Motorcycle Accident Reconstruction: Applicable Error Rates for Struck Vehicle EDR-Reported Delta-V

I’m in the midst of preparing two articles and a presentation for the upcoming EDR Summit in Houston (March 4-6, 2019). My topic is the use of struck vehicle EDR-data when reconstruction motorcycle-versus-passenger vehicle collisions. I thought I would put a short teaser introducing the topic I’ll be covering. I hope even this short teaser will give you some useful information. I’d love your thoughts on this topic. Read this post and then reach out to me at nrose@kineticorp.com or leave comments on the post. Thanks and I’ll see you in Houston!

Introduction

A common motorcycle crash scenario occurs when a passenger vehicle equipped with an event data recorder (EDR) turns left across the path of a motorcycle and is struck by the motorcycle. The EDR data on the passenger vehicle will often be accessible with either the Bosch Crash Data Retrieval (CDR) system or the Global Information Technology (GIT) system. In these instances, pre-crash EDR data can be useful for establishing the specific characteristics of the left turn that preceded the collision. This data may include speed, throttle percentage, brake applications, and steering angles for the struck vehicle. In addition to that, an EDR-reported change in velocity (delta-V) from the struck vehicle can potentially be used to infer the delta-V and impact speed of the motorcycle.

There are issues that can arise when using the struck vehicle delta-V to infer the delta-V and impact speed of the motorcycle, particularly related to the large weight ratio that often exists between the motorcycle and the struck vehicle. These issues are covered by Niederer [1990] and McNally and Bartlett [2002]. Suffice it to say here that small errors in the struck vehicle delta-V can potentially translate to large errors in the calculation of the motorcycle impact speed. For this reason, it is important to understand the level of error that may be present in a passenger vehicle EDR-reported delta-V.

How Accurate is the EDR-reported Delta-V?

I am not going give a full answer to that question in this post. For that, you’ll have to check out my presentation in Houston or read the articles in Collision that should come out not to long after the Summit. Here, I simply want to point out that the error in an EDR-reported delta-V can vary widely, depending on what error sources come into play.

The following error sources can affect the accuracy of the EDR-reported delta-V:

Error Source #1: During an impact, there can be a delta-V that occurs prior to the accelerations exceeding the event-triggering threshold (algorithm enable or AE). This is an error source that contributes to under-reporting of the delta-V.

Error Source #2: The recording window may be too short to capture the full delta-V. This error source contributes to under-reporting of the delta-V.

Error Source #3: The recording window may be too long and a delta-V experienced by the vehicle due to post-impact tire and dragging forces could be recorded. This error source could contribute to over-reporting of the delta-V. 

Error Source #4: The peak accelerations during a collision can exceed the capabilities of the accelerometer in the airbag control module (ACM) where the EDR resides, in which case the system will not capture the peak accelerations. This error source contributes to under-reporting of the delta-V.

Error Source #5: The ACM may reside some distance from the center of gravity (CG) of the vehicle. This is relevant because accident reconstruction calculations often calculate and utilize the CG delta-V. In instances where the collision induces significant rotation, the delta-V of the struck vehicle – which is measured at the ACM location – may need to be adjusted to accurately reflect the delta-V at the CG. Bundorf [1996], Marine and Werner [1998], Rose [2007], and Haight [2013] describe methods for making this adjustment.

Error Source #6: Physical damage to or displacement of the ACM can affect the accuracy of the reported delta-V. A vehicle involved in a collision can be split into two regions – a deforming or crushing region and a non-deforming region [Emori, 1968]. The occupants would ideally be contained within the non-deforming region of the vehicle and it is the delta-V of this region of the vehicle that the accident reconstructionist would typically be calculating in their analysis. When a collision is severe enough that the ACM ends up being within the crushing region of the vehicle, the accelerations and delta-V reported by this module will not be representative of the accelerations and delta-V experienced by the non-deforming region of the vehicle.

Error Source #7: The EDR could lose power before the collision is complete and some of the data may not get recorded. When this occurs, the system would under-report the delta-V. 

None of these are measurement errors, per se. The first four are hardware limitations and their effects will often be detectable through analysis of the CDR report. A reconstructionist will sometimes be able to correct for them. The fifth error source is simply something that the reconstructionist will need to account for in cases where there would be a significant discrepancy between the experienced at the ACM and that experienced at the CG. This will typically be instances when there is significant post-impact rotation. The sixth and seventh error sources amount to physical damage occurring to the measuring device itself or the power source. Errors in the reported in instances where there is actual damage to the ACM are not surprising and the reported may not be useable in those instances. For reconstructing motorcycle collisions into EDR-equipped vehicles, this list can be treated as a checklist of error sources that the reconstructionist could ruled out or account for through analysis. If these error sources can be ruled out through physical inspection of the struck vehicle and through analysis of the CDR report, then the potential error in the EDR-reported can be limited just to measurement error, which of course will be a smaller error than if one or more of these error sources is present.

In my presentation at the summit, I’ll cover studies that exhibit the effects of each of these error sources and address the potential magnitude of error in the struck vehicle delta-V if these error sources can be ruled out. I’ll see you there!

References

Niederer, P.F., “Some Aspects of Motorcycle-Vehicle Collision Reconstruction,” SAE Technical Paper 900750, 1990, doi:10.4271/900750.

McNally, Bruce F., Bartlett, Wade, “Motorcycle Speed Estimates Using Conservation of Linear and Rotational Momentum,” 20th Annual Special Problems in Traffic Crash Reconstruction, IPTM, Jacksonville, FL, April 15-19, 2002.

Bundorf, R.T., “Analysis and Calculation of Delta-V from Crash Test Data,” SAE Technical Paper 960899, 1996, https://doi.org/10.4271/960899.

Marine, M.C., Werner, S.M., “Delta-V Analysis from Crash Test Data for Vehicles with Post-Impact Yaw Motion,” SAE Technical Paper 980219, 1998, https://doi.org/10.4271/980219.

Rose, Nathan A., Beauchamp, Gray, Bortles, Will, “Quantifying the Uncertainty in the Coefficient of Restitution Obtained with Accelerometer Data from a Crash Test,” SAE Technical Paper 2007-01-0730, 2007, https://doi.org/10.4271/2007-01-0730.

Haight, S., Haight, R., “Analysis of Event Data Recorder Delta-V Reporting in the IIHS Small Overlap Crash Test,” Collision 8(2): 8-23, 2013.

Emori, R., “Analytical Approach to Automobile Collisions,” SAE Technical Paper 680016, 1968, https://doi.org/10.4271/680016.