Posts Tagged ‘accident’

Contributing Factors: The T-Bone

Friday, November 7th, 2008

There are a wide variety of contributing factors associated with vehicle accidents, and careful analysis is required to fully and accurately determine fault because of these many factors. For instance, let’s consider the case of a “T-Bone” type collision.

In this hypothetical collision, there is a Chevrolet traveling south on Hwy 11 when a Nissan pulls out from a side road into the path of the Chevrolet, and the Chevrolet impacts into the passenger side of the Nissan. Who is at fault in this collision? That can depend on several factors. Below is a partial list of possible contributing factors for each vehicle.


Nissan Chevrolet
Disregarded a Stop Sign Speeding
Failed to Yield the Right of Way Inattentiveness
Headlights Not Illuminated Headlights Not Illuminated

Only through a careful and thorough investigation can fault be accurately assigned. Our accident reconstructionists can typically confirm or disprove which of these were in fact the cause(s) of the accident. The following are various questions about contributing factors that you might have and the corresponding methodology we have available to answer those questions.

Question: Did the driver disregard the stop sign?

Answer: Based on impact speed and acceleration calculations we can determine whether it is reasonable for a vehicle to reach its speed after stopping at the stop sign.

Question: Was the vehicle speeding?

Answer: We can do various calculations to determine speed based on damage patterns, departure angles, and final rest locations. Also, we may be able to download crash data recorder information that can answer this question.

Question: Was the driver paying attention?

Answer: Based on speed/time/distance calculations, we can determine the approximate location where and the time when the driver perceived the hazard of the other vehicle and began to react. Then, we can attest to whether or not this information is consistent with a typical and attentive perception and response.

Question: Were the vehicle’s lights on?

Answer: In most cases this can be determined by forensic evidence that can be documented and collected.

Other questions that we consider and have the ability to analyze and address include:

· Was fog present creating a sight distance and headlight issue?

· Did sun glare obstruct or limit the view of the driver?

· Were the ambient light conditions such that the vehicle would have been visible?

· Were the proper traffic controls in place and appropriately located?

· Did the environmental conditions call for a reduction in the reasonable speed to be traveling?

As you can see, it is not a simple matter to say one or the other party is at fault. It takes a conscientious consideration of many possible factors.

Jonathan McGehee

Preserving the Accident Scene

Monday, July 7th, 2008

Consider yourself in the following scenario: You contact us to investigate an accident that occurred a year ago. We arrive at the scene. We attempt to gather evidence, but there appears to be no evidence to collect. There are no skid marks, no gouge marks, no fluid spills and the road is repaved. Trees may have been cut down or trimmed. Road signs have been moved. The vehicles are gone, nothing was marked, and the police did not take any photographs. We attempt to locate the vehicles to inspect their damage and find that they have been repaired or salvaged. As you can imagine, our ability to replicate this incident has become increasingly difficult.


The previous example is extreme, but nevertheless emphasizes the importance of preserving evidence as soon as possible. In accident reconstruction, our analysis can only be as accurate as the evidence we gather. As time goes on the evidence degrades, and as the evidence degrades so does the ability to determine what happened with certainty. This fact puts us continually fighting against the clock because a significant amount of scene evidence is short-lived and fleeting. For instance, impending skid marks, ABS skid marks, debris patterns, and paint transfers are typically moved or nowhere to be found within a few hours or days.


After Longer Periods of Time the Following Can Also Be Altered or Removed:

  • Roadway Signs (especially in construction areas)
  • Roadway Drag Factor (further traffic degradation or re-pavement)
  • Sight Distance Obstructions (embankments, trees, parked vehicles, etc.)

This is just a partial list but it stands to emphasize the point.


When an accident occurs, consider the benefits of taking timely action.  Ideally we would be able to respond to the scene immediately after the original incident, especially if there is any indication that a comprehensive, professional investigation may be required. This gives us the opportunity to document the scene thoroughly and accurately.


We utilize various methodologies and technologies to document the scene including:

  • Quality Digital Photography
  • D.A.R.T. LX-2 Drag Sled
  • Crash Data Retrieval System
  • Sokkia Total Station
  • Nikon Reflectorless Total Station

VCE has been utilizing the Sokkia Total Station to measure accident scenes with great accuracy and precision for over 10 years. This instrument allows you to measure points at the scene in a three-dimensional framework based on distances and angles to produce X, Y, and Z coordinates that can be used with our software programs to create precise two-dimensional and three-dimensional scale diagrams, animations, and simulations. Just recently we have expanded our services to include a Nikon Reflectorless Total Station that allows measurements to be taken without the necessity of a reflector pole. This is a great asset when attempting to take measurements of a busy intersection or interstate where it would be impractical to stand in the roadway with a reflector pole. Furthermore it is ideal for measuring damage profiles of vehicles.


With our thorough scene investigation complete, the scene and evidence are recorded and saved. The facts of the particular case can be preserved for whatever future analysis might be necessary. With this information, you can make an informed decision about the requirement of further investigation based on our initial findings without the additional pressure of capturing the evidence before it is gone.


Jonathan McGehee

The Skid Mark / Crush Factor Method

Thursday, February 7th, 2008

When investigating an accident a common question that arises is, “How fast were the vehicles going?” If the skid distance of the striking vehicle and the maximum crush depth of the target vehicle are known, a simple estimate of the speed of the striking vehicle can be made. This is done by determining the impact speed of the striking vehicle by measuring the maximum crush depth of the target vehicle and inputting the distance in the Crush Factor Formula.

The Minimum Speed Formula uses the pre-impact skid distance of the striking vehicle and the roadway drag factor.

Both the Crush Factor Formula and the Minimum Speed Formula are then combined in the following way to determine the Striking Vehicle Start of Skid Speed.

30 - Mathematical Constant in the formula

d1 - Striking Vehicle’s Pre-Impact Skid Distance (measured in feet). Note: Measure the skid marks from the start to the point of impact (offset in the mark) and then subtract the wheelbase (front to rear axle distance of the skidding vehicle) from the skid distance.

f - The adjusted drag factor of the vehicle leaving the skid marks on the roadway surface. Note: if the vehicle is a passenger car, van, SUV or pickup truck and all four wheels left skid marks and the roadway was level, the roadway coefficient of friction is the vehicles drag factor. For a dry traveled asphalt surface the coefficient of friction is usually within the range .6 to .8 g’s. If the roadway surface is wet or has gravel on it, the coefficient of friction can be significantly less.

d2 - The target vehicle’s maximum crush depth (measured in feet from the normal undamaged position to the maximum permanent crushed position) of either the side or rear surface. Note: this calculation can not be used for head on collisions. This may only be used for t-bone or rear end collisions.

cf - The crush factor of the target vehicle is vehicle specific, but the average crush factor for the side and rear surface is 27. The crush factor values are based upon statistical analysis of 1000 vehicles involved in accidents where the speeds of the vehicles were verified by independent means.

The following example illustrates how the combined speed formula works:


The striking vehicle left 59.5 feet of pre-impact skid marks and had a wheel base distance of 9.5 feet. Subtracting the wheelbase distance from the total skid mark distance gives a pre-impact skid distance of 50 feet. The skidding occurred on a roadway that was level, dry asphalt. The drag factor was measured to be .7 g’s. The vehicle impacted into the side of another car and left 18 inches (1.5 feet) of permanent crush damage.

The combined formula was used to determine the start of skid speed.

This formula works as an approximation of the start of skid speed for situations where one car, van, SUV or pickup truck impacts into the side or rear of another car, van, SUV or pickup truck. This is a relatively easy way to determine if the vehicle was traveling in excess of the speed limit and to decide whether or not a more detailed accident reconstruction would be helpful. This simple estimate of the striking vehicle speed and the speed of the target vehicle can be confirmed by the conservation of linear momentum method. In collisions that involve vehicles that impacted either head-on or head-on at an angle, this skid mark / crush factor method can not be used. In those cases either conservation of linear momentum or some other method needs to be used to determine the speed of the vehicles.

Todd Hutchison