SKIDMARK ANALYSIS & BRAKING
(Determination of Speed from Skidmarks)
A vehicle travelling along a roadway ha an associated energy of motion. This "kinetic" energy is a function of the vehicle weight and velocity. The heavier the vehicle, and the faster it is travelling, the more energy it will have. In an emergency situation, this energy of motion must be transformed nto another form of energy in order to bring the vehicle to a full stop. Typically, this "other" form of energy is heat, which is a by-product of the skidding process.
"The Stopping Force"
A skidding vehicle is decelerated at a rate which is related to the frictional force generated between the skidding tires and the roadway. The friction is dependant on the weight of the "coefficient of friction".
Earlier we noted that the kinetic energy of a vehicle was, in part, dependent on the weight of the vehicle; the heavier the vehicle, the more energy it will have at a given speed. It might seem logical, therefore, that a heavier vehicle may require more distance to skid to a stop than a similar, but lighter vehicle. Contrary to this line of thought, the increased friction generated by a heavier vehicle in a skid, directly compensates for the fact that the heavier vehicle initially had more energy. A heavier vehicle may indeed be more difficult to "lock-up" than a lighter vehicle, but once in a skid, the heavy and light vehicles will require the same distance to stop from the same initial speed. For this reason, vehicle veight is not included in the skid-to-stop velocity formula.
The simple form of the skid-to-stop velocity formula requires known values for the coefficient of friction and the length of the skid marks. The pre-skidding velocity is given by:
V= / 255 m S
V is velocity (km/h)
m is the friction coefficient
S is skidmark length (metres)
The coefficient of friction, (denoted by the greek symbol ("m ") is a rating of the grip or "traction" between a road surface and a tire. The value of the coefficient of friction is a fraction, which must be between zero and one. The lower the value of the coefficient of friction of the roadway, the more slippery the roadway will be. For example, an icy surface may have a coefficient of friction in the range of 0.1, while a clean, dry asphalt surface may have a coefficient of friction of approximately 0.7.
Factors Affecting The Coefficient of Friction
The main factors affecting the friction coefficient are road surface texture and condition and tire composition.
Condition of the roadway refers to the presence of snow, ice, sand and other lubricants. The presence of any of these will make the surface more slippery, which reduces the coefficient of friction.
Texture of the road surface refers to the natural roughness of the asphalt. A relatively new asphalt roadway has a "sharper" surface with a high coefficient of friction. An older well-travelled asphalt roadway has a smoother, slicker surface with a lower coefficient of friction.
Tire composition and wear surprisingly plays a lesser role in determining the coefficient. It appears as though road surface texture and condition are the more important factors. Tests show that on a clean, dry surface, bald, worn out tires actually will skid to a stop more quickly than newer, treaded tires. However, on a wet roadway, treaded tires are preferred.
Temperature has a small but generally negligible effect on the coefficient of friction; as temperature increases, the friction coefficient reduces minimally.
Determining the Coefficient of Friction
Ideally, the coefficient of friction should be determined at the accident scene. When this is done, there is little opportunity for any of the factors affecting the coefficient to change. Unfortunately, tests are seldom performed by Police Officers at the accident site. However, the road surface conditions at the time of the accident are typically reported in the Motor Vehicle Accident Report. One should make every effort to conduct tests under conditions as close as possible to those present at the time of the accident.
Tests may be conducted in one of several ways:
This method of testing involves an automobile skidding to a stop from a known initial velocity. The resulting skidmark length, and the known initial velocity are plugged into a different version of the skid-to-stop formula:
µ = V2 / 255 S
When this method of testing is used, a shot marker mounted on the front bumper is used to help determine the point at which braking began.
A second method of testing involves the use of a portable device known as a Drag Box. The Drag Box is fitted with a portion of a rubber tire on its underside, and is pulled along the surface which is to be tested. A gauge measures the force required to pull the device at a constant speed, and the reading is used directly to determine the value of the coefficient of friction.
A third method involves the use of an on-board acceleration-sensitive computer. Typically, these devices are mounted to the floor or to the windshield of the vehicle which is to be tested. Better models give digital readout of initial speed before skidding, distance skidded, and of course the coefficient of friction.
Used correctly, any of the above methods will allow one to determine an accurate value for the coefficient of friction. When the coefficient of friction is accurately known, along with the reported length of skid marks, these values can be used directly in the skid-to-stop formula to determine pre-skidding velocity of a vehcile.
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