Expertly Speaking

Perception – Reaction – Braking

Hugh Borbidge, BSME, Director of Engineering Animations



We, as vehicle operators, avoid collisions every day. We stop for red lights, brake when a car cuts us off, and slow down if there are children playing close to the roadway. In these avoidance situations, there are three components: Perception, reaction and braking.

Perception is when we first notice a hazard or the need to react to something. Reaction is the time taken to respond to what was perceived. A typical daytime perception/reaction time for a vehicle operator is about 1.5 seconds, meaning that, when you notice a hazard you keep traveling at the same speed for 1.5 seconds before you start applying the brakes. Braking is the minimum time it takes to bring the vehicle to a stop. Every bit of time in this series of events equates to a certain distance traveled. For example:

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Watch Out for Off-Tracking!

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

Before departing a trucking facility, a loaded tractor-trailer parked in the “cars only” parking lot for office employees. The tractor-trailer operator went into the office. When he returned to his truck and pulled away, he struck the parked car of an office employee. Why did this happen? Besides driver error/inattention, the off-tracking of the trailer’s tires relative to the tractor’s tires led to the crash.

During a turning maneuver, a trailer’s tires will always track to the inside of the tractor’s tires. In other words, the trailer’s tires will follow a smaller radius than the path followed by the tractor’s tires while turning.

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Tracking Uber Rides

Timothy R. Primrose, Mobile Forensic Analyst

There are approximately three billion smartphone users worldwide. Smartphones can provide users with turn by turn directions to a given destination via applications such as Apple Maps, Google Maps, Waze, and others. When data is forensically extracted from a smartphone that used one of these applications, a list of GPS coordinates can be produced that will illustrate the exact route the cell phone user/vehicle operator took.

A recent data extraction conducted on an Apple iPhone 8 showed that the Apple Maps application was activated for a trip, which marked GPS coordinates every 1 to 4 seconds. Data from the same device, and it’s *Knowledge C database, provided details of when the phone was unlocked and when the Uber application was opened.

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Crush Analysis

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

Since 2013, for most vehicles equipped with event data recorders (EDR) that record 5 seconds of pre-crash speed, performing a crush analysis to figure out vehicle speeds has become increasingly unnecessary for performing a reconstruction. However, there are instances where a crush analysis is still warranted. For example, the vehicles involved may be older and/or do not have an EDR; the vehicles involved may have been repaired with key airbag system components replaced (EDR is typically a function of the Airbag Control Module (ACM)); or the vehicles involved may have been salvaged/scrapped/crushed and are no longer available for data extraction.

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Don’t Always Blame the Tread and Riser for the Fall

Robert J. Nobilini, Ph.D., Biomechanical Engineer

Case Synopsis: Plaintiff testified that she was in the process of descending a flight of stairs, one step at a time. She had stepped down onto the first tread with both feet and was stepping down onto the second tread with her right foot when her foot went out in front of her. The heel of her foot barely contacted the front edge of the second tread, and she fell backwards down the rest of the stairs.

Expert Analysis: Plaintiff’s expert opined that the stair treads were shorter than what was required by present code and that there were inconsistencies in the tread and riser dimensions. He concluded that the plaintiff’s fall was due to these issues.

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Ladder Expert Witness

Ladder Fall

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

Case Synopsis: While working on a commercial building, the plaintiff fell while descending a ladder from a first-floor roof. According to the plaintiff, he setup the ladder on a sidewalk outside of the rear door to the building the morning of the incident. The plaintiff reported the ladder was placed at the proper angle and was secured by several bundles of shingles at the base. While working alone atop the roof, he needed to obtain additional supplies, requiring him to climb down the ladder. As he started to descend the ladder, the base of the ladder began to slide, causing him to fall and sustain injury a knee injury. The plaintiff claimed that, after his fall, he realized the shingles he placed to stabilize the ladder had been moved. He alleged this was done by an employee of the building owner.

The roofing company supervisor arrived at the site after the incident and testified he saw the top section of an extension ladder laying on the ground in the area of the fall. He further testified the injured employee had been told earlier not to use the upper section of extension ladders during the project. The roofing employee’s attorney also presented an engineer’s report indicating the way the roofer testified he set the ladder would result in a stable ladder, but the removal of the shingles from the base would allow the base to slide out, as the plaintiff alleged.

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