Yaw Marks Precede Almost Every Vehicle Loss of Control Rollover

The public is provided with little education regarding how motor vehicle collisions occur and what could injure or kill them. Every day there are numerous postings by police and news media about the latest significant injury or death yet scant information is provided about the details. The result is that needless collisions keep re-occurring, in very similar scenarios, without any meaningful intervention. The public need not know the details of interpreting physical evidence for collision reconstruction however very basic interpretation skills can progress to a progressively better understanding.

So, for this present article we focus on the general evidence found in a simple, single vehicle rollover. This discussion was spurred by the recent Twitter (X) posting by the Ontario Provincial Police (OPP) of a single photo (reproduced above) of a vehicle rollover along Highway 401 in Ontario.

The above photo shows a very common result of a vehicle rotating out-of-control into a roadside embankment and then rolling over. A gouge in the earth can be seen where the vehicle struck that embankment and preceding that gouge are a set of converging tire marks visible in the grass and on the asphalt shoulder.

Below we see the same photo with some added descriptions of the evidence.

In almost very scenario of a vehicle loss-of-control and subsequent rollover the vehicle enters into a rotation about its vertical centre-of-gravity, or yaw. Yaw rotation is what happens if you were to pierce the roof a vehicle with a rod downward toward the ground and then rotate the vehicle about that rod. Some common descriptions of this rotation are “fish-tailing” or “drifting”. Newer technology exists in almost all modern light-duty vehicles to prevent this rotation because of its undesirable injury consequences. Thus Electronic Stability Control (ESC) and its derivatives uses automatic adjustments to the braking and acceleration of individual wheels to keep a vehicle travelling straight in the direction it is travelling. So one would think that the frequency of the results shown in the above photo should be diminished over time.

So the tire marks in the above photos are yaw marks. But how do we know? The area of the blue circle in the above photo shows a typical characteristic of yaw marks in that they contain striations that often run diagonally with respect to the length of the tire mark. These striations are caused when the tire is rotating while also sliding sideways. Investigators can often look at the change in angle of these striations to determine if a vehicle has been braked or accelerated while producing these marks. If one were to move backwards from this photo and one were to see a longer length of these yaw marks one would see that they would be arced as the vehicle changes direction and is slowed as it travels to the roadside.

Another very common characteristic of pre-crash yaw marks is that they demonstrate the angle of the vehicle as it moves through the site. This angle is identified by the divergence and convergence of the tire marks. When a vehicle is travelling straight ahead without rotation the rear tires follow the path of the front tires. But as yaw begins the rear tires begin to follow a different path from the front tires. This divergence shows the initiation of the yaw rotation. As the rotation progresses the vehicle reaches a point where it is sliding sideways and on approach to this sideways position the tire marks converge: the left-front converges onto the right-front and the left-rear converges with the right rear. So when we see that this convergence reaches a point where only two tire marks are visible we know that the vehicle has reached a point where it is sliding sideways. So in the above photo we see that the vehicle is in an advanced stage of rotation because the tire marks have converged so much they the four tire marks have almost come down to just two. We leave this discussion now for fear of losing the readers’ attention with too many details.

In summary, almost all instances of vehicle loss-of-control rollover result in some form yaw rotation that is very often evidenced by visible yaw tire marks. These tire marks have very distinctive characteristics. Much like all physical evidence in a motor vehicle collision a detailed focus on the characteristics of the evidence can help to explain what transpired even when persons reporting the “facts” do not provide an accurate description of what occurred.