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Errors in
Accident Reconstruction
by Joseph E. Badger
It has
recently come to my attention that some police academies � for whatever reason(s)
� limit their instruction regarding accident investigation to a two-hour segment
on how to fill out the crash report form. Another statistic I heard was that �
at least for the aforementioned recruit schools � out of approximately 100 basic
students, only 14 to 20 take a 5 day crash investigation course.
Trust me on this, even a 5 day course � nay, not even a 5 week course � makes a
qualified accident reconstructionist. Neither does a 5 year physics or
engineering course. It not only takes training in the mathematics and
engineering principles involved, but also years of practical hands-on,
in-the-field experience.
Some agencies mandate its officers to NOT take photographs unless there is a
fatality. Yet we occasionally see reports of fatalities and serious bodily
injury where only the standard crash report is completed. No photographs, no
measurements.
We find that some officers, including those who have had no more than a couple
of weeks of reconstruction instruction, are holding themselves out as accident
reconstruction experts, yet make grievous blunders. Sometimes without even
realizing it.
Yaws
For instance, just because a tire mark is curved doesn�t necessarily mean it is
a yaw mark. (And while I�m on the subject, I�m forever seeing police reports and
diagrams that mention �yawl� marks.
A yawl is a two-masted, fore-and-aft-rigged, sailing vessel similar to a ketch
but having a smaller jigger mast stepped abaft the rudder. Or it�s a ship�s
small boat, crewed by rowers. American Heritage Dictionary.)
A yaw, on the other hand, is a whole other subject, described in Al Baxter�s
Accident Investigation and Reconstruction Terminology as �The rotational or
oscillatory movement of an object about its vertical axis; a movement about the
z-axis of the vehicle axis system.� Yaws often occur when a driver makes a
hurried steering maneuver (a) to avoid an object in the road, or (b) to remain
on the pavement while entering a curve too fast or (c) while overcorrecting a
previous situation. �A yaw mark is that physical mark on the roadway caused by
the rotating tires of a vehicle slipping in a direction parallel to the axle of
the wheel during a maximum rate change of direction (turn).�
Speed determinations may indeed be made by correctly measuring genuine yaw
marks; however, if those are merely spin marks, created by one vehicle that is
impacted by another vehicle, then it is inappropriate to use equations that
assume the mark to be a real yaw. And for heaven�s sake, you don�t have to
measure the length of a yaw mark � unless you simply want to know how long it
is. The length has no bearing in critical speed formulae.
For the critical speed equation to work, the vehicle in question must be both
rolling in one direction and sliding in another; however, one �expert� used a
supposed yaw mark and came up with over 90mph, but the front tires were both
flat and the front wheels were crushed, not allowing them to roll.
Flips, vaults and falls
In one respect the yaw is like a vault or flip � circumstances that may befall
vehicles in certain accident scenarios. That is, a speed calculation determined
from that action is final. Whatever the vehicle does subsequently doesn�t
matter. It can skid, crash into a tree, or rollover. Once you�ve computed the
yaw (or fall or vault or flip) speed, you must not combine the speed following
any of those events with the speed calculated in the event. You may � and should
� use data from circum-stances following the yaw, fall, or vault to confirm the
originally calculated speed. If a collision or skid occurs prior to a yaw
(etc.), then fine, speeds may be combined.
In one case, the reconstructionist actually added two airborne speeds together.
Go ahead and calculate both (hopefully the second one will be less than the
first one) but don�t add, subtract, or combine them. Use the answer from the
equation in which you have the best data.
Misinterpreting evidence
Be careful when taking scene photos that you don�t inadvertently create evidence
that isn�t there. Watch those shadows! Likewise, be careful when examining
photographs taken at accident scenes. Shadows created by overhead utility lines
have an uncanny way of appearing as yaws or curved tire marks on the pavement.
Combining speeds
Speaking of combining speeds, there�s a particular way of doing that.
�Combining� doesn�t mean �adding�. If a car, say, skids 50 feet on concrete, 25
feet on blacktop, then 40 feet on grass, and smashes into a 75-year-old oak
tree, there�s a reasonably easy way to determine how fast the car was going when
the driver first applied the brakes. The not-quite-so simple part involves first
a crush analysis, then backing things up wherein you compute the speed for each
surface. But you don�t add all those speeds together. It�s like Pythagoras Plus.
Square all the speeds, add those, then take the square root of the answer. That
way, 36 mph and 23 mph and 24 mph and 20 mph doesn�t equal 103 mph; it�s 52 mph
and some change.
In a motorcycle case, a Ph.D. testified that a motorcyclist�s speed, based upon
post-impact throw distance, was 30 to 35mph. He then said that the speed of the
motorcycle, based upon post-impact sliding distance, was 30 to 35. Sounds great,
right? No! He combined them and said that the speed of the motorcycle was
approximately 46 mph.
Incorrect science
Acceleration rates are interesting. Car magazines print data regarding how fast
certain cars can accelerate from 0 to 60, or the fastest speed attainable in a
quarter-mile. Be cautious using these data in inter-section collisions. There�s
the case where an opposing expert used an acceleration rate for 0 to 10mph to
show how quickly it could accelerate from 55 to 65 mph.
Drag factors
Much of the time, assigning a precise value for drag factor isn�t necessary. In
the overall scheme of things, there isn�t a lot of difference between the speeds
generated from a 0.65 drag factor as with a 0.75 drag factor. For one reason,
you end up taking the square root of the value. As an example, if a car skids to
a stop in 100 feet on a 0.65 surface, the slide-to-stop speed is around 44 mph.
On a 0.75 surface, every-thing else being equal, the speed equates to about 47
mph. What�s three miles an hour among friends?
Furthermore, if you perform a skid test (whether with a drag sled, bumper gun,
or electronic accelerometer) on the surface where the accident skidding occurred
and in the direction skidding happened, you should not add or subtract the
grade. The grade was �built in� when you completed the test.
But drag factors do make a difference when tractor-semitrailers are involved,
and especially when bob-tail units skid. (�Bob-tails�, by the way, are truck
tractors designed to pull semitrailers, but which are traveling without one.) On
a road-way surface that may be 0.80 for a car, it may be only 0.20 for a bob-
tail tractor. And for that same 100-foot skid, the respective speeds would be 48
mph versus 24 mph.
Drag factors are different for heavy trucks for a number of reasons, including
weight shift (no, not load shift, that�s something else), tire pressure and the
idiosyncrasies inherent with air brake systems.
There was an actual case: An officer used a 0.7 for friction, because �all
streets in the city have a 0.7�. He then adjusted for the 0.03 grade. Next, he
checked the striking vehicle and discovered that the rear brakes did not
function. He determined the speed of the striking vehicle by using momentum,
then combined the impact speed with the speed loss from pre-collision skidmarks.
He used the full friction (0.73) for both pre- and post-collision movement of
both vehicles, even though he had determined that one vehicle did not have
working brakes. He used the 0.73 for the post-impact movement of the other
vehicle, even though it was going downhill on grass, with only one wheel locked
from damage.
Similarly, there are officers who use the full drag factor � meaning 100%
braking � for vehicles that are free-rolling from impact to final rest.
Then there are some who mishandle drag sleds (usually by pulling them way too
slowly) and come up with drag factors of 1.20 on level blacktop surfaces. This
may approach possibility on some brand new asphalt that has bits of embedded
glass particles and a sled with really soft tire material, but beware anytime
you come up with a factor of 0.95 or more.
Computer programs
Beware also of accident reconstructionists bearing software. Paraphrasing
Charles Grosvenor, former Representative from Ohio, �Computers won�t lie, but
liars will compute. �I�m acquainted with two engineers who, on separate
occasions, used a particular program (with which I�ve had years of experience
and special training), one to calculate that a vehicle was astraddle the
centerline during a collision; the other who figured how two women pushed a car
up a grade at over 10 miles per hour.
And I�ve seen police officers who have used sophisticated mapping and measuring
software to calculate the departure angles of cars in intersection crashes based
on the center of mass on the cars at final rest, rather than the angle at
separation.
Rounding down
They always tell you to �give the benefit of doubt to the violator�, but wait,
you don�t need to give them the farm. Truncating the energy equation is one way
to do that. Sure, round down, but wait till you�re finished, then round down the
final answer.
Missing units
This sort of error crops up in measuring critical speed scuffs or curved
roadways. If you measure a chord in feet, measure the middle ordinate in feet
also, no inches. Likewise, if you�re dealing in feet per second, don�t use miles
an hour as formula input. Imperial and metric don�t mix very well either.
Don�t mix prices with pounds. This actually happened when an engineer checked a
NADA book for the weight of a used vehicle, and wrote down the retail price of a
pickup truck instead of its weight. Then he entered that number in the momentum
formula.
Bottom line
Before you hold yourself out as an accident reconstructionist, be sure you can
back up your methodology. Such support is not simply saying you read it
someplace or some teacher told you.
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