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This work is very basic, it does not address many of the factors involved in
determining vehicle speeds from skids that are addressed by a professional
traffic accident reconstructionist in calculating vehicle speeds. It is intended
to give the reader an understanding of how vehicle speeds are determined from
skid marks and some of the limitations of the calculations. This work is limited
to determining speeds from cars. Trucks, cars pulling trailers, and motorcycles
have a number of factors involved that are not addressed and this material
should not be applied to situations involving those types of vehicles. First, let's define a skid mark. A skid mark is a tire mark on the road surface produced by a tire that is locked, that is not rotating. A skid mark typically appears very light at the beginning of the skid getting darker as the skid progresses and comes to an abrupt end if the vehicle stops at the end of the skid. There are other types of tire marks including scuffs, scrubs and yaw marks. These must not be confused with skidmarks. A skid mark is left when the driver applies the brakes hard, locking the wheels, but the car continues to slide along the road. Steering is not possible with the front wheels locked. Skid marks are generally straight but may have some curvature due to the slope of the road. A car skids 60 feet. How fast was it going? The answer is, "It depends." Several things must be determined before you can determine the vehicle's skid speed. You must know the skid distance, a drag factor for the road surface and the braking efficiency of the vehicle. 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. If all four wheels lock at the same time, and the vehicle is skidding in a straight line, the marks from the rear wheels will overlap the marks from the front wheels. Rear wheel skid marks can be identified by the dark center while skid marks from the front wheels can be identified by two distinct thin lines on the outer edges. If four distinct skid marks can be found, they should be measured individually. To get the average skid distance for the vehicle, add the four measurements together and divide by four. This is the "average skid distance." If three skid marks are found, add the three together and divide by three to get the average skid distance. The same applies to two marks. If only one mark is found, measure the entire length and use this as the skid distance. If two skid marks are found, but it cannot be determined where the front wheel skid marks begin due to being overridden by the rear wheel skid marks, measure the entire length of the two skid marks, subtract one-half the wheelbase of the vehicle from the total and divide by two. The result is the average skid distance. 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. To provide some examples of drag factor values, drag factor ranges for various typical road surfaces are:
Portland Cement: 0.55 to 1.20 Third, 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. With the above, you have the three variables required to complete the minimum skid speed formula:
Where - An example: A car skids to a stop, leaving four skid marks with an average length of 60 feet. The road is asphalt. Skid tests reveal a drag factor of 0.75. Since all four wheels were braking, the braking efficiency (n) is 100% or 1.00. The value for "D" is 60. The value for "f" is 0.75. Insert the values into the formula and a speed of 36.7 miles per hour is determined (S = 36.7). It is important to understand this is a MINIMUM speed for the vehicle at the beginning of the skid. It is not possible to find all of the skidmark given it starts out as a light shadow becoming progressively darker. And this formula assumes the vehicle comes to a stop at the end of the skid without hitting anything, like another car or tree or bridge abutment. If there is a speed value at the end of the visible skid, as when the car strikes something, the residual speed value must be combined with the calculated minimum skid speed. You must not add a residual speed to the calculated minimum speed. For example, a car skids 73 feet on an asphalt surface and then strikes another car broadside at 28 miles per hour. A drag factor of .85 is determined and braking efficiency is 1.00. The minimum skid speed is determined as 43.14 miles per hour. To combine this speed with the impact speed:
There are many factors and situations that these basic formulas will not fit. What has been provided here would apply to the most simple of situations. Vehicle speeds can be determined from a number of variables, including fall distance, vault distance, yaw marks, crush and momentum. If an analysis of vehicle speed is desired, it is recommended that a qualified traffic accident reconstructionist be retained. How far a car will skid on various road surfaces from different initial speeds -
Notes on this table: The terms used to describe the road surface (asphalt,
concrete, snow, gravel) and the associated drag factors (f), are very general.
They do NOT apply to all roads. These values were selected as representative,
not absolutes. For any accident case where skid speed is at issue, a skid test
should be conducted, if practical, to determine the actual drag factor.
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. |
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