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Crash Dynamics and Accident Reconstruction Q & A's

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Crash Dynamics and Accident Reconstruction Q & A's

By: Patrick Sundby, Accident Investigator

Specializing in Low Speed and Catastrophic Crashes


1. “How do airbags work and why do they deploy in some cases and not others.”

Almost all airbag equipped vehicles contain an airbag control module.  The module monitors various vehicle systems and has a predetermined threshold for deployment; in simpler terms, this means the collision has to meet certain settings to deploy an airbag.  While each car brand’s system is specifically different from the next the concept is the same.

The module constantly monitors a vehicles speed and when a collision occurs the module can tell the change in speed is happening faster than if the car was slowing by brakes alone.  IF the collision, as calculated by the module, is extreme enough it will deploy the appropriate airbag(s).  The module has the final say in why an airbag is deployed, this is truly vehicle specific as well as module software & hardware dependent.

The module can knows, via onboard accelerometers, of changes in the vehicles direction and speed.  The module constantly calculates these changes and when it “sees” a change beyond preset thresholds it begins to monitor, very closely, the changes (this is called algorithm enablement).  If it determines the changes meet the criteria for airbag deployment it will deploy the appropriate airbag(s).

Many vehicles also have failsafe sensors mounted in the vehicle which are designed as a secondary mechanical and/or electrical triggering system.  These sensors are mounted on the front of the vehicle, usually under the radiator, when crushed or damaged they force an airbag deployment.    

Occasionally, someone will ask if how a vehicle knows if a seat is occupied.  The driver’s seat is obvious, beyond this, the front passenger seat has a pressure sensor in it which can tell when a predetermined amount of weight is on it, and the rest of the seats use the seatbelt latch (vehicle specific).  When you are driving a vehicle the module also monitors the status of seatbelts and the pressure sensors, it uses this data to make the best decision possible about which airbags to deploy and when.


2. I’m often asked about a specialists report, but the most common subset questions are about the lack of support for findings in the report.  I have chosen to address this question because it’s of personal & professional interest to me.

“I got this collision expert’s report but there doesn’t appear to be any explanation for his findings, is this normal?”

Yes and No.  Yes, this happens; no, it’s not acceptable standard.  One of the reasons I have chosen to work with Dr. Studin is his tenacious commitment to research.  If you have seen Mark present you know he has scholarly research to back up his points.  Mark and his colleagues have been through accredited and standardized training based on a lot of scholarly research.  All professional fields of post primary education are all based in accredited & scholarly formal standards. 

Collison reconstruction specialists are no different.  While not necessary part of an undergraduate or graduate program, the training and education they have is based on the same accredited & scholarly formal training and education - because of this correlation, the same standard should be applied to collision reconstruction specialists.  Scholarly research is based on objective methods of testing and investigation, peer review, and rigorous scrutiny before being accepted. 

When an expert offers an opinion without citing supporting scholarly documentation it’s not worthless, but rather it stands alone; it is only his opinion.  Conversely, when an expert offers and opinion with appropriate supporting scholarly documentation, all the work, expertise, and research is offered with his opinion.   

3. Often times an appraisal for repairs is used to justify “low speed” by citing minimal costs.  There are a few points regarding them to consider so the question is: 

“Is the listed cost on the appraisal an accurate reflection of damage?”

The short easy answer is “no”.  The long answer starts with understanding who did the appraisal and what is there background?  Usually, appraisers are trained by the insurance company – as such, minimizing the costs and expenses of repair is in the insurance company’s interests.  Secondly, most appraisers do not disassemble a vehicle to determine if there is any hidden damage, particularly in low speed collisions.

The next problem is when replacement parts are needed where do they come from?  Original Equipment Manufacturer (OEM) parts cost substantially more than Equal or Like Quality (ELQ) parts, as such, ELQ parts are the preferred choice of insurance companies.  It would cost the industry millions more to use OEM parts instead of ELQ parts when making repairs.  Along this same line, the quality of paint also varies.  Paint manufacturers offer paint systems which will meet the OEM specifications and are very durable paints, however, they also offer more economically friendly paint which is not as durable or closely color matched to the original, and as expected, it costs less.

The last problem to discuss is job downtime.  The longer a vehicle is in for repairs the more it costs the insurance company in rental fees.  While a shop can, and will, have a minimum amount of time to fix the vehicle the insurance company is going to keep them on this timeframe and constantly press for the vehicle to be completed.  Sometimes this drive can create an environment where the repair facility will sacrifice quality of workmanship to complete the job faster for a better profit margin.

The above variables greatly dictate the final number making it too subjective for a reliable point to support the threshold of injury; in other terms, the use of “low cost” as a justification for no injury is not appropriate as no causality relationship exists.  If a breakdown of the repair bill is provided, you could objectively price the repair parts and effectively show the bias towards reducing the cost of the repair.  


Patrick Sundby has decades of experience in the automotive industry including several years in law enforcement collision investigation. He has also been a driver training and firearms instructor in law enforcement and a police officer for 9 years before specializing in accident investigations. He has had the privilege of participating in both learning and teaching at Prince William County Criminal Justice Training Academy in Virginia and studied at the Federal Law Enforcement Training Center in Georgia. His specialty is low speed and catastrophic crashes and has testified over 500 times at various level. He can be reached at 571-265-8076 or

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