Cast Tire Track on a Rural Gravel Road
A cyclist is killed by a hit-and-run vehicle on a rural gravel road. Investigators cast a 4.2 m tire track in mud at the edge of the road. The track has a continuous center rib and shoulder lugs; the track width is 1.58 m. A suspect pickup truck is seized two days later. Its rear tires are a different brand from the front tires; the rear tires have a continuous center rib and shoulder lugs, and the rear track width is 1.60 m. Tread depth on the rear tires is 4 mm; the new depth is 12 mm. The truck has been driven about 2,000 km since the collision. The cast does not show a sidewall imprint or any obvious cuts. The laboratory must decide whether to report that the track was made by the suspect vehicle, by a vehicle with the same tire model, or that the comparison is inconclusive.
1. What class and individual characteristics are available, and what conclusion is appropriate?
Read the worked answer
Class characteristics include tread design (continuous center rib and shoulder lugs), approximate tire size, and track width. The 1.58 m cast versus 1.60 m vehicle measurement is a 2 cm difference, which can fall within measurement error from cast distortion, tire pressure, loading, suspension geometry, and the points chosen for measurement; it is not an exclusion by itself. Tread depth of 4 mm versus 12 mm new shows wear, but wear changes the appearance of the design rather than creating individual identity. Individual characteristics would be cuts, embedded stones, unusual wear, or sidewall marks; none are present in the cast. The cast therefore supports a class association with the tire model and design. The suspect vehicle cannot be excluded, but the track cannot be attributed to that specific vehicle. A conclusion of identification to the vehicle is not supportable; a conclusion that the track was made by a vehicle with the same tire model is defensible. Compare all four tires and check for replacement.
2. How would you document the comparison and address the 2 cm track-width difference and the 4 mm tread depth?
Read the worked answer
Document the cast with scale, lighting, and measurements at multiple points; record the substrate and cast condition. Measure the suspect vehicle's track width at the center of the tire treads, with tires at specified pressure and normal load, and compare like to like. Make test impressions by driving or rolling the suspect tires through similar mud and gravel, or use inked test prints on paper for tread design. For the 2 cm difference, report the range of measurement error and the variables; do not call it a mismatch. For the 4 mm tread depth, make test impressions at current wear, not from a new tire; note that the cast was made before 2,000 km of additional wear. Check tire pressures and vehicle load. Also document tire brands and sizes and whether front and rear differ. The report should state that the comparison is class-level and that the vehicle cannot be individualized without accidental characteristics.
3. If the suspect replaced the rear tires the day after the collision, what effect does that have on the examination?
Read the worked answer
If the rear tires were replaced the day after the collision, the seized tires may not be the crime tires. The cast can still be compared to the replacement tires for class characteristics, but if the replacement model differs, the comparison may exclude the current tires without excluding the vehicle at the time of the crime. Investigators should obtain the old tires through service records, purchase records, or surveillance; if unavailable, the evidence is limited to the cast and vehicle records. The examiner should not infer that the vehicle is excluded because current tires do not match; the relevant tires at the time are unknown. Report the limitation and consider whether another vehicle with the same tire model could have made the track. The replacement fact may be relevant to intent but is not a forensic conclusion.