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 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.

In car accidents, skid marks are caused by rubber being deposited on the road, much like that of an eraser leaving pieces of rubber on a paper. Skid marks can also come about when a car suddenly accelerates on a slippery surface, or takes a hard corner. Skid marks on concrete are not made by temporary melting of the road surface, as can be shown by skid marks made on concrete by a bicycle, with children often having competitions about who can create the longest skid mark. The rubber of car tires heats up with sliding friction, degrades and disintegrates at the road-tire interface, and is deposited on the road surface.

Skid marks can also be formed where a ladder suddenly slips and the user falls to the ground, especially if the ladder feet are faulty or absent. Loss of the rubber feet or foot causes the aluminum stile to make contact with the ground, and if a hard surface like concrete or tiling, a skid mark shows how and why the ladder slipped.

Such marks are important evidence for vehicular accident reconstruction, when their size and shape can reveal much about vehicle speed and forces of acceleration or deceleration. They are one form of trace evidence. They represent a form of contact evidence produced according to Locard's exchange principle. The length of the skid mark is usually closely related to the vehicle speed at the instant of braking, so measuring the marks yields an estimate of original speed.

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