100% efficient. Let's change the scenario a little bit. What happens if both carts absorb 10% each, totaling 20% of the speed lost to crumple zones?
Again, if the change happened in .1 seconds the red cart occupant would experience just over 5 g’s at the lumbar spine.
The math here is greatly simplified for a demonstration of concept, (energy absorbed) is a complex process detailed in other programs. The above is not a proven method for actual collision reconstruction.
In both examples 10 miles per hour is a very striking low speed in the grand scheme; it’s a common collision speed among parking lot and heavy traffic collisions. The g-forces experienced at the cervical spine are two-three times that of the lumbar spine. In both examples, the cervical spine would experience at least 10 g's to 15 g’s of force, which is well above the injury threshold at either end of the spectrum.
Let’s add some real-world context to this concept. Below are photographs from the NHTSA database. The listed change in velocity for this vehicle is 12 miles per hour.
This vehicle was also rear-ended, the listed change in velocity is 8 miles per hour.
In both cases, the database reports (See the links to the NHTSA below) at least a cervical spine injury, but as you can see, there is little damage to the vehicles. While several factors must be considered to determine if the injury is a result of a car collision, at the same time, the lack of damage or appearance of speed doesn't negate the possibility.
Injury can, and often does, occur in what seems to be the most miniscule collisions. There is no validity to “low speed = no injury” as that is nothing more than an “urban myth” or dogma.
Low Speed Crashes
and Missed Vehicle Damage
Standards to Demand in a Vehicle Inspection
By: Patrick Sundby, Accident Investigator
Specializing in Low Speed and Catastrophic Crashes
Mark Studin DC, FASBE(C), DAAPM, DAAMLP
One of the most common problems with low speed collisions is determining the extent of the damage. The common real world practice is to visually examine the exterior of the vehicle and document any damage. The problem with this is not knowing the extent of the damage behind the exterior panels. Very few cases have had through and complete vehicle examination. The question is why?
The crash Reconstructionist has a tedious job ahead of him when facing a collision with what appears to be minimal damage at first glance. Happer et al (2003) acknowledges different vehicles will have different damage, or appearance of damage, at the same speed due to different designs. The goal of the paper was to provide a sound method for determining the severity of a collision. Happer et al (2003) also states the physical evidence remaining after the impact must be reviewed and this process begins with dividing bumpers into three categories. In this writing we will focus on the second one, reinforcement beams with a polymer absorber. These bumpers are categorized as having a metal reinforcement beam with a polymer absorber behind a plastic or urethane cover; this is the bumper to focus on as the vast majority of the vehicles on the road today are constructed in this manner.
Below is a picture of one such polymer structure. In this instance a vehicle struck a guardrail (during a training event) in a glancing motion and the polymer structure was pushed out of the bumper cover.
In the above photograph the damage is obvious, what we need to focus on are the collisions where damage appears minimal. The purpose of the polymer structure is to crush under a predetermined load to reduce the damage to a vehicle during lower speed collisions. This structure eliminates or reduces the damage to structural parts and thus will reduce the cost of repair as well. The energy it takes to deform the polymer structure also reduces the likelihood of injury to the occupant – IF it deforms. When the polymer structure doesn’t deform what are the consequences? Consider the photograph below:
The vehicle in the photograph was the striking vehicle in a rear end collision. The bulge & paint scraps in the bumper cover, the misalignment of the front fascia to the hood and fender (next to the headlight), and a small crack in the edge of the fascia near the grill are all external signs of damage. In this case, the vehicle was deemed to have “minor” damage but no further structural analysis was completed. How can one be sure the damage is limited to just what we see?
Let’s take a minute and draw a comparison. Dr. Studin has often spoke of strain/sprain. As a quick recap, there are three levels, primary stretches the tissue & fibers, and secondary begins to tear the tissue & fibers, and tertiary is a complete tearing of the tissue and fibers. When a patient has a complaint of pain and there are no outwards signs of trauma, i.e.: no scrapes, bruising, or other wounds some form of medical imaging is ordered. The imaging is ordered to see inside of the patient and determine if there is any internal injury. Trauma to a disc in the cervical spine (neck) is an example of an injury which would not be expected to show up on any external physical exam but should be easily seen in a good medical imaging process and with correct image reading or interpretation.
The concept of examining a patient completely to determine the source of the problem is the same template which should be applied to a vehicle inspection. A complete inspection demands measuring all the structural components against known factory specifications, this process could entail the removal of the bumper cover, grill, headlights, and other parts to ensure accuracy. Further, as time passes secondary systems can elude to undisclosed damage. As an example, if the geometry of the suspension changed due to a collision the alignment could be off resulting in uneven tire wear. It would take some time for the wear patterns in the tires to change.
The concern for this level of detail can be summarized by saying any energy which is absorbed by the vehicle, but not accounted for, will reduce the final calculated speeds. If you want to be sure about the results, you need to be sure about the facts and must have all the internal protective structures analyzed, not just the “skin of the car.” A perfect understructure can indicate significant energy transferences to the occupant, where “crushed” understructures can mean the car absorbed or deflected the energy and protected the occupant.
Not knowing is leaving the final answer to rhetoric vs. the truth, with physics to back it up.
Happer, A., Hughes, M., Peck, M., and Boehme, S., "Practical Analysis Methodology for Low Speed Vehicle Collisions Involving Vehicles with Modern Bumper Systems," SAE Technical Paper 2003-01-0492, 2003, doi:10.4271/2003-01-0492.
Patrick Sundby has decades of experience in the automotive industry including several years in law enforcement collision investigation. He has also been a driver training and firearms instructor in law enforcement and a police officer for 9 years before specializing in accident investigations. He has had the privilege of participating in both learning and teaching at Prince William County Criminal Justice Training Academy in Virginia and studied at the Federal Law Enforcement Training Center in Georgia. His specialty is low speed and catastrophic crashes and has testified over 500 times at various level. He can be reached at 571-265-8076 firstname.lastname@example.org
Dr. Mark Studin is an adjunct associate professor of chiropractic at the University of Bridgeport College of Chiropractic, an Adjunct Professor of Clinical Sciences at Texas Chiropractic College and a clinical presenter for the State of New York at Buffalo, School of Medicine and Biomedical Sciences for postdoctoral education, teaching MRI spine interpretation and triaging trauma cases. He is also the president of the Academy of Chiropractic, teaching doctors how to interface with the legal community (www.DoctorsPIProgram.com). He teaches MRI interpretation and triaging trauma cases to doctors of all disciplines nationally, and studies trends in health care on a national scale (www.TeachDoctors.com). He can be reached atDrMark@AcademyofChiropractic.comor at 631-786-4253.