SCIENTIFIC METHOD APPLIED TO ACCIDENT RECONSTRUCTION
Biomechanical analyses have long played a role in vehicle accident investigation, typically performing injury analysis and the determination of occupant kinematics and impact forces to the body. Here, we explain how the scientific method can be adapted to accident reconstruction from a biomechanical and human factors perspective, incorporating by reference various aspects of accident investigation and engineering analyses for the purpose of testing hypotheses.
Determining what happened in an accident to a reasonable degree of scientific or engineering certainty is driven by a process of accident reconstruction that utilizes biomechanics, injury analysis, and human factors as critical analysis components. The process utilizes the scientific method as a framework for testing compatibility or consistency of different aspects of data or information that has been gathered about an accident (Fig. 1). Preliminary information provides context for understanding the general circumstances surrounding the
accident and guides the overall direction of scientific inquiry. From this information, hypotheses can be generated about specific occurrences or a sequence of events. Usually, these hypotheses are posed as “testable” statements (formulated as either a null hypothesis or in the affirmative) or as questions, where the answer meaningfully directs the analysis toward conclusion. Many times, answers are most useful when they are exclusionary, in that a specific event or sequence can be ruled out. The most important step is ‘testing’ the hypotheses. In this step, a protocol or technique is devised that will identify and gather the appropriate data to analyze to either support or refute the testable statements, or answer the questions. In investigating accidents that have already occurred, these tests may be physical experiments and/or demonstrations of scientific principles, but routinely this is not a practical approach. This is particularly true in human injury analysis where physical experiments can be problematic or impractical. Instead, analyzing research literature (often regarding previously published physical experiments) and other sources of information is the most common way to test these hypotheses. Reasonable care should be used with witness data. In situations where the witness’s version of events is in conflict with the laws of physics or the verifiable physical evidence, those aspects of the testimony must be rejected.
In utilizing the scientific method, a multitude of data from various sources is obtained in order to test the working hypotheses. To ensure it is useful and complete, this data must be organized and considered in a systematic way. One method is a modified Haddon matrix, where the ‘man’-related data is separated into injury data and human factors data, and both are potentially modified by medical factors (Fig. 2). Data are separated into the temporal categories: Before, During, and After the accident, if deemed appropriate for the specific accident reconstruction. The categories provided in the columns of Figure 2 are discussed further below.
Injury As Physical Evidence
The focal point of this method is using the ‘man’-related data, and in particular the injuries, as physical evidence that must be reconciled in order to have an accurate accident reconstruction. The injuries (and other human interactions) in the accident are used as a signature of the where, how, and when the person was placed within the accident sequence. The biomechanical physical evidence can be just as essential as the other physical evidence gathered relating to the product/machine and accident environment (Fig. 2, Col. 1).
To apply this method, the injuries are described by type, location, appearance, and severity from a review of the medical records (which may include review of X-rays, CTs, and MRIs), photographs, witness statements, medical examiner reports, and other similar sources. Second, the injury mechanism for each injury is determined. Injury mechanism information is often ascertained through comparison with specific data documented in the injury research literature. The mechanism provides information about the nature of energy transferred to the body (e.g. mechanical, thermal, electrical) and describes the specific method and means required to create the injury. For example, a spiral fracture of a long bone requires a torsional force component applied about the long axis of the bone. For a mechanically mediated injury, describing the body movements and the forces on and within the body that create the injury allows for an understanding of necessary physical interaction of the human body with the environment. Finally, determining the injury threshold or human tolerance for a particular type of injury can be important in accident analyses, since it provides context to the severity of the incident and the magnitude of force, acceleration, deflection etc. that typically produces injury. For many injury mechanisms, normal biomechanical tolerance has been established in the injury research literature, and provides a comparison to the specific injury or event being analyzed. In many cases, comparison of the accident exposure characteristics is made with biomechanical data from activities of daily living or voluntary human exposure research (see Medical Factors section below in situations where relevant pre-existing conditions/injuries exist).
In accidents where multiple injuries have been received, their pattern and distribution create a ‘constellation of injuries’ that can provide a unique insight to the accident. The steps are repeated for each injury, and those with similar mechanisms or locations are matched. Even what are typically considered minor or superficial injuries can provide important evidence to properly place a person in the reconstruction of an accident scenario.
Human factors data can be another critical component in the accident reconstruction. Human factors is a discipline that evaluates how people interact with their environment and encompasses physical and psychological aspects of human performance, capabilities, characteristics, and interfacing with tools, machines, and the environment. From an accident reconstruction perspective, this application is typically focused on the events leading up to and during the accident sequence in order to evaluate what and how it happened, and in some cases why (Fig. 2, Col. 2). Where appropriate, an individual’s anthropometry should be identified, including one’s height, weight, and segment lengths, which are all aspects of physical evidence that are relevant for addressing a person’s position, posture, and fit within the accident environment. Anthropometric and human factors data are available that describe a wide range of human measurements and provide context for accident specific evaluations. Surrogate studies are an additional means by which human factors considerations can be addressed.
Important to note are medical factors that could be potential modifiers of both the injury and human factors aspects (Fig. 2, Col. 1 & 2). A person’s health and/or medical condition may have a direct influence on their ability to resist trauma. The presence of disease or other pre-existing conditions, and the use of alcohol, drugs, or medications can have an impact on a person’s physical capabilities as well as their sensory perception and reaction. In these situations, an aspect of the analysis may include a determination of potential exacerbating influences, and whether a particular event is a significant contributor to the existing condition. This determination must follow the same methods described herein. In these circumstances, the mechanism of injury must still be present (e.g. the appropriate direction of force, acceleration, etc.), and comparison of an event with reasonable activities of daily living may be useful.
Product/Machine and Environment
Similarly, data must be gathered about other accident circumstances in order to put the injuries and human factors into context (Fig. 2, Col. 3 & 4). The geometry and layout of the accident site create physical evidence in the form of constraints, boundaries, and specific conditions (e.g. lighting, slip resistance). In accidents where products or machinery are involved, a description or knowledge of such things as the size, shape, materials, construction, controls, movement/action directions, speeds, and other characteristics of the equipment is important to understand the potential human interactions. Particular attention is given to documenting the damage, failed components, and/or witness marks that resulted from the accident. This gives key physical evidence about the nature of physical interaction between the man, machine, and environment. A range of failure analysis techniques are available to determine these interactions. In cases involving vehicles, the analysis can include a determination of the vehicle kinematics, as well as an assessment of the principle direction of force (PDOF) and velocity change (delta V) experienced. This information can then be used to determine occupant kinematics. It is not the intent of this paper to describe all the detailed analyses that are conducted on the product/machine or environment, but the data from these components is key to the accident reconstruction.
TEST AND ANALYSIS OF THE DATA
At the root of every accurate and complete accident reconstruction is consistency with the laws of physics and accounting for all the available physical evidence. When analyzing the data gathered and developed from the man, product/machine, and environment, the physics of all interactions between and within these groups must be consistent. This is identified in the bottom row of Figure 2. By regarding the injuries as physical evidence, they become not just an outcome of the accident, but an additional component (resource) to use in testing accident reconstruction hypotheses. In situations where there are inconsistencies between the available information, one must side with the physical evidence. Where there are apparent inconsistencies in the physical evidence, the data must be reexamined to resolve them. In this sense, the process can be iterative.
By following this method, the biomechanical accident reconstruction conclusions are founded in science and consistent with the laws of physics. Not all reconstructions will lead to a single answer. The available data that are gathered or developed may not be able to exclude all possibilities but one. This is usually a function of the ability to gather or develop sufficient data for this purpose. These efforts still have tremendous value, since knowing what did not happen can be just as important as knowing what did happen.
*This article is adapted from the following peer reviewed publication. Please refer to the publication for additional details and case examples.
Knox EH, Mathias AC, Stern AR, Van Bree MP, Brickman DB. “Methods Of Accident Reconstruction: Biomechanical And Human Factors Considerations.” Proceedings of the AMSE 2015 International Mechanical Engineering Conference and Exposition. Houston, Texas: November 13-19, 2015.