Delta-v is a widely used term in the world of collision reconstruction but is often misunderstood or unknown entirely by the general public. Rightfully so, it may have no relevance to you unless you’re an insurance adjuster or lawyer trying to establish liability or cost resulting from a vehicle collision.
As collision reconstruction consultants, our clients in the insurance and legal industries call upon us to provide them with the information they require to determine responsibility or liability for a given collision. This typically includes a determination of vehicle speeds at impact, a process that starts by first calculating the collision severity, a term often described and often misunderstood. While the calculation of collision severity can be quite complex and is beyond the scope of this short article, we can describe it here in simple terms so that the reader has a better idea of what it is and how we use it in our assessments.
Collision severity is defined as the change in velocity, or delta-v, from immediately before to immediately after a collision that is experienced by a vehicle. This delta-v occurs over a very short time time frame, typically 0.15 seconds or less — faster than a blink of an eye. To put this in context, we will refer to an example: for its crash worthiness and safety mandate, the National Highway Traffic Safety Administration (NHTSA) crash tests vehicles travelling at 56 km/h into a rigid wall (Figure 1); for our example, we’ll assume that it’s 60 km/h. During these tests, the vehicles experience a delta-v of 60 km/h, ignoring any sort of rebound off the wall. In other words, vehicles stop pretty much immediately as they hit the wall. These crash tests typically last for less than a tenth of a second; the collision shown in Figure 1 lasted for about 0.08 seconds.
Figure 1: NHTSA crash test of a 2018 Tesla Model 3 into a rigid wall at 56 km/h.
The collision described above is considered a very severe collision that happens very fast. As crazy as it may sound, you probably experience a delta-v this high every day. How so? Well, imagine you’re driving your car at 60 km/h down the road and approach a stop light. You slow down and eventually stop for the red light. In this sense, your speed reduced from 60 km/h to zero, and you experienced a changed in velocity (delta-v) of 60 km/h! But you weren’t in a collision, and you didn’t feel any sort of “severe” impact. The difference between this scenario and the crash test example given above is that you probably slowed down to a stop in roughly 8 to 9 seconds. That’s about two orders of magnitude (100x) longer than the crash test impact. So, it’s the time frame that makes the difference. The shorter the time, the more severe the change in velocity. When applied to a collision, the term delta-v is used.
Understanding Discrepancies in Collision Severity
While delta-v refers to collision severity, it is unlikely that both vehicles in a collision will experience the same delta-v. For example, consider a large passenger sedan colliding head-on with a small passenger sedan. Such a collision was tested by the Insurance Institute for Highway Safety (IIHS) (Figure 2), where a Toyota Camry collided with a Toyota Yaris, which it outweighed by about 400 kg. During the test, both vehicles were travelling at the same speed. As a result of the collision, which by-the-way lasted for about 0.15 seconds, the heavier Camry experienced a delta-v that was 21 km/h or 38 % lower than that of the Yaris, even though both vehicles still experienced a severe collision.
Figure 2: IIHS head-on crash test between 2009 Toyota (red) & 2009 Toyota Yaris (blue).
The reason for the discrepancy in the collision severity in the example above is because the masses of the vehicles were substantially different, and everybody knows that heavier objects are harder to move. The science behind this stems from a phenomenon first explained by Isaac Newton in his first law of motion, the law of inertia, which dictates that heavier objects are more resistant to changes in motion. This simple concept can be used to help insurance and legal folk better understand the severity of collisions, which may affect collision avoidance potential or even the possibility of occupant injury. But that’s another discussion altogether.
VANCOUVER TORONTO OTTAWA