Skid_mark

Skid marks on an asphalt road

In motoring terms, a skid mark is the mark a tire makes when a vehicle wheel loses traction and skids on the surface of the road. More generally, any solid which moves against another can cause visible marks, and is an important aspect of trace evidence analysis in forensic engineering. In car accidents, skid marks are caused by rubber being deposited on the road, much like that of an eraser leaving bits on a paper.^[1] Skid marks can also come about when a car suddenly accelerates on a slippery surface, or takes a hard corner. Some have purported that skid marks are the temporary melting of the road surface. This is not true as can be evidenced 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.

Accident reconstruction

Such marks are important evidence for vehicular accident reconstruction, when their size and shape can reveal much about vehicle speed and braking forces. 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.

Skid mark from faulty ladder

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 aluminium stile to make contact with the ground, and if a hard surface like concrete of tiling, a skid mark shows how and why the ladder slipped. In the picture shown at right, the rubber foot in the stile end in the foreground had worn so severely that the remnants were pushed back into the hollow stile, so that it became redundant and allowed the ladder to slip, with catastrophic results for the user. The worn foot should have been replaced long before, and the employer had to pay damages to the injured painter. Ladder accidents are among the most common industrial and domestic accidents causing severe injuries both in the USA and the UK.

See also

• Forensic Science
• Forensic engineering
• Trace evidence

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