A freeway chain reaction

Oct 1, 2011 by

The case of the month involves a four-vehicle series of impacts that occurred on a freeway before sunrise. The first vehicle, a Ford F150, stalled. It displayed no vehicle or hazard warning lights. The second driver in a Camry failed to see the Ford quickly enough and rear-ended it in the darkness. Our client’s son was driving his dad’s Mercedes-Benz, and he, too, failed to slow for the stopped vehicles, neither of which displayed any lighting. At the last moment when he did see them, he swerved to the left in an effort to avoid them and, in so doing, crossed into the path of an overtaking fourth vehicle, a Chevrolet Tahoe. The Chevrolet then struck the Mercedes-Benz.

The California Highway Patrol investigated the incident and found both the second and third drivers at fault, as per CVC §22350: unsafe speed for conditions. The Mercedes-Benz was also found to have made an unsafe turn on a freeway, per CVC §22107.

GEI was assigned to make an independent examination of the supplied documents and photographs, to determine the impact speed and g-forces, and likelihood for injury in this incident. We were also asked to determine the sequence of events and to evaluate if the Mercedes-Benz’s driver’s statement, given in a summary of facts in the police report, was consistent with the evidence.

Our expert examined the supplied vehicle photographs, repair estimates, and the police report for this incident. He researched the weights and measurements for the Chevrolet and the Mercedes-Benz. He evaluated the National Highway Traffic Safety Administration stiffness data that relates to the vehicles’ resistance to impact damage and equated it in terms of kinetic energy. He researched the position of the moon, its luminance, and the sun’s position and lighting for the date, location, and time of the occurrence.

The photographs and repair estimate for the Mercedes-Benz showed the right rear-end to be more impacted than the center or left side of the rear-end. The Mercedes-Benz weighed about 3,293 pounds. The Mercedes-Benz has a bumper system rated to accept a strike of 5 mph, and the Chevrolet has no requirement to be rated. The Chevrolet weighed about 4,828 pounds. The Chevrolet was struck on the right front fender. The repair required three hours of frame rack time. There was bumper, grille, right suspension, air cleaner, headlamp, fender, and wheel damage to the vehicle.

The expert compared damage to landmarks contained in the weight/measurement data, and estimated the crush damage to both vehicles. He used this information and NHTSA stiffness data to estimate crush energy to the vehicles. He then totaled the damage energy and attributed it to the Chevrolet’s impact speed-differential.

He determined that the Chevrolet was going at least 13 to 22 mph faster than the Mercedes-Benz, at impact. He used this range of 13 to 22 mph in order to construct a worst-case scenario. Using 13 to 22 mph as an impact speed, and accounting for the coefficient of restitution and the great weight difference in the vehicles, the speed change (Delta V) for the Mercedes-Benz was about 9 to 16 mph. This meant the Mercedes-Benz was accelerated by the impact to a speed 9 to 16 mph greater than before the collision.

The speed change for the Chevrolet was found to be in the range of 6.3 to 11 mph. The Chevrolet was slowed when it hit the Mercedes-Benz.

In order to calculate the applied force, one must determine the time over which the majority of the force is transmitted. This is known as the impulse time. Generally speaking, data shows that impulse time is .085 to .100 seconds (100 milliseconds). When this value was factored into the mix, it was calculated that the applied force to the Mercedes-Benz was about 4.9 g to about 7.5 g.

Vehicle g-forces are not a direct measurement of forces to the occupant. Of more importance is the force transmitted to the affected areas of an occupant’s body. Seat back restitution, interior padding, seat belts, air bag restraints, and headrest positioning are all factors that can influence occupant g-forces. The force, as calculated for the acceleration to the head of the occupant of the Mercedes-Benz, was about 12 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 14 g to produce complaints of pain to healthy test subjects.

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 incident provided no mechanism for the required speed/force changes. This was not an injury-producing event, as the forces and speeds exchanged between the Chevrolet and the Mercedes-Benz were less than those reported in the literature.

The driver of the Mercedes-Benz stated that it was dark, and there was no moon. Astrological data showed that there was 81 percent illumination of the moon. It was below the horizon and was in the western sky. The accident occurred at 5:45 a.m. and civil twilight began one minute later at 5:46 a.m. Sunrise was at 6:11 a.m., twenty-six minutes later. This indicates it was not dark, but the ambient lighting was crepuscular in nature.

Per his statement, the driver of the Mercedes-Benz was watching people on the side of the road. Even if both vehicles of the preceding collision had no illuminated rear lights, California Vehicle Code §24607(a) requires all vehicles to be equipped with red reflectors visible from 350 to 100 feet from the rear when directly in front of lawful upper headlamp beams. Had the driver of the Mercedes-Benz been observant and driving at a safe speed, he could have seen the required red reflectors on the stopped vehicles in his path.

While the Mercedes-Benz driver was charged with the Primary Collision Factor, there were Associated Collision Factors that may be considered as well.

California Vehicle Code §24250 requires the vehicles stopped in front of the Mercedes-Benz to have lights in operation after the hour of darkness. CVC §24252 requires a driver to maintain his lights. CVC § 24002 requires the driver to not drive an unsafe vehicle, and CVC § 24252(a) requires a stopped driver to have a battery system capable of operating the vehicle’s lights for a period of at least fifteen minutes, if the vehicle becomes disabled.

In conclusion, the Mercedes-Benz driver could have avoided the accident, had he been alert and traveling at a safe speed, yet the drivers of the Ford and the Camry shared some responsibility for the accident by not keeping their lights on, post-collision. The good news was that there was no mechanism for injury at the speeds of this crash.

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