Commentary

Motor vehicle accidents at various speeds, and in particular, low-speed motor vehicle accidents, represent the basis of many thousands of current accident injury claims and lawsuits nationwide. This writing addresses the basic technical terms, issues, and concepts encountered in the reconstruction of motor vehicle accidents and associated injuries.

The Low-Speed Motor Vehicle Accident and "Delta V"

The low-speed motor vehicle accident and the question of the causation and validity of injuries to the vehicle's occupants have been the subjects of widespread research and debate over the last several years. In the fields of mathematical accident reconstruction, Biomechanics, and Automotive Medicine literature, the "low-speed" motor vehicle accident has historically varied in meaning from accidents with pre-collision speeds of the vehicles being as high as 30 mph to those with pre-collision speeds as low as 5 mph.

However, it is now common to define "low-speed" accidents in terms of the levels of velocity changes that the individual vehicles experience during a mutual collision. The term commonly used by accident reconstructionists that refers to a near-instantaneous change in velocity of a vehicle along the Primary Direction of Force ( PDOF) of the collision -- from the first Point of Initial Contact (POIC) to the Point of Maximum Vehicle Engagement (POME) -- is "Delta V."

Accident reconstructionists now generally refer to a collision as "low speed" if the vehicles have respective values of Delta V's in the area of 5 to 8 mph.

During the period of mutual engagement of the vehicles in a collision, the vehicles experience distinct velocity changes. For a brief moment, these colliding vehicles become ONE single, unified mass, moving jointly together in a one direction that may be different from their original pre-collision directions. For most collisions, this mutual vehicle engagement characteristically actually lasts for only about one-tenth of a second. Then, after the maximum, mutual engagement, the colliding vehicles actively separate in new directions. This physical phenomenon is governed by the magnitude and direction of the respective pre-collision momentums (Masses x Velocity) and physical properties of the individual vehicles.

It is during this very brief period of mutual engagement time that each vehicle's individual change in velocity -- its Delta V -- is most significant. Biomechanics and impact kinematics (i.e., collision-motion) researchers have observed that the levels of injuries to occupants in motor vehicle collisions do appear to correlate highly with the levels of Delta V. That is, higher values of a vehicle's Delta V in a collision usually result in more serious injuries to the occupants.

We do know though that many motor vehicle collisions with inherent Delta V's substantially higher than 5 mph occur daily in which occupants walk away without any short- or long-term injuries. As an extreme example, well-protected race car drivers "hit the wall" at speeds greater than 200 mph. In reality, the Delta V vector component in the direction of the wall in this collision is actually only 50 to 60 mph -- not 200 mph. However, this 50 to 60 mph is still a massive instantaneous speed change for a vehicle to experience in one-tenth of a second. Yet, race-car drivers frequently walk away from these collisions uninjured.

On the other hand, traffic accident and medical records have demonstrated that occupants in vehicles in rear-end, front-end, oblique, and other types of modest collisions with vehicle Delta V's components along the PDOF less than 5 mph have, in fact, been seriously injured -- with occupants complaining of a variety of objective and subjective symptoms.

Essential Truths about Motor Vehicle Collisions

There are often diverging positions in the sworn statements and viewpoints of motor vehicle collision litigants and witnesses regarding the facts of an accident. On the basis of these statements alone, it is often difficult on the surface to decide what the actual facts of a collision are, and where the liability actually lies.

To assist the search for the physical truth about the accident, there are several irrevocable and fundamental principles that are common to all motor vehicle collisions -- and yield conclusions that are indisputable. Three of the six truths are provided by a single individual who did not witness the collision or any of its evidence -- having, in fact, been dead since 1727.

These Six Truths include:

Newton's First Law - an object in motion in a given direction will remain in motion in the same direction until acted upon by an outside, unbalanced force.
Newton's Second Law - the acceleration of an object is directly portional to the force applied and inversely proportional to its mass.
Newton's Third Law - for each applied force in which there is no motion, there is an equal and opposite force applied.
The Conservation of Linear Momentum - in a collision of objects, linear momentum will be conserved (i.e., not lost). That is, the total of the linear momentums of the objects before the collision will be equal to the total of the linear momentums after the collision.
The Conservation of Angular and Rotational Momentum - in a collision of objects, rotational momentum will be conserved (i.e., not lost). That is, the total of the rotational momentums of the objects before the collision will be equal to the total of the rotational momentums after the collision.
The Conservation of Energy - in a collision of objects, the total of all forms of energies will be conserved (i.e., not lost).

Utilizing these six physical laws, the motor vehicle collision's physical evidence and data can be analytically reconstructed, objectively evaluated, and its voice heard.

Using Data and Results from Biomechanical Research

It is currently commonplace in litigation settings to have a Biomechanical Engineer or Injury Reconstructionist involved in the evaluation of motor vehicle collision cases in which there are questions regarding the source of occupant injuries. However, we should be aware that the large volumes of Biomechanical data and experimental results found in current research publications -- often cited with vigor in litigation -- are most likely of only limited validity for a particular case in litigation.

In fact, the nature of specific Biomechanical research results -- with the inherent experimental and methodological problems and profound lack of face validity -- often eliminates any possibility that the research results can be applied to specific real-world collisions in litigation. Issues that are particularly significant include the following:

Biomechanical research and staged collision results are often based on collisions under laboratory conditions with extensive instrumentation attached. In these staged collisions, the human volunteer subjects have complete awareness of the impending experimental collisions. The researches also have the volunteer's permission to use them as live crash test dummies. The motivations of these volunteers varies. In some cases, the volunteers have offered their services in order to demonstrate how they can withstand a collision's effects. Clearly, no individual will deliberately volunteer for participation in an experiment in order to be injured. In the real world, there is often limited or no awareness of the impending collision on a relaxed occupant, with surprise and panic resulting. The body motions of a braced individual are unlike that of a relaxed individual.

Biomechanical research and staged collision results are often based on unrealistic and unlike collision circumstances. To avoid the possibility of injuring volunteer subjects in staged collision experiments, Biomechanics researchers must greatly limit the accident speeds and associated forces applied to human subjects.

Biomechanical research and staged collision results are often based on the use of cadavers. These are not realistic human subjects.

Biomechanical research and staged collision results are often based on the use of crash test dummies. These are not human subjects, and have limited face validity.

Biomechanical research and staged collision results are often based on the use of live farm animals. These are not even human subjects, and have very limited face validity.

Biomechanical research and staged collision results are often based on using denuded human tissues taken from cadavers for strength and tolerance tests. These are not realistic human tissue structures, and are not realistic tissue environments.

Biomechanical research and staged collision results are often based on inappropriate and unlike human subjects' body sizes, ages, genders, nationalities, physical features, pre-collision positions, etc. in the experiments.

Biomechanical research and staged collision results are virtually always based on inappropriate or unlike vehicles with different physical properties selected for the experiments.

Biomechanical research and staged collision results are often based on inappropriate or unlike exterior and environmental factors in the staged experimental collisions.

Therefore, it is difficult -- if not altogether incorrect -- to extrapolate the data and results from Biomechanical research literature conducted in very restricted staged collisions to highly specific, complex, real-world accidents with unlike subjects and unlike circumstances.

The Concept of the "5 mph Delta V"

Nonetheless, the accident investigation and accident reconstruction communities seem to have embraced an idea that motor vehicle collisions with values of Delta V less than 5 mph (some studies report as high as 8 mph) are characteristic of accidents in which no injuries can

occur. As a prime example of the type of Biomechanical experiment that has lead to this position, one oft-quoted experimenter used a grand total of TWO young, healthy, athletic males as test subjects in amusement park bumping-car collisions (1). In addition to the obvious experimental problems in the selection of these subjects, these two subjects had full awareness of the impending collision impacts. The conclusion offered by the experimenters was that the 5 mph value of Delta V often seen in motor vehicle collisions is approximately the same as the Delta V occurring in the collisions of bumping cars at amusement parks -- and clearly no injuries can occur there. The inference is that the levels of vehicle acceleration and/or deceleration in these collisions at less than 5 mph and the associated forces (per Newton's Second Law, F=ma) are just not sufficient to cause injuries to a vehicle's occupants.

As a result of these and other similar studies, the meaning of the "5 mph Delta V" has been extended to become an almost rigid Mason-Dixon line between "injury" and "no-injury" motor vehicle accidents. Biomechanics Experts on both sides of cases typically furnish charts and graphs at trial depicting experimentally-generated curves with "Injury" on one side of the curve and "No-Injury" on the other. They will then indicate on which side of these curves the subject accident falls. Unfortunately, the fact is that it is very likely that the subjects and circumstances that formed the basis of these particular curves have little or no resemblance to the accident subjects and circumstances in the litigation.

A great deal of the current motor vehicle litigation concerns injury to the spine and its supporting soft tissue. For individuals greater than 30 years of age, it is very common to have one or more pre-existing degenerative spinal diseases and still be completely asymptomatic with no previous complaints of pain. In fact, research has demonstrated that 50% of all individuals over the age of 40 years have one or more disk degenerations, and 20% of asymptomatic individuals over the age of 40 years have had actual disk herniations.

Alvin Hyde, M.D.(3) , in his basic and lucid primer on the subject of motor vehicle accidental injury, has indicated that about one-third of the asymptomatic adults that he has examined have been found to have abnormal lumbar spines. He has diagnosed herniated disks, facet degenerations, and spinal stenosis to be the most common. He also observed that one-half of the asymptomatic group of individuals examined had abnormal CAT scans. The obvious question then arises, with particular relevance to litigation: "After age 40, what then is considered to be a "normal" spine?"

Regarding disk herniation, Biomechanical research involving the forceful loading of vertebral bones of cadavers (with the interconnecting vertebral disks present and with its other supporting tissues stripped away) has demonstrated that the vertebral bones will actually fail structurally before the disks fail. Consequently, these Biomechanical researchers have concluded that in a forceful vehicular collision, the collision tends to be the culminating and precipitating event in the herniating of an occupant's spinal disk -- not its originating event. That is, the herniation of a spinal disk is the climax of a long-term degenerative process for a motor vehicle collision occupant. In this culminating event, the annulus fibrosus (i.e., the disk's outer fibrous windings) decomposes, frays, or breaks further allowing the nucleus pulposus (i.e., the disk's liquid-center) to bulge or protrude and pressurize the surrounding tissue.

It has been observed by medical practitioners that selected spinal disks of an occupant in rear-end, front-end, and oblique collisions can decompose at an increased rates after a collision and whiplash. As a consequence, occupants can almost inexplicably begin to complain of high and low-back pain even weeks after the collision. The validity of this particular claimant-phenomenon is often questioned. At the very least, it can be stated with certainty that a motor vehicle occupant with a pre-existing symptomatic or asymptomatic spinal disease is more vulnerable in a motor vehicle collision. Given this statement, the concept of the "eggshell plaintiff" becomes particularly relevant in the assessment of claimant's damages.

Crush damage and the Potential for Occupant Injury

Crushing metal and plastic components of motor vehicles in a collision requires that force, momentum, and energy be applied from one motor vehicle to another motor vehicle -- and vice versa. Notably, the forces in the vehicles in a collision are equal and opposite to each other along the PDOF (per Newton's third Law). Although the forces must be the equal and opposite, the vehicle with the weaker components will demonstrate more physical damage. The occupants in both motor vehicles in the collision will be subjected to similar levels of force.

As the velocity of a motor vehicle increases, its level of momentum (Mass x Velocity) and kinetic energy ( x Mass x Velocity squared) also increase accordingly. The vehicle's kinetic energy is a measure of its ability to do work (i.e., force times distance), or in the case of a collision, to generate physical crush damage to itself and to other physical matter. As stated above, a vehicle's crush potential increases by the SQUARE of the velocity of a vehicle. Therefore, as a vehicle's speed increases, its ability to crush increases dramatically.

Motor vehicles traveling at higher speeds have a higher potential for causing human injury due to the increased ability to accelerate (and decelerate) human occupants' tissues past their acceptable elastic tissue ranges. However, complicating this obvious relationship between vehicle speed, crush, and potential for occupant injury are the particular mechanical properties of the materials of the motor vehicle enclosing the occupants. The stiffer and harder the motor vehicle's components in a collision are, the more resistant these vehicles' components are to crush deformation in a collision. The crush energy being applied to the vehicle by another vehicle in a collision may then instead be transferred and applied to the vehicle's occupants. This can result in serious injuries to the vehicle's occupants rather than in crush to the metal or plastic parts.

Therefore, in a vehicle with very hard and stiff components, much of the applied kinetic energy received in a collision from another vehicle may be transferred and applied to the occupants of the vehicle. In this type of vehicle, it is possible for relatively serious injuries to occur with comparatively little structural damage to the vehicle.

The energy-absorbing bumper systems including gas-filled isolators (i.e., the horizontal shock absorbers inside the bumpers) and a rigid metal plate bumper covered by plastic/ vinyl materials on many of the newer passenger vehicles are designed to prevent the momentum and energy from being transmitted to the occupants, and to the rest of the metal and plastic components of the vehicle. This absorption is performed by slowing and spreading the transfer of energy through this energy management system. Unfortunately, there is tremendous variability in the abilities of bumpers on current passenger cars to absorb 5 mph of vehicular collision energy without noticeable damage. The amount of crush damage to a vehicle is, therefore, only a rough indicator of pre-collision speeds in a collision.

No Daytime Speed Limit in Montana

As an interesting side note, on December 9, 1995, the State of Montana removed its daytime specified speed limit on certain roadways. This change allowed each driver to individually judge the meaning of the term "reasonable and prudent speed" for daytime highway conditions. There was an immediate surge in the traffic accident frequency rate. An article subsequently appeared in the Whitefish (MT) Pilot on September 19, 1996. It cited the Montana Highway Patrol's curious explanation for this abrupt rise in the traffic accident frequency rate in the Flathead Valley. Quoted below are excerpts from that article:

"Speed is not a factor in accidents, MHP says Although traffic accidents in Flathead County increased have increased more than 100 percent during the first three months of this year, Montana Highway Patrol officials say that weather and driver error rather than the elimination of Montana's speed law may be the cause. ... (and that) ... 90 to 95 percent are definitely (due to) human error ... Alcohol was the major cause of accidents during the hours of darkness ... while driver inattention was the major cause of accidents during the day."

The article cited the numbers of motor vehicle accidents in the Flathead Valley during a several month period in 1995 and the same period in 1996. Notably, the percentage increases per calendar month were calculated and cited incorrectly. The numbers of motor vehicle accidents between similar months in 1995 and 1996 cited actually indicated increases that actually ranged from 25% to 30% -- not the 100% increase as the article stated.

Notwithstanding, coinciding precisely in time with the onset of the removal of Montana's daytime speed limit, 25% to 30% is a massive increase in traffic accident frequency. The MHP's statement that motor vehicle speed was not a causal factor in the rise of the frequency rates of these collisions -- despite obvious epidemiological evidence to the contrary -- is interesting. If the root cause of this rise in the accident frequency rate is actually reduced semantically to "... human error ..." as the MHP states, then this human error is the driver's decision to drive too fast.

Evaluating the technical aspects of driving, higher allowable motor vehicle speeds inherently have the following features.

Higher vehicle momentums.

Higher vehicle kinetic energies.

Longer internal vehicle braking reaction distances.

Longer vehicle braking and skid time and distances.

Decreased time for human reaction in panic situations.

Higher likelihood of cognitive dissonance resulting from driver panic -- increasing the human reaction time well upward from the normal 1.5 seconds.

Greater variability between the speeds of adjacent and following vehicles.

Greater need for driver-to-driver interactions in decision situations.

Greater injury potential.

Each of the features associate with higher speeds can increase the hazardousness and risk of driving. Most notably, as the velocity of a motor vehicle increases, its level of momentum and kinetic energy also increase. The level of a moving vehicle's kinetic energy determines its ability to do work in the form of physical crush damage to itself and to other physical matter. Higher speeds therefore enable higher levels of crush damage to vehicles.

More importantly, however, a very significant issue that was not treated in the MHP's frequency rate response is the increase in the SEVERITY rates that accompany higher-speed collisions. The vehicle's kinetic energy and its ability to do crush damage is a function of the SQUARE of its velocity. Therefore, as a vehicle's speed is allowed to increase, its ability to inflict crush damage increases drastically. As a result of higher speed collisions, it is an absolute certainty that there has also been a substantial rise in the severity of both property damage and the human injuries.

After substantial pressure applied from various entities including the Federal government, the State of Montana later reinstated posted numerical speed limits.

References:

1. Siegmund, G. P. and Williamson, P. B. Speed Change (Delta V) of Amusement Park Bumper Cars, Proceedings of the Canadian Multi disciplinary Road Safety Conference VIII, June 14-16, 1993, Saskatoon, Saskatchewan.

2. McConnell W.E., Howard, R.P., Guzman, H.M., et al.. "Analysis of Human Test Subject Kinematic Responses to Low Velocity Rear-End Impacts", Society of Automotive Engineers, 930889, International Congress and Exposition, Detroit, MI, March 1-5, 1993.