Forensic Physics: Tire Tread and Skid Mark Speed Estimation

This medium-difficulty UGC-NET Forensic Science Paper II drill covers tire tread classification, TWI (Tread Wear Indicator) bars, tire impression casting, and the physics of skid mark analysis used in traffic accident reconstruction. The opening questions map the five tread families: highway (rib-and-groove, quiet ride, low rolling resistance), all-season (asymmetric or symmetric pattern, M+S rated), mud-snow (aggressive open-block lugs), performance (directional V-groove, high-speed rated), and off-road (deep mud-terrain lugs). TWI bars are raised rubber bridges at 1.6 mm depth in the main grooves: when the tread surface wears flush with the bar the tire is at the legal minimum under Rule 93 of the Central Motor Vehicles Rules 1989, which implements the Motor Vehicles Act 1988 Section 161 safety standards. Brand-specific patterns from MRF, Apollo, Bridgestone, and JK Tyre appear as distractor anchors. Casting technique questions distinguish dental stone (preferred for outdoor impressions: 1-kg stone to 280 mL water, 30-min set) from Hydrocal and silicone rubber used on vertical or delicate surfaces.

The second half applies accident reconstruction physics. Skid marks (continuous black rubber deposit, brakes locked), scuff marks (diagonal striations, steering input under braking), and yaw marks (sideways-crescent striations from lateral slide) are distinguished by morphology. Drag factor mu values are tested for dry asphalt (0.65-0.85), wet asphalt (0.40-0.50), gravel (0.40-0.60), and ice (0.10-0.20). The skid-to-stop formula v = sqrt(2 * mu * g * d), with g = 9.81 m/s2 and d in metres, gives speed in m/s; conversion 1 km/h = 0.2778 m/s is required. Questions also cover pre-skid reaction distance (perception-reaction time typically 1.5 s), total stopping distance, and the ABS caveat: anti-lock braking systems prevent continuous skid marks, so modern reconstructionists use yaw marks, scuff marks, and vehicle data recorders instead. Indian legal anchors include Motor Vehicles Act 1988 Section 161 (duty to give information and render assistance) and BNS 2023 Section 106 (rash and negligent driving causing death, replacing IPC Section 304A). CFSL Chandigarh's accident-reconstruction section uses SWGTREAD guidelines and SAE J2944 terminology.

Topics covered:

• Tire tread families: highway, all-season, mud-snow, performance, off-road
• TWI bars: 1.6 mm legal minimum, Indian Motor Vehicles Rules 1989 Rule 93
• Impression casting: dental stone technique, Hydrocal, silicone rubber
• Skid mark vs scuff mark vs yaw mark morphology
• Drag factor mu: dry asphalt, wet asphalt, gravel, ice
• Speed from skid: v = sqrt(2 * mu * g * d), g = 9.81 m/s2
• Pre-skid reaction distance and total stopping distance
• ABS vehicles: no continuous skid mark; EDR and yaw mark alternatives

Work through all 30 questions before reviewing explanations, and cross-check answers against Bodziak, Brach, Saferstein Chapter 5, and SWGTREAD guidelines. Allow 30 minutes.