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You've probably seen it in movies: tire tracks in the snow, in the mud, and dirt. They're left by a criminal at a crime scene, and an investigator is able to identify the suspect thanks in part to the tire tracks linking the suspect's car with the crime scene. How do investigators go about preserving this kind of evidence, and why is it important? This lesson goes over some of the important fundamentals behind this.
Why bother with tire mark forensic investigation? Well, a lot of crimes from burglaries and robberies to stalking and kidnapping are committed by people who must have a means of getting to what will later be the crime scene. Vehicles provide not only a means by which a criminal gets to the crime scene but also a means by which they can quickly escape from one. The vehicle also gives the criminal some security and secrecy.
As a result, a crime scene, or a nearby area, may have tire marks that can help investigators zero in on a suspect. They might match the tire marks at a crime scene with those coming from a suspect's car. Tire marks thus help investigators eliminate or confirm a suspect. They are not, by themselves, definitive evidence that someone committed a crime; however, they may be a very big piece in the puzzle of solving a crime.
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Before you can examine tire impression evidence recovered from a crime scene and compare it to tires of a suspect's car, you need to document, recover, and preserve the tire impression evidence. There's a lot more that goes into this than you may think. Here are just some of the many important things to consider.
As soon as you get to the crime scene, you must establish which areas may have tire marks that are of value to you. Keep in mind that oftentimes the important tire-track evidence may be somewhere away from the crime scene area in a location that isn't as obvious as you might think. For instance, a criminal might park several hundred feet away from the house he will be breaking into. Once you've established an area of interest, establish an outer perimeter in order to ensure other people or vehicles don't enter this area.
Next, walk around the outer edges of this area. Staring at the area from one angle isn't enough, because light may reflect differently from one side to the next, rendering tire marks more or less visible at different angles. If doing this at night time, use halogen lamps to recover tire impression evidence.
If you see a lot of tire tracks, then try to delineate fresh versus weathered tracks. The fresh ones might be more likely what you're looking for. Also, pay attention to footwear impressions. This may indicate someone entered or exited a vehicle. Put identifiers next to each impression within the area in order to pinpoint its location. Identifiers are numbered or lettered markers of various sizes and colors that pinpoint key areas in a crime scene.
Take photographs of tire impressions along with the identifiers. Make sure to take both general scene photographs and examination quality ones. General scene photographs are what they sound like; they are photographs that document the location of the tire impression relative to the entire crime scene. These photographs are not used for examination purposes. The examination quality photographs are taken with a tripod, with the camera directly over the top of the impression, and with a scale (like a ruler) placed parallel to the tire impression. Once you've finished documenting everything and photographing what you need, you can cast 3D impressions using dental stone, a type of plaster that sets quickly.
Ok, so now you've got your evidence. Now it's time to examine and compare the evidence recovered from the crime scene to the tread from a known tire and its impression. One of the most obvious things to compare is the general tread design. It's either a match or not in the sense that one tread design on a tire isn't going to leave a different tread design on the ground. Look for tread wear indicators as well. These can help you ascertain which specific part of a tire may have made the impression at the crime scene.
Pay attention to something known as deflection variables. For example, let's say that you have a cast of a deep impression that was left on soft ground at the crime scene. You're comparing this with an inked impression collected on a hard surface. Even if the same tire made both impressions, it may not look exactly the same, because tire tracks on soft substrates leave impressions that are a bit longer than those from a hard surface. This may help explain why you can't precisely match up the two tire impressions.
Along the same lines, you need to be aware of what kind of soil the impression was collected from. For instance, tire impressions left on very loose soils or unstable soils may not record accurately, or they may even change after the impression has occurred. You should also consider inflation differences. Tires may deflate over time, and this can lead to minor changes in the length of an impression.
There's way more to the recovery and examination of tire marks. Every idea in this lesson can be expounded upon quite a bit, but at this point you should have a good general understanding of the purposes and methods of tire marks forensic examination. With that in mind, let's review a few things.
Investigators examine tire marks found at crime scenes because they may help to catch a criminal. Many crimes nowadays are committed by people using a car as a method of transportation, partial anonymity, and security. When recovering the evidence, make sure to:
When examining the evidence:
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Criminal Justice 106: Forensic Science14 chapters | 104 lessons
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