A Door Dent

The Accord driver admitted to backing into the left side of an unoccupied Silverado pickup. The pickup owner said he was inside the vehicle and was injured. Did that really happen? Due to the claim timeline, the client needed a fast turnaround so GEI analyzed the accident and produced a report in under a week.

Typically this kind of analysis is a three-step process. First, the vehicle speeds are estimated based upon vehicular damage. Second, forces upon the occupants from the estimated speeds are calculated. Third, those forces are compared to the reported injuries to see if the forces experienced in the accident are congruent with the observed injuries.

The 1998 Honda Accord had some cosmetic scrapes to its rear bumper cover on the left rear corner. There was no other damage visible or reported to the Honda.

 

The 2001 Chevrolet Silverado pickup had a dent in the lower portion of the left door. The dent extended from the bottom margin of the door upward to the trim line. The dent was deepest under the left rearview mirror and tapered off to zero depth about midway in the door.

 

Our expert agreed that the damage to the vehicles was consistent with the left rear corner of the Honda making contact with the soft sheet metal of the left door of the Chevrolet, creating a shallow dent a few inches in depth along the lower portion of the door.

According to published standards, the Accord weighed 3,089 pounds, and the Silverado weighed 4,123 pounds.

When two vehicles collide, the difference in their speed and direction results in the total kinetic energy of their collision system. This kinetic energy is partitioned between the two vehicles inversely proportional to their weights with the lighter vehicle receiving the greater portion of the energy. The energy is dissipated through the bending of metal, the fracturing of rigid parts, plus heat and noise. The damage to the vehicles is the signature of the expended kinetic energy and also reflects the energy partition with the lighter vehicle showing more damage than the heavier vehicle. In this collision, the heavier Chevrolet had an advantage over the Honda. The Honda dissipated 57% of the collision energy while the Chevrolet dissipated 43% of the collision energy. While the damage to the Chevrolet appeared to be more severe than the damage to the Honda, the Honda was dissipating its energy through an energy absorbing rear bumper while the Chevrolet dissipated its energy through soft sheet metal that is not designed to attenuate energy. The damage to both vehicles was so minor that the energy partition was actually a moot point.

The Honda was equipped with a 5 mph energy absorbing bumper system. These bumpers are designed to absorb the energy from an equivalent barrier collision of 5 mph without allowing damage to structures beyond the bumper system. Ordinarily, when an energy absorbing bumper system approaches the limit of its resistance, there is evidence of permanent deformation to the bumper. In this collision, there was no evidence of permanent deformation to the rear bumper of the Honda. The damage to the rear bumper of the Honda was superficial cosmetic scratches to the plastic bumper cover. Lack of deformation to the Honda’s rear bumper system indicates the bumper was not experiencing a speed change close to its limit of 5 mph. Based upon this and the damage profile to the Silverado door, our expert concluded that the speed change experienced by the Honda was less than 3 mph and the speed change experienced by the Chevrolet was therefore also less than 3 mph.

Injuries in a traffic collision occur when the occupants riding in a collision vehicle undergo a very rapid change in speed or direction. That change must be so rapid that the body cannot move to compensate for the forces acting on the body. For example, anyone who stops at a traffic light from 30 mph undergoes a speed change of 30 mph. However, that speed change occurs over several seconds and the body easily adjusts to the change in inertia and no injury occurs. If that same 30 mph speed change occurs over a tenth of a second the human body cannot compensate for the high force acting on the body, which results in tearing of tissue and fracturing of rigid body structures. Before the human occupants in a collision vehicle can experience such a violent speed change, first the vehicle they are riding in must also undergo a similar speed change.

Absent a significant speed change to a collision vehicle, there is no potential for injury to the occupants in the vehicle. In this low speed contact event there was not enough energy represented in the damage to the Chevrolet to sustain a conclusion that the Chevrolet was moved off its footprint. At most, the Chevrolet experienced some rocking on its suspension but it was not significantly accelerated nor was its direction significantly changed. Absent any significant change in speed or direction of the Chevrolet, there cannot be a significant change in speed or direction to the occupants in the Chevrolet. The speed change experienced by the occupants in the Chevrolet was less than 3 mph. There was not enough intrusion into the left side of the Chevrolet to produce a bodily contact injury from the collision.

Based upon thousands of peer-reviewed studies, the minimum speed threshold for expected injury to occupants riding in colliding vehicles is a speed change of 5 mph or greater. In this accident the speed change experienced by the occupants of the Chevrolet was less than 3 mph.

There was no need for a discussion about or analysis of the nature of the Chevrolet driver’s injuries, because whatever they were, the backing of the Honda into the Silverado did not cause them.

A Backing Accident

It was summer camp time. Moms were dropping off their kids at about 9 am. Unfortunately, two of the drivers backed into each other. There were two different versions of the collision, plus the story of an unrelated witness. GEI was assigned to evaluate which version the evidence supported and to address the likelihood of injury in this incident.

The vehicles involved were a brown Ford Flex and a silver Chrysler Sebring.

The photographs and the repair estimate for the Chrysler showed the rear right corner was damaged.

The strike was on the rear face of the bumper but the energy absorption element was not damaged. The impact was delivered to the very corner of the bumper, a place that has no underlying support. This portion is similar to an empty plastic box, as it offers no meaningful resistance to impact.

The Ford was damaged differently. The rear bumper face had evidence of a strike on the rear bumper face just left of center. The bumper had only cosmetic damage. The rear license plate and frame were not bent, suggesting an impact in the one to two mph range.

So who hit whom?

The Ford driver parked nose-in on a driveway near the camp area to offload. The Chrysler then pulled up parallel to the curb into a parking space that was on the same side of the street, past or after the driveway where the Ford was parked. The witness stated that the Chrysler was parked with the rear of the vehicle hanging out into the driveway. There was an allegation that the Chrysler backed into the Ford as the Ford was backing out of the driveway, but the independent witness did not support that assertion. He said the Ford backed into the parked Chrysler, striking the part that was sticking out into the path of the driveway. After the impact, the drivers got out and recognized the other driver as another parent at the camp. They exchanged information and left. The police were not called or involved.

A scene investigation confirmed that both parties would have had a clear view of each other’s vehicles, and that there were signs prohibiting street parking. The witness also stated that the Ford driver was talking on her cell phone.

What about injuries?

The Chrysler’s bumper is rated at 2.5 mph, but when a bumper is loaded on only one side, the bumper will accept only 1.5 mph worth of impact energy without damage. The damage pattern to the Chrysler suggested an impact in the one to two mph range.

Recall that the impact was from right-to-left, and had no forward component. This was not a whiplash incident as normally considered.

The applied force to the Chrysler was about .9 g. The forces experienced by the Chrysler were so low that the occupant g-forces in this event were less than those required for injury. The occupant received forces that were, at most, about 2.7 g.

How do these data 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. 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 in all directions of about 2.13 g are the norm.

The conclusion was that the speeds involved were far under the published standards of threshold for injury.

In addition to being parked in a no parking zone, the Chrysler’s partial blocking of the driveway was in violation of District of Columbia Regulation §18-2405.2(a), which states one cannot park within 5 feet of a public or private driveway.

So in this collision, neither party was blameless. The Chrysler driver illegally parked, partially blocking a driveway. The Ford driver, talking on her cell phone, failed to look behind her, and backed into the stationary Chrysler.