Heavy Trucks
Traffic accidents involving commercial vehicles often result in serious personal injury, large property damage, and great amounts of media attention due to the severity of the accidents. Perhaps no area of traffic accident reconstruction lends itself to more erroneously made assumptions and improper applications of the basic dynamic equations than that of heavy articulated vehicles. Some investigators may attempt to apply the same reconstruction procedures associated with passenger cars and pickups to commercial vehicle accidents without modification. Often, the appropriate vehicle parameters necessary for an accurate reconstruction are not collected and/or documented. Specialized training is required for the correct analysis and reconstruction of commercial vehicle traffic accidents, and Donan Engineering’s team of experts has the knowledge and tools necessary to understand the complex circumstances that are typically involved in heavy vehicle accidents.
The Vehicle Study
A thorough post-crash inspection of a commercial vehicle is often essential to determining if the mechanical condition of the vehicle contributed to the cause or severity of the crash. A partial or complete North American Driver Standard Level 1 inspection may be conducted on both the tractor and trailer after a severe traffic accident. If relevant to the particular accident, the condition of the headlights, turn signals, side marker lights, wheels, rims, and tires may be studied and documented. The steering, suspension, and air system may also be studied for pre-crash conditions that may have contributed to the subsequent crash.
One of the most important post-crash inspections that the investigator conducts will be of the vehicle’s air brake system and its components. It is vital that the investigator is familiar with the different types of foundation (service) brakes used on commercial vehicles and the individual components that make up the air brake system. The condition, type, and size of the air brake components should be documented. The pushrod stroke should be measured and documented at full service brake application at 90 to 100 psi application pressure to check if the brakes are in proper adjustment. In order for the air brake system to be effective, the air pressure must be converted to mechanical leverage, and the service brakes must be able to generate enough brake force to effectively slow the vehicle.
Photo 1: Post-crash inspection showing the measurement of the pushrod stroke of the air brake system of a trailer.
In addition to measurement of the pushrod stroke of the air brake system, a wheel-off inspection of the components of the foundation brakes may also be required. There are three basic types of foundation brakes used on commercial motor vehicles: drum or s-cam brakes, wedge brakes, and disc brakes. A wheel-off inspection can reveal excessive wear of the brake friction components, or mechanical failure of a component of the brake itself.
Photo 2: Air brake components (brake chamber, pushrod, & slack adjuster) with drum or s-cam brakes.
A post-crash inspection of the tires is often required to determine if the tires were a contributing factor to a traffic crash. Often, the driver of the vehicle will attribute a loss of control to a tire failure. It is important to inspect the overall condition, tread depth, and inflation pressure of each tire on the vehicle. Examination should be made for continuity of the steering system. A discontinuity may either be the result of the collision or a contributing cause to a loss of control that resulted in the collision.
The Site Study
A careful and thorough examination of the accident site is an important step in the reconstruction of any traffic accident. Additional considerations should be made during the survey of an accident site involving a commercial vehicle. Since a commercial vehicle may have as many as 18 or more tires capable of leaving tire marks on the roadway, a thorough study and documentation should be made of any tire marks left on the roadway by a commercial vehicle. Since the acceleration capability of a loaded commercial vehicle is substantially lower than that of a passenger car, pickup, or motorcycle, special attention should be made of the sight distance available at the accident site, particularly in cases involving a commercial vehicle attempting to make a turn at an intersection from a stop. Often, investigators will improperly attribute an accident to a failure to yield the right-of-way to a commercial vehicle when the actual cause is driver inattention from the opposing vehicle operator.
Photo 3: Accident site map of a commercial vehicle accident.
The area of the impact between a commercial vehicle and a passenger car is often evidenced by numerous gouges and scrapes in the roadway surface, collision scrubs and tire marks from the passenger car tires, and fluid splatter as the passenger car is forced downward into the roadway by the taller commercial vehicle. An accurate survey and thorough documentation of the physical evidence at the accident scene, along with a thorough study of the vehicles, is necessary for the most accurate analysis and reconstruction of the traffic accident.
The Analysis
The methods available for vehicle accident reconstructionists to mathematically calculate the collision speeds in a commercial vehicle and passenger vehicle traffic accident are somewhat limited by the large weight difference between a loaded commercial vehicle and a passenger car/pickup. In the car-truck crash analysis, the insensitivity of the calculation to the difference in the weights of the vehicles may result in the inability to accurately apply the Conservation of Momentum principle as a tool in the analysis of the vehicle speeds. A Conservation of Energy analysis can be used to determine the initial speed of a commercial vehicle with a fair degree of reliability, depending on the availability of physical evidence as well as the characteristics of the traffic collision. The Conservation of Energy analysis requires a thorough study and survey of the physical evidence at the accident site and a thorough study of the mechanical condition and adjustment of the air brake system.
Photo 4: Collision between a tractor-trailer and passenger car at an intersection.
Photo 5: Closer view of the tractor-trailer and passenger car involved in the collision.
Since the late 1980s, manufacturers of heavy truck engines have been gradually replacing the mechanical devices that control engines with electronic devices. One standard feature on today’s heavy truck engines includes Electronic Control Modules (ECMs) which not only control and maintain functions of the engine in selected operating conditions, but also record data such as event/incident data. When an operating condition such as unusually hard braking or a collision is met, instructions in the ECM transfer data from a temporary buffer to permanent storage. Data transferred from the ECM that can be very useful to the reconstructionist includes: vehicle speed (mph), engine rpm, brake status on/off, clutch status on/off, engine load in %, throttle in %, cruise control on/off, and diagnostic code.
Photo 6: ECM Data being retrieved from a Cummins Engine.
Accessing the event/incident data can be accomplished using software and components purchased from the engine manufacturers. Event/incident data can be downloaded from the ECM on newer engines of the following engine manufacturers: Caterpillar, Cummins Diesel, Detroit Diesel, and Mack. The event/incident data can assist the reconstructionist in the analysis of the traffic accident. The data does not represent the only evidence in the same manner as the “Black Box” data obtained from passenger cars, but the data should be part of a complete reconstruction. Without an understanding of the evidence at the scene, the data from the ECM may be misinterpreted and lead to erroneous conclusions.
Donan Engineering’s team of Vehicle Accident Reconstruction experts have the skills and experience to successfully determine the cause of these accidents and successfully defend those findings in subsequent legal hearings.