Most forensic accident reconstructionists have the ability to download the data saved from an EDR in many of today’s passenger vehicles, but heavy (or commercial) vehicles have several key differences that set them apart.

Differences:

-          Heavy vehicles can be outfitted with engines from different manufacturers, which results in different methods and required equipement to download the EDR.

-          Records data based on events such as “hard stop”, or “last stop”

-          These events can be written over by subsequent events

-          Records total distance travelled, fuel consumption, cruise time, percentage of time in each gear, and more

-          Heavy Vehicle ECMs have configurable settings that can be changed by the user. For instance, transmission, axle gearing, and tire sizes programmed into the ECM may be different than what is actually on the vehicle.

Similarities:

-          Can record information useful to accident reconstruction such as traveling speed at per second intervals

-          Has limitations that need to be addressed by an expert

Downloading this data can be extremely helpful to your file when used in conjunction with other physical evidence. Our crashteams experts have specialized software that allows the extraction of this data, and more importantly the training required to do so without risk of data spoliation. For example, one major engine company EDR is preset to purge data after it has been downloaded. This feature is convenient for the mechanic who will diagnose and fix engine malfunctions, but will result in lost data in the case of an inexperienced forensic “expert”. Similarly, units that record “last stop” data can be overwritten, so educating drivers in proper accident site protocol will result in the best chance recovering valuable EDR data.

Identified the marks as skid marks. Not all tire marks are caused from braking. Yaw marks for example, are created when the vehicle is traveling at an  angle different than the direction it is facing. (this is called the slip angle)

Skid Marks

Yaw Marks

Yaw marks are differentiated from skid marks by the alignment of dark bands within the mark. They are aligned at an angle to the direction of travel for yaw marks, and parallel to the direction of travel for skid marks.

Skid Marks have Parallel Dark Bands

Yaw Marks have Dark Bands Aligned at an Angle (click photo for larger resolution version)

Measured or assumed a friction factor, and explained why. Different surfaces call for different friction factors, and these friction factors may vary even on a single surface. For example, passenger car tires on travelled asphalt range from 0.6 to 0.8. (“Friction Applications in Accident Reconstruction”, Warner et al.) Alternatively, if the surface is suspected to be different than reported in research, or relevant research can’t be found, acceleration tests can be performed with a similar vehicle at the site.

Measured the skidmarks correctly. This is the most common error made during the reconstruction. Skid marks may be of equal or different lengths, or one or more tires may not produce skid marks. The reconstructionist must explain the lack of any skid marks. For instance, a tire was not braked (many inexperienced motorcyclists fail to use their front brakes in an emergency situation), or there was a brake failure (which can be revealed from a vehicle inspection). This article only addresses the situation where all tires leave skid marks (of equal or unequal length).

When all (4 for passenger vehicles) skid marks are of equal length, then that length is used. If the skid marks are of unequal length, then there are two methods to calculate the possible speed of the vehicle.

use the average length of the 4 skid marks. This method produces a conservatively low estimate of the vehicle’s traveling speed.

use the length of the longest skidmark. This method produces a conservatively high  estimate of the vehicle’s traveling speed.

Pitfalls to avoid

The following examples come from real cases we have worked on, and represent an extreme lack of understanding regarding the fundamentals of skid mark analysis. If your reconstructionist does any of the following, contact us.

Adding skid mark lengths together.

Confusing yaw marks and skid marks. See the photos above for examples.

Using friction factors to calculate a vehicle’s acceleration. A vehicle’s acceleration (increase in speed) is not generally equal to its deceleration (decrease in speed, or braking).

How this can help you as a litigator

If you spot any of the above errors, or just think that the speed estimate given by the other side doesn’t “feel right”, contact a Crashteams reconstructionist. They will be able to review the other expert’s work, and provide you with a verbal or written report detailing exactly what that expert did wrong, along with the correct method and correct results.

Speed  from skid mark calculations

Simple skid mark calculations can be automatically performed at the following link. Be careful to avoid making any of the mistakes discussed above!

Skid Mark Calculator

This common statement is often made by the driver of a vehicle who has just failed to yield the right of way to a motorcyclist. In fact, in one study 62% of accidents involving another vehicle were contributed to by “traffic scan error”. The National Highway Traffic Safety Administration reports that motorcycle rider fatalities have increased for the seventh year in a row from 1997 to 2004  to 4008 in 2004.

Why do these types of accidents happen? How is it even possible for a person to not see something that was clearly in front of their eyes?

The “Maids” report is an exhaustive study of motorcycle accidents in Europe, involving 921 investigated accidents and 923 base level motorcyclists (not involved in an accident). They found that:

• 54% of accidents took place in intersections
• 62% involved a traffic scan error by the other vehicle
• Excessive motorcycle speed contributed to 18% of accidents
• Automobile drivers with a motorcycle license are almost 4 times less likely to have “failed to perceive motorcycle” listed as a contributor to an accident. (50% vs 13%)

How would you react to this type of situation? Watch the video, and then click on the link below to tell us how you did. (Hint: Click on “see previous responses” after filling out the form in order to see a summary of the answers.)

Tell us your results, and see the summary

winter is coming

With winter fast approaching, it is time to adjust our driving habits to compensate for the worsening conditions. This commentary looks at the effect friction has on the handling abilities of our cars.

First of all, we all know that snow and ice is slippery, but how much? This study (1) found that dry pavement friction was reduced by 61% for compacted snow, and 81% for ice. This means that at 50mph on dry pavement, you will stop 130 feet after pressing the brake pedal. However, you would require 330 feet to stop on snow, and 685 feet if on a thick sheet of ice.

Friction Circle

Which brings us to the question, on winter roads, how do we safely travel through corners? Consider the following diagram, commonly called a “friction circle”. At the top of the diagram is your maximum braking potential, on each side, is your maximum cornering ability. The circle represents the fact that the friction you are using is a finite resource. As you can see, you cannot brake and corner without compromising both.

Driving Line

Race car drivers have the solution: brake before reaching a corner, in order to use all of the available friction for cornering later on. This is because braking in a corner decreases your cars ability to steer. Therefore, for the fastest (not to mention the safest!) way to complete a corner, brake before it comes up, and don’t start accelerating until after you reach the apex of that corner. In the figure to the left, brake in the red portion, then steer, and finally accelerate in the green portion.

Do you have a winter road condition case? Call or email us to discuss how we can help you.

(1)    – Tire-Road Friction in Winter Conditions for Accident Reconstruction

"some days I love my job"

Crashteams CEO Tim Leggett, PE just returned from dynamic limit testing of a commercial vehicle in order to ascertain its extreme boundaries. With him were  local Crashteams reconstructionist  Martin Boisvert, and trucking expert James Sloan. The tests included a wide range of measured variables including braking distance, acceleration capability, vehicle handling characteristics, and vehicle geometry. We used sophisticated measurement equipment such as accelerometers and rugged weigh scales, but also a few novel tools such as can be found in your smartphone. Call us to discuss how we can implement the same tools to assist you.

The Current Solution: measuring the scene of an accident is generally done by a surveyor with a Total Station. This Total Station reads in one point at a time, when the surveyor points it at an item of interest. The resulting file, using containing dozens of points, along with a description of each point, is then fed into diagramming software such as AutoCad, or ARAS 360, and converted into a scale diagram.

New Tech: Survey systems such as the Leica ScanStation using a laser to precisely measure a point, in much the same as before. However, this laser moves in an automated fashion, and records 50,000 points per second. The result is a data file containing millions of points, which when put together represent the scene scanned. In addition, each recorded point has a color value, which makes visualization of the scene, as well as using the points in an animation, that much easier.

This technology was pivotal for a file that recently achieved settlement. First the scene was measured and imported into 3d Studio Max. Note, for this scene 8 laser surveys were combined and merged to create one long scene.

Then, details such as construction barrels that had been removed from the accident scene at the time of the scene survey were reintroduced. As well, construction that had long since been completed was digitally reversed to the original state at which it existed at the time of the incident.

The final animation was completed in less time, and with a higher quality than ever before. Using the point cloud data allowed us to create a scene that included numerous secondary objects such as buildings and trees, all of which were precisely modeled and located automatically. This corresponded to an animation that was immediately more immersive, while still presenting the data in an objective way.

Meanwhile, Story House Productions has found another accident for us to look into. Tim Leggett will be traveling back to Ohio to take a look at the scene and figure out what happened. Keep an eye on out website for the air date of this story.

Here’s the video showing a glimpse of what happened.

http://www.youtube.com/watch?v=OOyAUpLcblo

The typical conclusion reached when a windshield fracture like this is seen is “that guy was not wearing his seatbelt.”  Before coming to this conclusion please contact your local Crashteams Expert to examine the seatbelts for evidence of use and for functionality.

Careful examination of a seatbelt’s webbing can reveal deformation of the webbing as shown to the right.  This occurs when the webbing is deformed under tension by the wearer during the collision and is usually located where the webbing sits, when in use, in the seatbelt’s latch plate or D-ring.  These structures may also show evidence of use in the form of striations and melted plastic deposits as shown in the two photographs of a latch plate below.

Your Crashteams Expert will also test the seatbelt’s emergency locking retractor (the ELR) and buckle mechanism for proper functionality.  If the seatbelt’s ELR was not functional additional webbing would spool out during a collision rendering the seatbelt virtually useless.  Likewise, debris in the buckle mechanism, or a broken mechanism, may allow the user to “buckle up” the belt but not actually lock the latch plate in the buckle.  During a collision the latch plate could then pull free of the buckle and again render the seatbelt useless.

During the vehicle examination your Crashteams Expert will also measure the crush to the vehicle and the remaining occupant space.  This will allow him or her to determine the impact severity commonly termed the speed change.  Research into the effectiveness of seatbelts to restrain an occupant can then be used in conjunction with the speed change and the occupant space dimensions to ascertain whether an occupant would have made contact with the windshield, for example, even if wearing a fully functional seatbelt.