Car accidents occur between various types of vehicles, at a range of speeds and angles and typically involve two moving vehicles. However, most standard government enforced crash tests do not necessarily exhibit what typically occurs in a real-world crash. This is why one of the objectives of our upcoming Crash Conference is to further study intersection collisions, with respect to the crash dynamics, mechanics, accelerations and forces involved.
In order 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. 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 163º before coming to rest. Scenario B is similar to 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 130º before coming to rest. The lateral speed change of the Oldsmobile in scenario B is 3.5 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.
Figure 1: A comparison of post collision dynamics: Scenario A (left panel) – Chevrolet is stationary (red vehicle) and Oldsmobile is moving (blue vehicle), Scenario B (right panel) – both vehicles are moving.
In scenario A, the occupants of the Oldsmobile would initially move primarily forward as a result of the collision with the Chevrolet. Comparatively, as a result 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 130º more 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, which 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 degrees per second. Therefore, the occupants of the Chevrolet would have initially moved forwards and to 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.
Therefore, 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 wherein there are two moving vehicles in order to better understand the crash dynamics, vehicle response, damage and movement post-impact to have 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 particular crash scenario in question.