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 experts@forensicDJS.com or via phone at 215-659-2010.
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