Expertly Speaking

Reverse Engineering the Crash that Never Happened

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

He was on a routine drive home just before midnight on a Saturday night. Traveling westbound, he stopped at a red light at a T-intersection. The light changed to yellow … one-one thousand, two-one thousand, three-one thousand, four-one thousand, then to red for the roadway running north/south, at which he was stopped … one-one thousand, two-one thousand… Then, the light changed to green, and he proceeded into the intersection to turn left. One-one thousand, two-one thousand, three-one thousand, four-one th… From his left, a speeding car ran the red light, and plowed his vehicle on the driver’s side. That can’t be good, can it? Miraculously, he survived, without a scratch. How do I know? How is this possible?

Well, I am the “he” that I write about; and, I survived without a scratch, as the crash never actually happened. Two weeks earlier, I was at that same intersection, at that same time of night on a routine drive home. Two weeks earlier, the light changed to green, and I proceeded into the intersection to turn left, and I did, in fact, turn left. No incident occurred. Nothing to speak of even. Fast forward two weeks later when I was at that intersection again and the light turned green. I started to proceed, but then stopped abruptly – almost as soon as I had started, as I saw the speeding car coming from my left, not stopping. After the speeding car blew through the intersection, I proceeded to make my left turn and continued home, uneventfully. But don’t think for a moment that I didn’t realize how lucky I was. I thanked my “lucky stars” for keeping me safe that night.

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The “Oh Crap” Moment

Justin P. Schorr, Ph.D., Principal Collision Reconstruction Engineer
Animation by Hugh Borbidge, BSME, Director of Engineering Animations


In collision reconstruction, the “oh crap” moment is the location, in both space and time, which represents a vehicle operator’s final opportunity to perceive, react, and avoid a hazard. Initially, you may think this is the same as the time required to perceive, react, and stop your vehicle – however, it is a bit more intricate than that. Identification of the “oh crap” moment requires an understanding of the time-space continuum and, while this may seem complicated, you don’t need to be Sheldon Cooper to understand.

In the accompanying animation we have an “observer” (the approaching vehicle) and a “hazard” (the pedestrian). We start by calculating how much distance is required for the observer to perceive, react, and stop based on:

  • The speed of the observer and the type of vehicle (often passenger vehicle or heavy truck)
  • A typical perception plus reaction time (often 1.5 seconds during the day and 2.0 seconds a night)
  • A typical coefficient of friction for the roadway (often 0.7 for a passenger vehicle on a dry roadway)

Now we know the location where a collision goes from avoidable to imminent. From here, the next step is to determine how long the hazard needs to be visible in order for the observer to have sufficient time and distance to perceive, react, and avoid it. We make this determination by calculating the time it would take the observer to travel from this distance without performing any evasive action (stopping, slowing, swerving, etc.). If the hazard is visible for longer than this time – then the laws of physics establish that the observer has not yet arrived at their “oh crap” moment. Bazinga!

Justin P. Schorr, Ph.D., Principal Collision Reconstruction Engineer and Hugh Borbidge, BSME, Director of Engineering Animations with DJS Associates, Inc., can be reached via email at or via phone at 215-659-2010.

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Mobile Forensics

Cell Phone Evidence Repair

Timothy R. Primrose, Mobile Forensic Analyst

There are three components of a cell phone that need to be functioning in order to extract data from the device: the screen, the charging port, and the battery/power.

The Screen: If a cell phone screen is damaged, be it the result of a car crash or voluntary destruction to hide evidence, it may appear that the phone no longer functions. The internal hardware where data is stored, however, may still be intact. Most smartphones require certain permissions to be accepted or selected on the device before data can be accessed or extracted, thus a working screen is imperative to selecting these permissions. Replacing a phone screen or a backlight fuse may suffice for this circumstance.

If more components were damaged in addition to the screen, forensic evidence repair technicians will analyze all of the internal components and connectors to determine the problem.

Charging Port: Forensic investigators typically need access to the charging port of a cell phone for cable connection and data extraction. Small repairs may entail component fixes with the use of a soldering iron or a heat gun. However, if the port is crushed or damaged beyond use, it may require swapping the damaged port for another.

The Battery: Before plugging in or powering on a damaged cell phone, the battery must be inspected. If a battery is damaged or has a bubble in it, it may catch fire, which can damage a device beyond repair. In the event of a fire, the phone and battery should be placed in sand to extinguish the fire, not water.

Cell phones have liquid damage indicators that are designed to turn red in the presence of moisture. If the liquid damage indicator is red, the phone is corroded, or there is visible moisture, it does not mean the cell phone data cannot be extracted. A wet cell phone is not the end of the world, as long as it is not powered on or used while it is wet. If the device is powered on while it is wet an electrical current will cause components on the circuit board to short or burn out. So, if a cell phone is discovered in water, it is best to leave the cell phone in water until a repair technician can properly dry the components. This will assist in preventing corrosion due to the exposure of oxygen, though forensic technicians can remove corrosion by cleaning and careful treatment.

If a device is damaged beyond repair, a simple method of data extraction will not be available. A destructive process involving the removal of the data chip from the logic board will be required. This data chip contains all of the data from the cell phone. Data is retrieved from this chip via a chip reader. This data extraction method, called Chip-off, is a last resort because the chip cannot be placed back on the logic board of the device.

Timothy R. Primrose, Mobile Forensic Analyst with DJS Associates, Inc., can be reached via email at or via phone at 215-659-2010.

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Apartment Complex

Flawed Repair Technique Leads to Injuries

Lt. Col. Bryan Smith, PE, Commercial and Residential Site Safety Expert

Case Description: Maintenance work was being performed at a multi-unit apartment complex to address several weather and damage issues on exterior staircases. A second-floor tenant was injured while descending her stairs. One stair-tread end section broke free from the improperly repaired support bracket as the tenant stepped on it, resulting in her ten-foot fall. She suffered severe injuries.

Expert Analysis: An analysis was performed after reviewing site photographs provided by the plaintiff’s attorney, as well as copies of the apartment complex’s maintenance/repair invoices. The evidence showed that the original stair-tread support brackets had been severely rusted, while the repair technique chosen by the contractor called for the installation of new Simpson Strong-Tie brackets as reinforcement to the existing rusted brackets.

Instead of removing and replacing the rusted brackets on the underside of each tread end (the existing stairs were constructed of only two stringers – one at each tread end), the repair contractor left the existing rusted brackets in place and installed the new brackets on the top side of each tread. This placed the attaching bracket screws of the new brackets in tension to pull out of the tread top surface, instead of being in shear while the new brackets carried the load. A rusted bracket subsequently gave way on one tread end as the tenant stepped onto it, thereby transferring the full load of the tenant onto the new bracket’s screws. As the wooden tread itself was weather-pressured treated wood, the new bracket’s screws easily pulled out of the top tread surface and failed. The tenant, with trash bags in her hands, fell through the opening created by the fallen tread. The tenant then fell headfirst down the remaining stairs. The analysis proved that the contractor’s repair technique was flawed.

The apartment complex owners relied upon the contractor’s skill to make proper repairs instead of engaging a design professional to review/approve of potential structural repair techniques, and thereby better protecting their interests as well as their tenants. Additionally, a recommendation was given to the apartment complex owners to add middle stair stringers (in addition to correcting the Simpson brackets installation) in order to increase the stringers’ ability to resist a similar failure in the future.

Result: Case settled

Lt. Col. Bryan Smith, PE, Commercial and Residential Site Safety Expert with DJS Associates, Inc., can be reached via email at or via phone at 215-659-2010.

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Thinking Outside the ‘Box’ for Pedestrian Safety

Robert T. Lynch, PE, Principal Collision Reconstruction Engineer

Vehicle manufacturers market their advanced safety features to keep occupants safe, but what about the safety of those who are outside the vehicle?

Advanced driver assisted technologies such as pedestrian detection systems and automatic emergency braking are designed to help mitigate pedestrian collisions or eliminate them altogether. But these systems are limited in their ability to actually detect pedestrians, especially at night.

Over the past decade, pedestrian fatalities have increased by approximately 50% at night (and 15% during the day).

Current systems rely on cameras and radar to determine if there is something in front of the vehicle. In low-light environments, the cameras, like our eyes, struggle and radar systems have a high degree of false positives and false negatives to pass as being consistently reliable.

To improve upon the current systems, lidar sensors can be installed on vehicles to map the environment in real-time, and then software can predict the paths of both the pedestrian and the vehicle. If a collision course is imminent, automatic emergency braking will intervene. Lidar sensors have a clear benefit to camera-based systems because they are not sensitive to light and work equally well at night as in the day. However, installing lidar sensors in vehicles is still cost-prohibitive. As manufacturers continue to push the limits of partial automation, lidar systems are expected to become more and more prevalent in vehicles. Perhaps then we will see a significant decrease in pedestrian impacts at night.

Robert T. Lynch, PE, 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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Tractor Trailer Fire

What Investigating Fires on Commercial Tractor-Trailers Requires

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

Because commercial tractor-trailer combinations possess a wide variety of electrical, mechanical, and hydraulic systems and components, investigating fires that occur on such a vehicle requires not only an understanding of fire science, but knowledge of the systems unique to commercial trucks. Experience has shown that one of the most common causes of commercial tractor-trailer fires is related to the braking system.

Brake Chamber

In particular, commercial tractor-trailers utilize what are commonly referred to as “spring brakes.” Spring brakes, or perhaps better described as parking brakes, engage when air pressure is removed from the parking brake actuator. Normally, this is what happens when a driver stops the truck and applies the parking brake. By activating the parking brake valve, air pressure is vented from the brake chamber and a powerful spring within the chamber extends. The brake shoes expand to hold the brake drum and wheel from rotating.

Failures within the system can occur that cause the spring to partially extend while the truck is moving without the parking brake applied. As a result, the brake shoes contact the rotating brake drum with just enough force to cause rubbing friction between the shoes and drum. Over time, this friction creates heat, which in the extreme case, can result in what the industry calls a “wheel fire.”

Wheel Fire

Wheel fires can often be confirmed through witness testimony and physical evidence. In addition to exterior fire, heat, and smoke patterns, wheel fires resulting from dragging brakes can be confirmed by removing wheel and brake drums for a detailed examination. During this removal, other potential causes, such as improperly lubricated wheel bearings, can be examined. With this information, fire investigators can identify whether factors such as manufacturing or vehicle maintenance are in any way relevant.

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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