04-04-2012, 03:30 PM
Determining Vehicle Speeds From Skid Marks
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Measuring skid marks:
The skid speed is the speed of the vehicle at the beginning of the visible skid mark. This will be
a conservative value as the wheels do not lock-up instantly. There is some "shadow skid," a
light mark produced as the wheels begin to slow and just before they achieve full lock. Shadow
skid and clearly visible skid should be considered as one continuous mark for any given tire.
Cars have four tires, two in the front followed directly by the two in the rear. The wheels on most
cars, assuming the brake system is functioning correctly, will tend to lock at nearly the same
time. Current brake design includes pressure limiters that prevent the rear wheels from locking
before the front wheels lock.
Drag factor:
A drag factor is the term for the tire/road surface interface when determining vehicle speeds.
There are several ways to find the drag factor but the most accurate is to conduct a series of
test skids with an exemplar vehicle equipped with a recording accelerometer and chalk bumper
gun. Unless you have formal training in conducting skid tests, it is not recommended that you
attempt skid tests.
braking efficiency:
Each wheel on a car provides a certain amount of the total brake force available. If all four
wheels are braking evenly, leaving four distinct skid marks, then braking efficiency is 100%, or
1.00. If the rear brakes are not functioning at all, then 40% of the brake force is not available,
leaving a braking efficiency of 0.60 for the car as it was skidding. For rear wheel drive cars, the
brake force can be assumed to be 30% for each of the front wheels and 20% for each of the
rear wheels.
Reaction Distance
Skid distances do not include perception-reaction time distances. This is the distance a vehicle
will travel during the time period a hazard first becomes visible to a driver and the driver takes
action, such as stepping on the brake pedal, to avoid the hazard. A perception-reaction time of
1.4 seconds for a nominal hazard, with an alert driver may be used for some situations.
By performing skid tests at the scene, resulting in visible skid marks of the same length as found
at the accident scene, and using a chalk bumper gun to record the actual skid distance and
speed during the tests, it can be shown with great certainty how fast the car was actually going
to leave visible skid marks of that length.
If you have a case where the calculated speed from skid marks is at issue, contact Harris
Technical Services. We can perform a skid speed demonstration for use at trial with
photographs and videotape as part of the reconstruction