Expertly Speaking

Airbag Deploy

To Deploy or Not to Deploy – That is the Dilemma

Robert T. Lynch, P.E., Principal Collision Reconstruction Engineer

The purpose of an airbag in a vehicle is to offer an increased level of protection to occupants in a crash. To be able to do this the airbag needs to deploy in crashes where added protection is needed, but not to deploy in crashes when the risk to the occupant would outweigh the benefit. Generally frontal airbags will deploy if the Delta-V, or change in velocity of the vehicle, exceeds 8-14 miles-per-hour (MPH). However, there are scenarios when airbags have deployed in collisions where the Delta-V was less than 8 MPH, or airbags did not deploy in collisions where the Delta-V was above 14 MPH. In these latter instances, a claim for failure to deploy is often made by an injured party.

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Dealer or Buyer: Who Controls the Used Vehicle?

R. Scott King, BSME, CFEI, Principal Automotive/Mechanical Engineer

Are you in the market for a used vehicle? If so, and if you’re thinking of financing that purchase, you may want to continue reading. What you learn here could stop you in your tracks.

In lecturing and authoring on the topic of onboard crash data recording technology for over twenty years, I have come to learn that many people simply do not welcome the concept of their vehicle recording and storing data describing their driving habits and actions – the “Big Brother Bristle,” I have come to call it. However, more recent vehicle technologies do nothing to assuage those concerns. This is because most new vehicles can now facilitate two-way, remote communications between the vehicle and, frankly, anyone that’s listening.

Telematics: In very broad terms relates to the ability for onboard vehicle technologies to send and receive data anywhere, anytime. First used routinely by trucking companies to assist with GPS routing, load procurement, and driver communications, telematics provided improved efficiencies, safety, and customer satisfaction. Although originally installed as after-market solutions, truck chassis manufacturers soon incorporated telematics into their original build configurations, providing new opportunities for real-time vehicle diagnostics, crash data transmission, and more. Since then, the use of telematics has expanded into the passenger vehicle segment, providing similar capabilities, among the most remarkable of them is the ability to control a vehicle remotely.

GM’s OnStar system provided the first mass-produced “telematically controlled” vehicle. To be precise, the potential control that the OnStar system had on a vehicle was limited. Specifically, the system could limit a vehicle’s speed by taking control of its computerized fuel and ignition systems. The benefit here was for law enforcement, assisting with stolen vehicle investigations and child abductions. Most recently, however, used vehicle finance companies have begun using telematic technology to help combat payment delinquency and loan defaults. Here is how it works:

A used vehicle is purchased and financed, typically by subprime lender. As part of the finance agreement, the purchaser consents to the installation of a telematic device that can not only track and pinpoint the vehicle’s location but can disable its starter as well. Systems vary on functionality. For example, most vehicles provide warnings of varying duration, and the scope and resolution of GPS data collected can vary. However, those are the essentials: default on a loan and the car will not start. Where and when a planned disablement occurs is not certain nor is the workmanship involved in the system’s installation. Deficiencies in either could lead to unintended consequences and unique forensic investigative strategies can be deployed to evaluate their potential relevance to a specific circumstance.

R. Scott King, BSME, CFEI, Principal Automotive/Mechanical Engineer with DJS Associates, Inc., can be reached via email at or via phone at 215-659-2010.

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School Bus Lift

Wheelchair Lift Won’t Quit Its Run

John R. Yannaccone, P.E., Senior Mechanical Engineer

Case Synopsis: One morning, a wheelchair-bound student was being picked up by a school bus equipped with a commercial wheelchair lift. As the lift containing the wheelchair-bound student was being raised up to the bus, the lift continued to run and then began to fold to a stowed position. The attendant inside the bus was able to get the student safely into the bus as the lift was folding. The driver, who was operating the lift, caught her arm in the mechanism while attempting to prevent the student from falling as the lift folded.

Expert Analysis: Inspection of the wheelchair lift revealed it was in good condition, but did have some damage from extrication of the driver’s arm. The electrical components of the lift were inspected and tested, which failed to show any problems that would explain why the lift continued to run when the operator released the controller. Additional inspection of the solenoid, which provides power to the motor for the hydraulic pump, showed evidence of arcing and possible welding on the contacts. Welded/sticking of this contact would continue to supply power to the pump motor, and cause the lift to continue to run.

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Tiger Woods Crash Reconstruction

Initial Tiger Woods Reports Are Misleading

Justin P. Schorr, Ph.D., Principal Collision Reconstruction Engineer

This analysis is based on publicly available data as of February 25, 2021.

Narrative 1: Dangerous stretch of roadway, lots of curves, lots of crashes. FALSE

The first piece of physical evidence consistent with a “loss of control” of the Genesis is found on the center median strip separating the northbound lanes of Hawthorne Boulevard from the southbound lanes. Taking this as the first point where we can say that the collision sequence had begun, the approach to this location is more or less a straight shot for just under 900 feet. The tip of the median is at the start of a long, gradual bend in the roadway to the right – though this bend has a critical speed in excess of 130 miles per hour (the maximum speed at which passenger vehicle operators could, if they so desired, negotiate the turn without leaving yaw marks) and accordingly is not of the nature which would cause an operator to lose control of their vehicle.

It should be mentioned that Hawthorne Boulevard northbound, in this immediate area, is on a downgrade which approaches 10%. This downgrade, while steep, can still be safely and easily navigated consistent with data from the California Statewide Integrated Traffic Records System (SWITRS) – a statewide database maintained by UC Berkeley – which shows that there were no other substantially similar collisions within an approximately ½ mile radius for the most recent 10-year period in which crash data is available (2010 – 2019).

[*This database includes, apparently, all California collisions where an injury, including minor ones, was reported (approximately 720,000 collisions between 2010 and 2019 in Los Angeles County). Within the noted radius (which does NOT include the intersection of Hawthorne Boulevard and Palos Verdes Drive), there were no reported collisions in the northbound travel direction (Tiger’s direction of travel) and there was one reported collision in the southbound travel direction. The southbound collision occurred in 2010.]

Here is a map showing the location of Tiger’s collision along with those in the Statewide Database between 2010 and 2019:

Tiger Woods Crash Analysis


Narrative 2: This was a “rollover” collision, as noted in many headlines.  FALSE

Based on the physical evidence available at this time, including photographs and video of the roadway markings, the damage to the vehicle, and the point of rest location and orientation – this data is NOT consistent with the current characterization as a “rollover.”

While the Genesis does indeed come to rest on its driver side, this is indicative of the vehicle overturning at the tail end of the collision sequence and not with an extended rollover event, as it is currently being characterized in the news reports.  There are no roadway markings consistent with the vehicle overturning prior to departing the west side of the roadway (this departure point being approximately 100 feet from where it ultimately came to rest and approximately 300 feet north of where the vehicle crossed the center median), and the damage pattern to the roof of the vehicle (where there is almost no damage) suggests that the vehicle did not complete a full rotation at any point. 

Rather, provided the principal direction of force consistent with the damage to the front of the vehicle (front to rear and centered towards the passenger side) and the damage to the foliage along the west side of the roadway (it appears a large tree/stump has been uprooted as a result of being struck by the Genesis), the data is consistent with the Genesis striking a tree/stump with its front, passenger side and then subsequently rotating in a clockwise direction consistent with the laws of physics.  This rotation, coupled with the uphill topography immediately west of where the Genesis came to rest, would introduce a substantive rotational force about the z-axis and promote the overturn onto the driver side.  

Here, it is important to point out that the majority of the damage to the Genesis’s front end and the only overturning in the collision sequence both occurred within approximately 20 feet of where the Genesis came to rest.  The data is consistent with the vehicle being upright from the point of the loss of control until it struck the tree – or approximately 95% of the 400+ foot travel path across the median and southbound side of the roadway much discussed in current news articles.

Tiger Woods Crash Reconstruction


What to expect moving forward:

One final misleading narrative I am hearing is that since there are no tire marks along the roadway, this is somehow evidence that there was no attempt to slow the vehicle at any point prior to or during the collision sequence.  The vehicle has anti-lock brakes which (by definition) prevent the wheels from locking.  Tire marks are left, typically, by wheels which are locked and sliding along the roadway.  As such, the absence of markings provides little to no information regarding the brake status prior to or during this collision.  With that said, the police have almost certainly already acquired what is known as “event data” from the vehicle, which will detail, at a minimum, the 5 seconds prior to airbag deployment through various data elements including speed, steering wheel angle, and brake status.  Depending on what that speed is (and there is nothing in terms of physical evidence which necessarily establishes the vehicle was traveling above the 45 miles per hour speed limit), this data may or may not capture the moments which preceded the initial loss of control.  However, depending on the capabilities of the police department conducting the investigation, there is likely additional pre-crash data which can be accessed through the infotainment system given that this is a brand-new vehicle with what I assume is the most advanced technology package Genesis has to offer.

Justin P. Schorr, Ph.D., Principal Collision Reconstruction Engineer with DJS Associates, Inc., can be reached via email at or via phone at 215-659-2010.

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SUV v. Gas Station Engineering Animation

Hugh Borbidge, BSME, Director of Engineering Animation



Synopsis: Engineering animations are an important and useful tool to help explain issues to a jury. This is especially true when there may be a language barrier. For this case, DJS was tasked with completing an engineering animation to demonstrate an incident that occurred when an SUV reversed and struck a gas station attendant.

Scene photographs provided points of rest and a trail of damage left from the event. Witness testimony contributed approximate initial positions of the SUV and attendant with regard to the fuel pumps.

To-scale computer models were built of the SUV, gas station attendant, and the gas station itself. With the available data, we reconstructed the sequence of events and created animation clips showing what had happened. After the motion was finalized, the incident could be viewed from any vantage point. The client utilized two perspective views for this case.

Result: Case settled out of court.

Hugh Borbidge, BSME, Director of Engineering Animation with DJS Associates, Inc., can be reached via email at or via phone at 215-659-2010.

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The Need for Speed: Obtaining Speed From Video

James R. Schmidt, Jr., BSME, Collision Reconstruction Engineering Analyst

You guessed it! If Jim Schmidt is writing something, it must be regarding speed from video! That’s because I’m passionate about what I do! They don’t call me “Captain Video” for nothing.

A new year. A new dash cam. A routine drive home in the evening. One thing leads to another, pushpins were plotted, speed was calculated, and wouldn’t you know it … the same results! Perfection!

Pushpins were plotted for 59 seconds and 0.6 miles of travel. A speed versus time profile was generated from this plotting, and a comparison was made with the GPS speeds shown on the dash cam display. Assuming for this example the GPS speed display to be the actual speed of the vehicle (which is a reasonable assumption for normal driving with gradual speed variations), the independently calculated speeds were within 2 mph of actual over this entire time and distance. Additional pushpins could be plotted to refine this even further.

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