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.
Tire Skid Marks & Causality
By: Patrick Sundby, Accident Investigator
Specializing in Low Speed and Catastrophic Crashes
Mark Studin DC, FASBE(C), DAAPM, DAAMLP
Not only do tires play a critical role in the performance of your vehicle, but a lot of information can be garnered about what happened before, during, and after a crash. We will explore tire marks and, generally, what those marks tell us.
First let’s talk about where the marks come from. Skid marks are created by the intense thermal relationship of a tire against the roadway surface during extreme stresses put on the tire, a simpler way to say this is a tire will “marks” when it nears, or exceeds, the limits of its relationship with the roadway. These marks happen because the oils in the roadway and/or the tire(s) are brought to the surface and “burned or melted” to the roadway. If a tire is heated enough it will be obvious as the surface of the tire will have changed, it will have a flat spot and obvious abrasions.
Types of skid marks.
There are three specific types of marks we will discuss, these are the most common four wheeled car and light duty truck marks. (Other vehicles, such as motorcycles, have different specific marks)
Light to dark or dark to light.
All marks can be put into two categories when referencing the direction of the vehicle which made them. Light to dark marks (in the direction the vehicle was traveling) support a vehicle making the marks during some form of deceleration (extra points if you wanted to read “negative acceleration”). Dark to light marks (again, in the direction the vehicle was traveling) support a vehicle making the marks during some form of acceleration, usually excessive wheel spin.
Darker in the middle, darker on the outsides, or uniform.
Marks which are darker in the middle indicate a tire which is overinflated, conversely marks which are darker on the outside edges indicate a tire which is underinflated. Marks which are uniform in nature indicate a properly inflated tire.
Please read the 2 articles on tires to understand the significance of either over or underinflated tires.
ABS verses standard marks.
ABS (Anti-lock Brake System) marks are lighter than standard marks and have more tire tread definition in them, Non-ABS marks rarely have tread definition in them. ABS marks are also shorter when compared to non ABS marks from a vehicle traveling at the same speed.
What else skid marks can tell us?
As you have already discovered skid marks can tell us about the tires inflation, ABS or non-ABS braking, and direction of travel. Skid marks can also tell us something else, when and where the decision to brake occurred. This is perhaps the most under-utilized and under explored aspect of collision reconstruction – even more so in lower speed collisions. Some basic calculations can be made, using various aspects of the skid marks, to determine where the driver made the decision to brake.
Why is this so important? Consider the following illustration.
This drawing is a classic teaching example used to show the value of skid marks. Consider this scenario, the blue car says he had the green light and was hit in the intersection. The red car says he also had the green light and saw the blue car so he braked hard. There is no other evidence or witnesses to further determine the cause.
Now the student would be asked to calculate the position of the cars when the decision to brake was made using the start of the skid marks, ultimately this would place the vehicles in the position labeled 1.
Now the obvious problem with the red car’s scenario now that we have used the skid marks to determine where he decided to brake, a building blocks his view of the blue car (position 1 for both vehicles). This begs the question as to why did he decide then to brake? The answer, the light was red for the red car and the driver was braking for traffic light, not the blue car making the red car culpable in this scenario as the physical evidence verifies the “at fault” party.
Another valuable piece of information is that rubber is biodegradable and there are naturally occurring nitrogen based bacteria that “eat” rubber. These bacteria are aggressive and will eat rubber in most environments, therefore if you are attempting to determine causality and the “at fault” party, it is in your best interest to take pictures of the roadway as soon as possible. Often skid marks are gone in a relatively short amount of time.
Skid marks are a valuable piece of evidence and a great tool for determining many factors in a collision; it’s very important none of them are overlooked or underestimated.