Tire Tread and Tire Track Evidence: Recovery and Forensic Examination

1. What they did

Criminals use vehicles to reach and leave most major crimes, and their tires often leave tracks and impressions at the scene. William Bodziak, who examined tire and footwear evidence at the Federal Bureau of Investigation (FBI) Laboratory for nearly 25 years, wrote this handbook for the crime scene staff who find and recover tire evidence and for the examiners who compare it with suspect vehicles.

The book covers:

• How tires are built and labelled, from early wooden wheels to the modern radial tire, and the information on a tire’s sidewall.
• Track evidence: what the paths of all four tires reveal about a vehicle, and how to measure them.
• Finding, photographing and casting tire impressions at crime scenes, including in snow.
• Making known test impressions of suspect tires, and how tires are manufactured, retreaded, designed and worn.
• Individual characteristics that make one tire different from all others.
• A step-by-step examination procedure, how to report results, and how to present tire evidence in court.
• Resources and databases, three case studies, and the tire evidence in the 1995 Oklahoma City bombing case.

2. What they found

Reading the tracks of a vehicle

• When a vehicle drives straight, the rear tires run over and wipe out most of the front tire impressions. Because front and rear track widths usually differ slightly, a thin strip of the front track often survives along one edge.
• When a vehicle turns, the rear tires track to the inside of the front tires, and the gap grows as the turn tightens. This is the same whether the vehicle goes forward or backward.
• Track width, wheelbase, turning diameter and rolling circumference can all be measured at the scene, and can include or exclude a suspect vehicle. Turning tracks make the front track width look narrower, so it cannot be measured usefully from turning tracks alone.
• Wobbly, not quite straight tracks on off-road surfaces point to the steering tires; signs of wheel spin show a drive tire; a track that dead ends shows the vehicle stopped and changed direction; tracks that cut through others were made later.
• Shoe prints near a vehicle should be recorded, including whether they were made before or after the tire tracks.
• The best place to evaluate track evidence is at the scene. Working later from photographs alone can be seriously limited.

Recovering the evidence

• Tire evidence is often away from the centre of a crime scene and is only found when crime scene staff are aware of it and search for it. The first officer should protect any area whose surface could hold tire impressions.
• A full turn of a car tire is about 6 to 8 feet long, and the tread blocks change in size and arrangement all the way round. A 1-foot section of a 6-foot impression is “like recovering 2 inches of a 12-inch shoe impression.” The minimum is photographs and at least one 3-foot cast of each potentially important track.
• The author gives examples where poor or missing measurements made good snow tracks next to a homicide victim “essentially useless.”

Comparing tires and impressions

• A new tire leaves an impression like any other tire of the same design and size. With use, tires gain wear and individual characteristics (cuts, tears, stones held in grooves, nails, plugs) that can make them unique.
• Results range from elimination, through “similar design” and “design and dimension correspond,” to identification of the one tire that made the impression.
• Tread depth in a three-dimensional impression can be misleading, because soil, mud or snow can fall back into or clog the grooves.

3. What they concluded

• Tire evidence has long been a reliable way to link vehicles to crime scenes, but its full value depends on scene staff recognising it, protecting it, and recovering enough of it properly.
• A tire examination is a simple application of the scientific method: did this tire make this impression or not? Examiners should stay as objective as possible and base their opinion only on clear, confirmable observations.
• Some subjectivity is part of any forensic examination, but a trained and experienced examiner following a structured method can reach reliable conclusions.
• In the case studies, tire tracks helped place vehicles at scenes. In the Oklahoma City bombing case, dual tire tracks in front of a storage locker rented by Timothy McVeigh and Terry Nichols matched an identical Ryder rental truck.

4. How tracking was or was not used

The book is about tracking vehicles rather than animals. It teaches how to read and interpret tire tracks and impressions at a scene: the direction of travel, turns, stops and reversals, which tires steer and which drive, the order in which tracks were made, and how shoe prints relate to the vehicle. It also covers measuring tracks (track width, wheelbase, turning diameter, rolling circumference), photographing them, casting them in soil and snow, and comparing them with suspect tires down to individual characteristics.

6. Other technologies, skills or innovations

• Casting with dental stone, with guidance on powder-to-water ratio, pouring, drying (about 48 hours) and cleaning.
• Casting in snow: dry casting, Snow Print Wax and Snow Impression Wax sprays, and sulfur casts.
• Examination-quality photographs, and film versus digital cameras.
• Known test impressions with ink or powder on clear film or white board, white adhesive lifts and the Identicator inkless method.
• Tread design guides, the Tread Assistant CD-ROM, track databases (Michigan, Royal Canadian Mounted Police, Europe) and SICAR TreadMate.