Car accidents involve various types of vehicles—typically two—moving at a range of speeds and angles. However, most standard government enforced crash tests do not necessarily exhibit what typically occurs in a real-world crash. Therefore one of the objectives of our Crash Conference was to further study intersection collisions, with respect to the crash dynamics, mechanics, accelerations and forces involved.
To illustrate the effect of a crash (or a crash test) on an occupant, consider the difference between the following scenarios in which the target vehicle (the vehicle being struck) is either stationary or moving when the collision occurs.
Scenario A
In Scenario A (below), a blue Oldsmobile T-bones a stationary red Chevrolet at the right rear wheel at an impact speed of 60 km/h. As a result of the collision, the Oldsmobile would experience a rearward speed change of 20 km/h and a lateral speed change of only 2 km/h. The Chevrolet would experience a longitudinal speed change of only 2 to 3 km/h, but a lateral speed change of 18 km/h in combination with a rotation of 163° before coming to rest.
Scenario B
Scenario B is like Scenario A in terms of the area of impact and impact speed of the Oldsmobile of 60 km/h. The difference in Scenario B is that the target vehicle (Chevrolet) is now also in motion prior to the collision and is travelling at 30 km/h when the impact occurs. As a result of this collision, the Oldsmobile now experiences a rearward speed change of 19 km/h, a lateral speed change of 7 km/h, and rotates 130° before coming to rest.
Discussion & Comparison
The lateral speed change of the Oldsmobile in Scenario B is three and a half times more severe in magnitude and experiences more post-impact rotation when compared to the collision that occurred in Scenario A where the Chevrolet was stationary. The longitudinal speed change of the Chevrolet in Scenario B is 40% more severe in magnitude, and the vehicle experiences almost twice as much post-impact rotation when compared to the collision that occurred in Scenario A where the Chevrolet was stationary. The vehicle dynamics can be seen in the figure below.
In Scenario A, the occupants of the Oldsmobile would initially move primarily forward because of the collision with the Chevrolet. Comparatively, because of the collision in Scenario B, the movement of the occupants in the Oldsmobile would be different than the motion experienced by the occupants involved in Scenario A despite the similarities between the crash scenarios. In Scenario B, the lateral severity was significantly greater, and the vehicle rotates 130° farther post-impact compared to Scenario A. Therefore, the occupants of the Oldsmobile would experience greater lateral severity and rotational forces, which would likely increase their movement and possibly increase their risk of injury.
As a result of the collision in Scenario A, the occupants in the Chevrolet would move primarily to the right, with some movement to left as the vehicle began to rotate. The lateral speed change in Scenario B would have been less severe than the collision in Scenario A; however, the longitudinal speed change was greater and is primarily responsible for the increased vehicle rotation (almost two times as much rotation). In fact, the initial rotation rate of the Chevrolet exceeded 300°/s. Therefore, the occupants of the Chevrolet would have initially moved forward and to the right. As the vehicle began to rotate, the occupants would be pulled left to a greater degree (compared to the occupants in scenario A), due to the increased severity of the rotational forces.
The risk of injury to the occupants of the vehicles does not only depend on the magnitude of the speed changes, but also on the rotations. Accordingly, it is important to study crash tests in which there are two moving vehicles to better understand the crash dynamics, vehicle response, damage, and movement post-impact. These details help provide a better understanding of the expected occupant motion in real-world intersection crashes. The accuracy of a biomechanical assessment of the crash injuries depends upon the applicability of the research and testing when dissecting the crash scenario in question.
Karla Cassidy is a Senior Engineer in J.S. Held's Accident Reconstruction Practice. Ms. Cassidy has been an active member in the biomechanics community since 2006 and in the accident reconstruction industry since 2010. Her expertise spans both biomechanical and mechanical engineering. She has been involved in hundreds of cases involving vehicles, pedestrians, motorcycles, farm equipment, and cyclists. Her specialty areas are biomechanics, personal injury, injury probability, seatbelt usage, slip, trip and falls, and determination of occupant position. Ms. Cassidy also conducts collision reconstruction and damage consistency analyses. She is a published author and has provided litigation support.
Karla can be reached at [email protected] or +1 416 977 0009.
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