Passenger Vehicles
The reconstruction of a traffic accident is a vital part of determining all causes of a collision. Contributory factors may include vehicle speed, sight distance, sightline obstructions, right-of-way issues, and roadway design. In some cases, it may not initially be clear which occupant was driving the vehicle at the time of the accident. Proper Accident Reconstruction techniques, along with the evolving “Black Box” technology, can provide an accurate representation of all of the contributory factors in an accident.
The Vehicle Study
An examination of the damage on the vehicles involved will reveal vital clues as to the cause of the accident. There are two types of damage: contact damage and induced damage. Contact damage results from contact with another object at that specific location. It is characterized by sharp deformations, scraping, tearing, and paint transfer. Induced damage is caused by the collision itself; however, no actual contact was made in that specific area. This is characterized by buckling, folding, or bowing of the bodywork. The direction of the deformation is important in determining the Principle Direction of Force (PDOF), which is the direction of the force transmitted onto a vehicle during a collision.
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| Photo 1: Damage on the left rear door is from contact with the lug bolts on a tractor-trailer during rotation. This is contact damage, indicating that actual contact between two vehicles occurred at this location. |
A vehicle accident reconstructionist will use the contact damage and PDOF to determine how a vehicle interacted with an object, which could include another vehicle, a pedestrian, a utility pole, or any other roadside object. These assist in positioning the vehicle with respect to the contacted object, and help to provide a clearer picture of the travel path and orientation of the involved vehicles. Measurement of the depth and width of the damage is necessary if an Energy Analysis is to be used in determining vehicle speed.
The Site Study
A careful examination of the accident site is an important step in the reconstruction of an accident. Evidence at the scene is useful in determining the location and position of the vehicles before, during, and after impact and is necessary in most reconstructions to determine vehicle speed and other potential causes of the accident.
Evidence at the scene includes gouges, scrapes, tire marks, ruts, and damage to stationary objects such as road signs or guardrails. Gouges in the roadway may indicate the area of impact of the point of maximum engagement between the involved vehicles. Scrapes, tire marks, and rutting give clues as to the movement of the vehicles after the impact. Pre-collision tire marks may also indicate a driver’s attempted evasive maneuver.
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| Photo 2: Gouges in the roadway indicate area of maximum engagement between the two involved vehicles. |
There are various types of tire marks; the two most common are straight-line skid marks and yaw marks. Straight-line skid marks indicate a tire that is not rotating, either due to braking or as a result of damage sustained during a collision. Straight-line skids are generally linear, and the striations within the tire mark are oriented parallel to the tire mark. An abrupt change in a straight-line skid mark indicates external force acting on the vehicle, such as an impact with another vehicle. This can be useful in determining the position and orientation of a vehicle at impact.
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| Photo 3:Pre-collision straight-line skid marks indicate locked tires during braking. |
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| Photo 4: Rutting in the grass median shows the travel path of the vehicle. Four ruts are visible, indicating that the vehicle was in a yaw as it traversed the median. |
A yaw mark indicates rotation of a vehicle, as would occur when a vehicle “spins out.” Yaw marks are generally curved, and the striations are at an angle to the mark. The orientation of the striations may change along the length of the yaw mark. Because the yaw marks are caused by the tires, they are used to determine the travel path of a vehicle before and after a collision.
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| Photo 5: Tire mark is a yaw mark – note orientation of striations. Yaw mark indicates vehicle rotation. Sudden change in direction of yaw mark indicates external force acting on the vehicle, such as a collision with another vehicle. |
Other evidence may also be present at the scene: rutting in the grass, a damaged guardrail, paint transfer onto a road sign, vehicle debris, and disturbed vegetation all help to reveal an accurate picture of how the vehicle moved and the location where it came to rest. Potential sight-line obstructions such as signs, tall vegetation, hill crest, parked vehicles, and other objects are noted and measured to be included in the analysis.
The Analysis
Once the position and orientation of the vehicle before, during, and after the accident are known, several methods can be used to determine vehicle speed. Some of the most common methods are: Momentum Analysis, Energy Analysis, and Slide-to-Stop. Equations of motion can also be used in cases involving a vehicle that falls, vaults, or flips.
Other contributory factors must also be examined once vehicle speed and movement are known. Possible sightline obstructions can be mapped out to determine what a driver could see while performing a maneuver. For example, a Time-Distance diagram can show whether a driver was able to see an approaching vehicle due to a hill crest in the roadway combined with the speed of the approaching vehicle.
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| Photo 6: A Time-Distance diagram shows the relative positions of the vehicles if Vehicle 2 had been traveling at the posted speed limit of 55 mph. |
Reconstructions may also involve various other scenarios: determining which occupant was the driver, whether a hit-and-run occurred, or whether the driver would be able to see a pedestrian in the roadway at night with sufficient time to take evasive action.
Event Data Recorders (Black Boxes)
Many vehicles are equipped with devices to record Event Data involved in an accident. The list of vehicles containing black boxes that can be downloaded by an individual other than the manufacturer is limited at this time, but generally includes: most General Motors vehicles from 1994 to current; many Ford vehicles from 2001 to current; and a very limited number of Isuzu vehicles. This list is expected to expand as more manufacturers allow third-party access to the data inside their Event Data Recorders.
The Black Box is the airbag computer which senses a collision, performs diagnostics of the airbag system, and commands the airbags to deploy when warranted. As a secondary function, a limited amount of data is recorded surrounding an accident. A sudden deceleration “event” is required to trigger a recording. A collision sufficient to deploy the airbags will trigger a recording. Less severe collisions may also trigger a recording if the deceleration is higher than a certain threshold.
The data available varies by vehicle manufacturer, model line, and model year. At a minimum, most vehicles will record the system status and the crash pulse, or Delta-V, experienced during the accident. More extensive data may be recorded as well, including: seat belt status, tire pressure monitoring system status, driver’s seat position, and much more. Many of the later-model GM vehicles also include approximately five seconds of pre-crash data, including vehicle speed, engine rpm, and throttle percent. Brake pedal position (on/off) may also be recorded for up to eight seconds prior to impact.
The data contained within the black box is additional information which may be useful as part of an accident reconstruction. The black box data should not be used as the only evidence, but should be part of a complete reconstruction. Without an understanding of the evidence at the scene, the data within the black box may be misinterpreted and lead to an erroneous conclusion.
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| Photo 7: Data downloaded from the Event Data Recorder, or “Black Box.” Example of Crash Pulse graph displaying Delta-V experienced during a collision. |
Donan Engineering’s expert team of Vehicle Accident Reconstructionists is highly experienced and trained to evaluate the wide array of data necessary to properly determine the cause of most traffic accidents.