Hit from behind: even low-speed rear-end auto crashes can lead to claims of severe injuries. A new study examines what really happens to drivers and their cars in a variety of circumstances. (Property/Casualty: Auto).

Injury claims resulting from rear-end automobile collisions can be anathema for insurers. By their nature, the claims are subjective. They usually involve whiplash-associated disorders, such as headaches, neck pain or back pain--the kind of symptoms that make the severity of the injury difficult to measure.

Exacerbating the problem for insurers is the frequency with which rear-end collisions occur. Rear-end crashes (categorized by first harmful event) accounted for 29.7% of the total number of motor-vehicle crashes in 2000, according to "Traffic Safety Facts 2000," a report by the National Highway Traffic Safety Administration. Rear-end crashes were second only to angle collisions (30.2%) in terms of the total number of crashes, and way ahead of sideswipe (7.1%) and headon (2.2%) collisions.

In addition, injuries from these crashes can be expensive. The Insurance Institute for Highway Safety reported that neck injuries resulting from rear-end crashes cost at least $7 billion per year.

Due to the increasing number of accidents, injury claims and incurred costs, accident reconstructionists and biomechanical engineers frequently are called upon to analyze rear-end automobile accidents and assess the potential for an injury to have occurred to the occupants of the vehicle. The key issues in these types of accidents are typically speeds and forces involved in the impact, and whether or not these speeds and forces were sufficient to cause injury.

Several studies done in the 1990s have attempted to address these issues by exposing human volunteers to rear-end impacts. Researchers at Boster, Kobayashi & Associates recently conducted a study to add to the growing body of research related to occupant kinematic response to rear-end impacts and to further quantify the vehicle dynamic response that would be typical of the bumper systems in use on modern passenger cars.

Why Conduct a Study?

The study had three objectives. The first was to add to the available body of knowledge by testing human volunteers in low-speed rear-end impacts at a level that was at or below the level currently associated with no significant risk of injury. Existing data indicated that, for a normally seated healthy adult occupant with adequate head support, a change in speed of 5 mph or less does not expose the person to any significant risk of injury. Thus, the new study was intended to include impacts approaching but not exceeding a change in speed of 5 mph to the target vehicle.

The second objective was to subjectively describe and characterize the severity of the impact that was experienced by the occupants. That is, in addition to the ability to characterize an impact in terms of change in speed or average or peak acceleration, the researchers were interested in determining whether the volunteer's subjective impressions of the forces experienced in certain low-speed rear-end impacts may be appropriately compared to head movements routinely experienced in daily activities.

A study published in SPINE in 1994 measured how the head moved during daily activities that included sitting down in a chair, hopping off a step and plopping backwards into a chair. Such activities are sometimes used by accident reconstructionists to describe and characterize the severity of a low-speed rear-end impact. This may or may not be appropriate, depending on the peak head accelerations experienced.

The third objective was to evaluate how vehicles respond to low-speed rear-end impacts. The researchers were specifically interested in determining average and peak vehicle accelerations, coefficients of restitution and duration of impacts. In addition, they were interested in examining what happens when a vehicle is hit from behind when the driver's foot is on the brake.

Because some vehicles can withstand impacts that produce a change in speed of 5 mph or more with little or no visible damage, factors other than vehicle damage alone may be useful in reconstructing the change in speed and impact speeds of a low-speed rear-end impact. Statements regarding how far a vehicle was knocked forward, whether the occupant's head contacted the head restraint, or whether the driver's hands came off the steering wheel can all be potentially useful pieces of information in quantifying the severity of a low-speed impact.

Test Results

Seven vehicle-to-vehicle rear-end impact tests were conducted at impact speeds ranging from 2 to 6.5 mph. Changes in speed to the target vehicle of 1.5 to 4.5 mph were produced. Seven healthy men and women volunteers between the ages of 29 and 61 participated in the tests. Two of them were seated in the target vehicle and one drove the bullet vehicle that would rear-end the target vehicle for each of the seven tests.

The majority of participants experienced no symptoms or injury as a result of this testing. Three of the participants that had multiple exposures (all testing was conducted in one eight-hour period) had minor neck stiffness, but not pain, that went away in one day without treatment. This is consistent with the published data regarding human volunteer exposures in which volunteers have been exposed to changes in speed of up to 5 mph without sustaining injury.

For impacts that produced a change in speed to the target vehicle of 3 mph or less, the nature of the impact in terms of force and severity can reasonably be related to activities of daily living. These impacts produced forces on the head, neck and torso that the test subjects would agree are within the range of forces that can be experienced in daily activities.

For impacts that produced a change in speed in excess of 3 mph, the people who participated in the tests consistently noted that their heads were forced backward into the head restraints. For weaker impacts, the volunteers either did not find that their head was forced back into the head restraint or the force was so minor that they barely noticed it.

Vehicle-to-vehicle coefficient of restitution values were calculated and found to be in the range of 0.3 to 0.5. The impact durations ranged from 0.090 to 0.124 seconds for these bumper-to-bumper impacts and tended to decrease with increasing impact severity over the range of speeds tested. Peak vehicle accelerations were between 1.5 and 2.7 times higher than the average vehicle accelerations.

Target-vehicle forward displacement was found to be a weak indicator of impact severity for this series of rear-end impacts. Test subject reaction and braking response to the rear-end impact significantly influenced target-vehicle rollout distance in some tests.

Study results, both subjective experience as well as experimental data, supported the findings from other tests that used real people. Normally seated, healthy adults, with adequate head support can withstand impacts producing a change in speed of 5 mph or less without significant risk of injury.

RELATED ARTICLE: Crashes Take Their Toll

Rear-end motor-vehicle crashes accounted for nearly one-third of the total number of crashes in 2000. Details of the overall crash picture that year, according to "The Economic Impact of Motor Vehicle Crashes 2000," a report by the National Highway Traffic Safety Administration, were as follows:

* The total economic cost of motor-vehicle crashes in the United States was $230.6 billion.

* 5.3 million persons were injured in 16.4 million motor-vehicle crashes, including 41,821 fatalities.

* 21% of these injuries occurred in crashes that were not reported to police.

* 27.6 million vehicles were damaged in motor-vehicle crashes.

* 23.6 million, or 86%, of those vehicles were damaged in incidents that resulted in property damage only.

* The remaining 14% involved injuries to occupants of the vehicle or to nonoccupants, such as pedestrians and bicyclists.

* Approximately half of crashes and a fifth of all injury crashes are not reported to police.

The Black Box Knows

Airplanes have been monitored by their flight data recorders for years, and now motor vehicles on the ground are getting similar scrutiny. Manufacturers are equipping many of their vehicles with a recordable air-bag module, sometimes called a "black box," which captures and stores data that can be particularly useful in reconstructing accidents.

General Motors, for example, is equipping nearly every new vehicle with a black box, more correctly known as the air-bag Sensing and Diagnostic Module. The air-bag module is the vehicle's "computer" that controls air-bag deployment. The module consists of an accelerometer, low pass filter, and microcomputer, which includes a data-recording feature, The most enhanced version of the module, in use on certain 1999 and nearly all 2000 and later model-year vehicles (including passenger cars, sport utility vehicles and light trucks) captures and stores five seconds of precrash data, including vehicle speed, engine rpm, throttle position and brake switch on/off status. In addition; detailed change-in-speed vs. time data, seat-belt status, ignition cycles and air-bag system status are recorded for the crash. Depending upon the particular vehicle, additional data may be captured.

The module records two different types of crash events. The first is the near-deployment event. A near-deployment event is an event severe enough to wake up the air-bag sensing algorithm, but not severe enough to deploy the air bags. It contains precrash and crash data. The module can store only one near-deployment event, and such event will be overwritten by any subsequent event having a greater velocity change, or it will be cleared from memory after the ignition has been cycled 250 times.

The second type of recorded crash event is the deployment event. It also contains precrash and crash data. A deployment event is an event severe enough to deploy the air bags. This causes the module to permanently store precrash and crash data. Deployment events cannot be over written or cleared from the module. and, therefore, can be retrieved at any time after an accident.

Data can be retrieved from the module in one of two ways. If the electrical system of the vehicle is intact, the data can be read by connecting to the vehicle's diagnostic link connector. Located underneath the dash, it is used by technicians to talk to the vehicle's on-board computer. If the electrical system is not intact, then the module can be accessed by connecting directly to its connector.

Although other vehicle manufacturers also have recordable air-bag modules, so far only GM has made its data readily accessible via the commercially available Vetronix Crash Data Retrieval system. Ford is currently working with Vetronix to allow the data retrieval system to access the modules on some of its vehicles.

Thomas A. Braun is a consulting engineer for Boster; Kobayashi & Associates, Livermore, Calif., and a co-author of the study.