Why Injuries Occur in Minor Vehicle Damage Collisions
by John J. Smith P.E., MSBMT, MSEE ( http://www.rpsa.biz/ )
Abstract
The reasons why injuries can occur in collisions with little or no damage have been examined. Analysis of the underlying math and physics reveals that reliance on vehicle damage alone to determine the severity of a collision is invalid. Discounting injuries based on lack of vehicle damage is not supported by either the laws of physics or the empirical data available. Additionally, the use of a threshold for injuries has been refuted by the principles of biomechanics and the empirical data.

Introduction
The reconstruction of many traffic collisions is significantly complicated by the absence of hard data. It is not uncommon for the accident either to be not investigated, or investigated only superficially. This is especially true in collisions where the property damage is minor. Compounding the problem is that many of these collisions appear to be noninjury crashes until a few hours after the police have left 1. Often, the only information available is the damage to the vehicles, the statements of the parties involved and the medical data. A thorough analysis of both the speeds involved and the injury potential of the collision requires the inclusion of all three of these items. However, there is not a quantitative link between the speeds involved and the injuries received. As such, speed determination and injury potential can be discussed separately.

PART1: Speed Determination in Minor Vehicle Damage Collisions
Speed determination of vehicles can be both qualitative and quantitative. Qualitative is usually subjective, and involves personal observations of witnesses or participants. Examples of this include statements about relative speed, "I was traveling at 20 m.p.h. and the other car was moving at the same speed", or statements about absolute speed; "The car was going about 55 m.p.h."

Quantitative determination of the speeds in traffic collisions involves the determination of a precise number or range of numbers and can be accomplished in two ways. The first is direct measurement. This includes the use of items such as RADAR guns and lasers. However, it is rare that direct measurement of speed exists in actual collisions.

The second quantitative method in speed determination is the application of the laws of physics to the physical evidence at the scene. This can include damage to the vehicles, preimpact evidence and post impact movement of the vehicles.

The quantitative method has significant limitations in minor vehicle damage collisions. Often there is minimal physical data on preimpact action or post-impact movements. For this reason, it is often necessary to combine quantitative physical evidence and the qualitative observations of the participants regarding actions and movement.

The most common qualitative attempt made is to look at the damage to the vehicle and attempt to divine an impact speed. From this, changes in velocity (delta-V) can be inferred 2. Determining an impact speed from damage in collisions where there is little or no damage has severe limitations. If a collision results in no visible damage, and the impact is bumper-tobumper, it is probable that the impact speed was over 10 m.p.h. This is based on hundreds of crash tests that have been run throughout the United States, Canada, Europe and Japan i, ii, iii, iv, v, vi, vii, viii. As an example, Figure 1 shows the rear bumper of a Ford Festiva that was just hit from behind at approximately 11 m.p.h. This vehicle had no damage despite the fact it had been involved in a frontal impact at approximately 7 m.p.h. previously that day.

Figure 1a - Ford Festiva After 11 m.p.h. impact Figure 1b - Ford Festiva After 11 m.p.h. impact
Figure 1a - Ford Festiva After 11 m.p.h. impact. Figure 1b - Ford Festiva After 11 m.p.h. impact


The qualitative method is also limited when the reconstructionist has only pictures of a vehicle, often taken in such a way that damage is not visible. Figure 2 shows a Dodge Intrepid that was just impacted at over 20 m.p.h.

Even with repair estimates it is difficult to ascertain anything less than a minimum impact speed. If damage is visible, a reasonable minimum is a 10 m.p.h. impact speed. If damage extends beyond the bumper, 15 m.p.h. is a reasonable minimum impact speed. It is rare to have an impact speed as high as 20 m.p.h. with no damage. However, the absence of visible damage in a poor photograph is not the same as there being no damage.

Figure 2 - Intrepid after impact in excess of 20 m.p.h. Figure 2 - Intrepid after impact in excess of 20 m.p.h.
Figure 2 - Intrepid after impact in excess of 20 m.p.h.


Another source of information that can be used, and is often overlooked, is frame or unibody damage. This is often the first damage that is visible in a rear impact and is captured in the repair estimates only a fraction of the time. It can be inferred through physical inspection of the vehicle and proven with inexpensive laser measurements. It is usually not visible in photographs taken by insurance companies. Figure 3 shows a vehicle with significant unibody distortion that required physical inspection to identify it. Figure 4 is a printout showing the unibody damage. This vehicle had distortion of 17 millimeters that was not visible in the photographs.

The common error in determining speed based on damage where there is little or no damage is to make an invalid comparison to a non vehicle-to-vehicle crash tests. The most widespread of these is comparison to tests run by the Insurance Institute for Highway Safety (IIHS). Almost as widespread, is the comparison to bumper standards. Finally, comparison to other non-IIHS crash tests is common.

Figure 3a - Vehicle with unibody damage Figure 3b - Vehicle with unibody damage Figure 4 - Printout of unibody damage
Figure 3a - Vehicle with unibody damage. Figure 3b - Vehicle with unibody damage. Figure 4 - Printout of unibody damage.


IIHS Crash Tests
Figure 5 - Honda which struck Intrepid in Figure 2
Figure 5 - Honda which struck Intrepid in Figure 2.
Every year the IIHS, an insurance industry funded group, runs numerous crash tests of vehicles into barriers at low speed. The IIHS then publishes the results as an indication of how much damage a vehicle will incur in a low speed collision. Usually these tests are run at 5 m.p.h. into walls or poles. Many times a reconstructionist will attempt to compare the damage in a vehicle-to-vehicle collision with the damage in an IIHS test. Based upon the principle of conservation of energy, this will grossly underestimate the actual collision speed (see the discussion of barrier impacts below). As an example, the Dodge Intrepid, (See figure 2) has a bumper rating of "marginal" from the IIHS, just above the lowest rating of poor.

Most midsize small Honda vehicles receive a bumper rating of "acceptable". Figure 5 shows the Honda Prelude that struck the rear of the Intrepid in Figure 2. The impact was initially bumper-to-bumper. While the IIHS did not provide a specific rating on the Honda Prelude bumper, based on barrier test, the damage should be reversed.

IIHS barrier test data does not reflect the performance of actual bumpers in vehicle-tovehicle collisions. The data only provides information on how bumpers will perform when running into walls.

It is worth noting that the IIHS implies that the reason they run barrier tests is to ascertain the performance of bumpers in vehicle-to-vehicle collision. Their web site ix has the following statement;
Bumpers should protect car bodies from damage in low-speed collisions, the kind that frequently occur in congested urban traffic. But many don't. The Institute assesses performance using the costs of repairing vehicle damage in a series of four crash tests at 5 mph -- front- and rear-into-flat-barrier, front-into-anglebarrier, and rear-into-pole.
IIHS does not test bumpers under conditions that match their implied goal. Their data does not establish the actual performance of bumpers in vehicle-to-vehicle collisions. The data collected by the IIHS has no relevance in determining the impact speed in a vehicle-to-vehicle collision, although it is commonly used in litigation in support of the insurance companies that fund the IIHS 3.

Bumper Standards
A misconception often propagated is that if a vehicle has a 2.5 m.p.h. bumper and the bumper is not damaged, the impact speed was under 2.5 m.p.h. This is a misapplication of the bumper standards x. The standard establishes the conditions under which bumpers cannot be damaged. It does not establish the standard under which a bumper must be damaged. Empirical evidence reveals that in vehicle to vehicle collisions xi, the bumpers will not be damaged until bumper standard speed has been exceeded by a factor of 2 to 8 times.

It is worth noting that the design of modern bumpers often prevents the direct observation of bumper damage without physical removal of the bumper.

Non vehicle-to-vehicle impacts
The most common of these are barrier impacts of the type described in the section on IIHS. A vehicle 4 is run into a barrier of some kind and the damage is compared to the actual collision. However, the physics of the collision does not support this. In a barrier impact, virtually all of the energy goes into damaging the vehicle since it cannot go anywhere else. In vehicle-to-vehicle collisions, most of the energy goes into moving the vehicle. Therefore, a car that is damaged in a 5 m.p.h. barrier test, may not be damaged in a 15 m.p.h. vehicle to vehicle collision xii.

TEST
TYPE
CLOSING
SPEED
IN M.P.H.
AVERAGE
CRUSH IN
MM
%
AVERAGE
CRUSH
%
KINETIC
ENERGY
VTB 34.8 637 100 100
VTV 60.1 571 90 298
VTV 55.6 567 89 255
VTV 54.9 570 89 249
Table 1 Damage comparison between VTB and VTV collisions.


Table 1 is a comparison of four Honda Accords from tests run by the National Highway Traffic Safety Administration (NHTSA). The first vehicle impacted a barrier. The other three vehicles were involved in vehicle-to-vehicle (VTV) collisions at significantly higher speeds. Even when the car-to-car collision had three times the energy of the vehicle to barrier collision (VTB), the damage was less. The percentage listed is relative to a barrier impact.

Computer Programs
Solving for the speed of a vehicle in an automobile accident often involves solving up to six simultaneous equations with over a dozen variables. For this reason, even when fewer equations and variables are involved, computers have been employed to help solve the equations.

Among the earliest attempts to solve this problem was the Simulation Model of Automobile Collisions, (SMAC) developed by Calspan under DOT contract. In this program, the initial conditions, such as speed, type of vehicle and directions were entered into the computer and the system would determine where the vehicles should end up. The investigator could then look at the results and alter the initial conditions until the solution matched the final results of the actual collision being investigated. The problem with this approach was the large amount of computer time required to run a single simulation 5. For this reason, a preprocessor was written in an attempt to get a starting point that was "close".

The preprocessor was the Calspan Reconstruction of Accident Speeds on the Highway (CRASH) xiii. This program used several approaches in an attempt to approximate a collision. One method used the post impact movement of the vehicles, another used the damage to the vehicles while a third used both. This program was thought to be accurate to within 10% in collisions with a change in velocity of under 15 m.p.h. However, full scale testing revealed that in this range, the program isn't reliably accurate to within 100% xiv and can actually return impossible answers.

Despite these shortcoming in the program, and the fact it was designed to be a first step in a reconstruction, the equations developed were copied and propagated throughout the accident reconstructioncommunity. Over time, the equations took many forms and became increasingly accepted as scientifically valid. Even when full-scale tests showed the equations to be invalid they weren't abandoned, they were modified and adjusted. This occurred in spite of the fact that the basic underlying assumptions were not valid for anything more than a first order approximation 6.

Table 2 - RICSAC Results
Table 2 - RICSAC Results (Click to enlarge.)


In an attempt to validate the approach, there was a full-scale study done in 1978 called RICSAC xv. The study consisted of 12 staged collisions of various speeds and crash geometry. These crashes were used to determine the validity of the CRASH program and its successors. An attempt was made to define the error of the estimate independent of impact speed. The problem with this approach is that the impact speed is in error if it deviates from the measured data from the RICSAC tests. Table 2 is a graphical representation of the RICSAC results 7. The table shows that for all cases, the calculated values are different from the measured values. In one case, the program overstated the actual velocity of the vehicle (measured at 31.5 MPH) by over 20 MPH and in another instance shows the vehicle going backwards at 6 MPH when in reality the vehicle was not moving. Another case showed the calculated vehicle speed (measured at 31.5 MPH again) was understated by 11 MPH.

Numerous other tests have been run in an attempt to validate the model and all have shown unacceptable errors. In fact, the User's manual from CRASH3 actually warns that the results may have no significance for any single collision. The problem this generates is that there is no mechanism for knowing when the program has given a correct answer.

Numerous other programs exist, some based on the CRASH3 model and some based on attempting to determine the energy involved in damaging the vehicle. The latter usually involves the determination of very low energy values for the damage. The problem with this is that thousands of ft-lbs of energy can be absorbed by the vehicles with no visible damage. This is complicated by the fact that energy absorption in collisions with no visible damage does not follow an identified pattern.

Other Errors
Other errors that are often seen but will not be extensively discussed include the use of bumper isolators and photographs to determine speeds and ignoring foam core bumper damage. Isolators are often used to assert a maximum impact speed. However, they only have some applicability in determining minimum impact speeds.

Photographs:
Photographs will also only provide reasonable minimum speeds. Figure 6 shows a typical low quality photograph often used to determine impact speed. The difficulty with this approach is that it is often hard to actually observe the damage.

Figure 7 shows the same vehicle but with a better picture. An analysis predicated on the first picture would tend to underestimate the actual speed.

Figure 6 - Damaged Vehicle Figure 7 Damaged Vehicle
Figure 6 - Damaged Vehicle. Figure 7 Damaged Vehicle.


This is informative because frequently the reconstructionist is only provided poor photographs from the insurance company. In "A Guide for Risk Managers and Claims Personnel 8", the following statement appears.
"The most common mistake for an accident investigator is to just take a couple of overall shots of the vehicle, at some unknown and oblique angle."
However, this is the most common type of photograph provided by insurance companies.

Foam core bumpers:
Foam core bumpers often show no visible damage even when the steel beam in the center is damage. In order to see this damage, it is common that the bumper must be removed. Figures 8 shows a foam core bumper with no visible damage. Figure 9 shows the damaged bumper of the vehicle in figure 8.

Figure 8 - Foam Core Bumper with No Visible Damage Figure 9 - Damaged Bumper of vehicle in Figure 8
Figure 8 - Foam Core Bumper with No Visible Damage. Figure 9 - Damaged Bumper of vehicle in Figure 8.


Part 2: Injury Potential in Minor Vehicle Damage Collisions

Location of applied forces
Magnitude of applied forces
Daily Activities
Injury Databases versus Injury Thresholds
Injury Study Data
Aggravating Factors
John Smith is the president of Raymond P. Smith and Associates. He has published and lectured extensively in the area of accident reconstruction and biomechanics. Mr. Smith was the supervising engineer for the low speed crash test depicted in the video "Four Speeds". His office is located in Parker Colorado. He may be reached by email at JJSakaJTM@aol.com. The Raymond P. Smith and Associates web site can be visited at http://www.rpsa.biz/.

  1. The delay of symptom onset has been well documented in other articles. The Association for the Advancement of Automotive Medicine has reported that over 50% of injuries are not identified for 6 hours. Full scale testing has also revealed this delay.

  2. Speeds can be calculated using the law of conservation of momentum and the coefficient of restitution.

  3. Due to the principle of conservation of energy, strengthening bumpers actually results in increased injury potential in lower speed collisions, opposite of IIHS's implied goals.

  4. Occasionally vehicles are modified from manufacture's specifications before the tests are run, altering the damage pattern.

  5. The program was written before the days of desktop computers when everything ran on a main frame.

  6. The derivation of the equations in the program required the use of numerous simplifying assumptions. However, when running the program, these assumptions are violated, invalidating the equations and any results.

  7. Analysis showed that R squared was .5752. Statistically this means the calculated results from the program are meaningless. CRASH3 was a later version of CRASH.

  8. "A Guide for Risk Managers and Claims Personnel" A Publication of Ruhl & Associates


  1. "Investigation of the Kinematics of Whiplash", Mertz, Patrick, SAE Paper 670919, 1967

  2. Human Occupant Kinematic Response to Low-Speed Rear End Impacts, Szabo, Welcher, SAE 1994

  3. An Investigation into Vehicle and Occupant Response Subjected to Low Speed Rear Impacts, Navin, Romilly, Proceedings of the Multidisciplinary Road Safety Conference VI, June 1989

  4. Human Subject Kinematics and Electromyographic Activity During Low Speed Rear Impacts, Szabo, Welcher, SAE 1996

  5. Engineering Report on Impact Tests, Smith, July 1997

  6. Characteristics of Specific Automobile Bumpers in Low Velocity Impacts, SAE 940916

  7. Analysis of Human Test Subjects Kinematic Responses to Low Velocity Rear End Impacts, McConnell, Howard, Guzman, Bomar, Raddin, Benedict, Smith, Hatsell, SAE 1993

  8. Head/Neck Kinematic Response of Human Subjects in Low-Speed Rear-End Collisions, Siegmund, et al, SAE 1997

  9. http://www.hwysafety.org/vehicle_ratings/low_speed_smcars.htm

  10. Federal Motor Vehicle Safety Standard 215

  11. ibid

  12. "Damage Only, It Doesn't Work; Minor Vehicle Damage Doesn't Mean No Injury", John J. Smith, Alan J. Mencin

  13. "User's Manual for the CRASH Computer Program", McHenry, R.R; February 1976, NTIS #PB 252115

  14. "Further Validation of EDCRASH Using the RICSAC Staged Collisions", Day, T.D., and Hargens, R.L., SAE Paper No. 890740.

  15. ibid


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