Case of the Month: A Chain Reaction Crash

Jul 1, 2014 by

This auto accident was described as a three-vehicles-in-a-row collision. The lead car was a Honda, the middle was a Jeep Grand Cherokee, and the third was a Ford. The accident description provided to us was that the Ford hit the Jeep, which was then pushed into the Honda. We were asked to determine if the vehicles could have been involved in the accident as described, and what were the speeds, forces and injury potentials for this event.

Vehicle Damage Analysis

There was damage to the front of the Ford and to the rear of the Jeep. The front of the Jeep was void of damage, while the rear of the Honda had damage, which required 24.4 hours of sheet metal repair work. The damage levels in this incident were not consistent with the offered scenario.

Our expert examined the repair estimates and the damage photos for all three vehicles. He researched the published weights and measurements for the involved vehicles. He compared the applied heights of damage to each vehicle, and he examined the vehicles for the direction of force that was applied to the impacted surfaces. He researched the National Highway Traffic Safety Administration default stiffness data for the rear surface of the Jeep. He researched the Insurance Institute For Highway Safety 20-mph rear impact head protection data for the Jeep and Honda. The Jeep was untested and the Honda was rated “Good”, the highest rating offered. This test imposes a load equal to a 20-mph impact to test the head protection offered to an occupant. This event saw an impact of about 1/10th of the IIHS tests.

The Ford had a non-rated bumper system and weighed about 3,850 pounds. There was damage to the front face bar and the two mounts. There was no other damage to the vehicle.

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The Jeep had a non-rated bumper system and weighed about 3,790-pounds. The rear bumper was pushed downward at the right rear corner. There was a shallow dent in the sheet metal skin of the rear hatch just above the top surface of the bumper and at the right side.

 

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There was no damage to the frame, the right quarter panel, or to the exhaust system.

The front of the Jeep was undamaged. The vulnerable front license plate was not crushed or even bent. An impact speed of about one to two mph is required to damage a license plate/frame assembly. There was no evidence that this vehicle was involved in a collision to its front surfaces at a speed greater than one or two mph, if at all.

 

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The Honda weighed about 2,525 pounds and had a 2.5-mph rated bumper. This means it must absorb an impact of 2.5-mph without damaging surrounding components or the fuel or exhaust systems, and the hood and trunk lids must operate normally. A load placed on only one side of such a bumper is required to and does absorb only 1.5-mph.

The rear of the Honda was damaged. The vehicle required 24.4-hours of sheet metal repair work in addition to the 17.4-hours of refinish work. The floor pan, the rear panel, and the right quarter were damaged. The rear sheet metal had to be pulled into place. The entire rear bumper system was damaged and required replacement. The right rear lamp was damaged, as was the trunk.

 

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The rear face bar suffered two indentations; one deep indentation in the center of the bumper face, and the entire right side was pushed forward. There were no matching counter-points on the front of the Jeep to coincide with the noted indentations on the rear of the Honda. The involvement of the Honda in the chain reaction, as described, just did not happen.

Occupant Injury Potential Analysis

There was no evidence supplied that showed a nexus between the front of the Jeep and the rear of the Honda. Since there was no connection between the Jeep and the Honda damage, our expert constructed a model that assumed the Jeep hit the Honda based upon the damage to the rear of the Jeep.

The Ford’s impact speed into the Jeep was about 4.8-mph (worst-case). This produced a speed-change to the Jeep of about 3.1-mph. This means the speed of the Jeep into the Honda could not be greater than the 3.1-mph Jeep Delta V. If the Jeep hit the Honda at 3-mph, the speed-change to the Honda would be 1.9-mph. This would produce a force to the Honda occupants of about .86 g.

Vehicle g-forces are not a direct measurement of forces to the occupants. Of more importance is the force transmitted to the affected areas of an occupant’s body. Seat back restitution, interior padding, seat belts, airbag restraints, and headrest positioning are all factors that can influence occupant g-forces. The maximum force in this event was less than 2.4 g. How did this calculation compare with published threshold injury data?

Research has determined that in a retroflex impact, an occupant needs a speed change of over 7 mph coupled with a force of over 14 g to produce complaints of pain to healthy test subjects. Lateral flex impact injury thresholds require a speed of about 10-mph with a g-force of about 4.5 g. Injury thresholds are described as pain lasting 12 to 24-hours. To give the reader a feel for what forces are involved, we can compare this event with those forces experienced by riders in amusement park bumper cars. Speed changes of about 4.7 mph, and g-forces of about 2.13 g in all directions are the norm. Federal NCAP data tells us that the act of simply stepping off a curb produces g-forces of about 5 g. This incident was well under these values.

In order for an occupant to suffer an injury, the occupant must either undergo a speed change or a direction change within a very short period of time. This particular incident provided no mechanism for these changes. This was not an injury-producing event, as the forces and speeds exchanged between the Ford, the Jeep, and the Honda, even if the scenario described was correct, were just too low.

 

Expert of the Month: William (Bill) C. Sommers

 

Mr. Sommers is a Fully Accredited Traffic Accident Reconstructionist (ACTAR #268), a Certified Drug Recognition Expert by the International Association of Chiefs of Police, a Certified Trainer of Drug Recognition Experts by the International Association of Chiefs of Police, and is a licensed private investigator. He is a member of several professional organizations and has more than forty years of experience in law enforcement. He holds an Associate of Science Degree in Police Science from Chaffey College in Alta Loma, California.

He has completed over forty special training courses in related police procedures. He has received several awards for excellence in his field. His expertise for GEI includes speed analysis, g-forces, occupant kinematics and injury mechanisms, human perception and reaction, vehicle momentum in collision systems, vehicle collision time and distance analysis, tire and wheel failure mechanisms, highway vehicle tire hydroplaning, seat belt analysis, and automotive lamp analysis. He is also a Bosch Certified Event Data Recorder Technician.

 

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