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Concept

Skid mark

A skid mark is the visible mark left by any solid which moves against another, and is an important aspect of trace evidence analysis in forensic science and forensic engineering. Skid marks caused by tires on roads occur when a vehicle wheel stops rolling and slides or spins on the surface of the road. Skid marks can be analyzed to find the maximum and minimum vehicle speed prior to an impact or incident. Skidding can also occur on black ice or diesel deposits on the road and may not leave a mark at all. Skid marks are divided into "acceleration marks" created on acceleration, if the engine provides more power than the tire can transmit; "braking marks", if the brakes "lock up" and cause the tire to slide; or "yaw marks", if the tire slides sideways. Each skid mark has a characteristic appearance, and an experienced accident reconstructor or forensic engineer can often determine what the vehicle was doing by examining the marks left by the tire. Different road surfaces produce different kinds of skid marks, sometimes also referred to as scuff marks or tire marks. On asphalt road surfaces, skid marks are usually the result of bituminous oils in the asphalt that are heated because of the friction of braking or accelerating and rise to the surface, leaving dark marks. This kind of mark on asphalt can last for months or even a year or two if they are particularly dark and the roadway is not well traveled. On other road surfaces, such as concrete, the marks result from the deposition of tire compounds onto the surface of the roadway. This kind of skid mark is typical of airport runways, which need to be periodically cleaned to remove deposited debris that reduce the frictional coefficient of the runway. The durability of these marks depends on a number of factors, including road use, tire compounds, weather, and frequency of road cleaning. Braking marks will vary depending on whether the car uses ABS or not. If not, there will be two lines. The darker marks on the outside are from the front wheels, while the back tires leave thinner marks.

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