The Traffic Accident Reconstruction Origin -ARnews-
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The Traffic Accident Reconstruction Origin -ARnews-
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Jerry Eubanks remarked:
>In 1966 D. Severy at UCLA did 9 pedestrian crash tests. In the data
>collected from these tests the initial ground contact was between 20-40 g's.
>They also found the pedestrian slding friction
>was between .8 - 1.2 on a nominla .7 asphaltic roadway.
In Severy's paper, he gives an example to show how he calculated the
pedestrian drag factor. From the dummy's test instrumentation, he got the
horizontal velocity at "touchdown" as well as the distance from there to
final rest. So far so good; then things get tricky. Severy noted that his
test dummy took a couple of substantial "bounces" evident on the film as well
as on the velocity traces. He decided to subtract the "bounces" from the overall
sliding distance, and calculated a drag factor of about 1.2.
Most of us don't subtract the "gaps" in a "skip-skid," yet that's just what
Severy did in arriving at his often-quoted (0.8-1.2) pedestrian drag factor.
It's hard to see how a reconstructionist would know how much to subtract from
a pedestrian's skid distance, since we're lucky if we even know where "touchdown"
was. If we re-calculate Severy's example using the total skid distance, we find
that the upper end of Severy's range is a little over 0.6.
In other words: Severy's data support Northwestern's range (0.45-0.6).
It's important to note that this calculation only gives the speed of the
PEDESTRIAN at the beginning of his slide. It doesn't tell you (directly) the
speed of the CAR, or even the speed of the pedestrian immediately after impact.
Looking at Severy's velocity traces, we see a significant loss of horizontal
velocity due (presumably) to aerodynamic drag in the interval between separation
from the vehicle, and pavement "touchdown." None of the trajectory models I've
seen published addresses aerodynamic drag explicitly. It appears that some of
them roll this speed loss into the overall drag-factor for the throw distance.
Hence, their numbers are different from Northwestern's, because they are measuring
different things. Nothing wrong with this approach, except that the underlying
assumptions may not be explicitly recognized.
>I can tell you that for a roadway surface dry not contaimnated for with a non ABS
>friction of .7 NWUTI's formula will always be 15-25% below the actual impact
>speed.
Sounds like you're comparing the known speed of the CAR to the speed calculated
for the PEDESTRIAN using TI's model that ignores both aerodynamic drag, and the
effect of incomplete speed-matchup. Your 15-25% can be viewed as an estimate of
the magnitude of these two factors ( others, perhaps), taken in combination.
Let me restate your observation from a different viewpoint:
Since TI's numbers are supported by the research from Severy Boise State, we
can argue that they give a pretty reasonable speed estimate for the PEDESTRIAN,
at the beginning of his SLIDE. Based on your empirical observations of the
aggregate effect of aerodynamic drag, partial speed-matchup etc., we can estimate
the impact speed of the CAR to be 18-33% higher.
Six of one, half-a-dozen of the other.
My point ( and I do have one) is that we need to be aware of the underlying
assumptions BEFORE we try to use ANY model.
* All "drag factors" are NOT created equal.
* Never violate the "Fruit Law" (Thou shalt not compare apples to oranges)
* You need to understand the model, so you'll know which type of
drag factor to use.
* The results of any analysis must be understood within the limits of
that method.
Sorry for all the bandwidth--say "hi" to Rusty for me!
Brian P. Griffin
<\lurk mode back on\>
Brian P. Griffin
Entertainment.Chairman@Otis.Elevator
For example, to continue this discussion look for a thread titled
Pedestrian Accident Reconstruction
If this thread does not exist in the current archive, you can begin another one by using that title.