Accident Investigation: Skid Marks, Collision and Hit-and-Run

A crash scene differs from most forensic scenes in one critical respect: time pressure to clear it. A homicide in a bedroom can be cordoned for 24 hours. A fatal collision on NH-48 outside Mumbai loses control of the lane in roughly 45 minutes. Skid marks degrade with the next monsoon shower, gouge marks fill with rubber dust by the next truck, and debris drifts every time a vehicle passes. The whole reconstruction depends on documentation made within the first hour.

The first-officer protocol is therefore aggressive about geometry capture before evidence collection. Photographs and a measured sketch come first; physical recovery comes second. Tyre-mark lengths are paint-marked in chalk or spray before the road is washed down. The point of rest of every vehicle and every body is fixed by triangulating from two permanent reference points (a kilometre post, a culvert edge, a survey marker on the carriageway).

A working checklist for the first 60 minutes:

• Block the lane. Two patrol vehicles upstream with hazards on; cones at 50 m and 100 m intervals. Indian state SOPs specify three cones at a closing taper for 60 km/h roads, six for expressway speeds.
• Mark every skid, yaw and gouge with traffic-grade paint at both ends and the midpoint, before any tow truck arrives.
• Photograph at four heights: knee-down for tyre marks, waist for debris distribution, shoulder for vehicle positions, drone for the overall geometry. Cross-link to the photo-log workflow in Forensic Photography.
• Triangulate point of rest for each vehicle and casualty from two fixed reference points; record the bearing and the distance.
• Recover debris radially from the point of impact outward; debris orientation is geometry, not just material evidence.

The collision type determines the reconstruction approach. Damage signature, resting positions, and casualty pattern all follow from the geometry of impact, and rapid classification from scene photographs is an essential field skill.

The pedestrian collision warrants closer attention because it accounts for the bulk of hit-and-run casework in India. The bumper-height contact bruise on the victim's leg gives an approximate height of the striking vehicle's bumper at the moment of impact, which narrows the candidate-vehicle list. The throw distance from impact to point of rest, combined with the victim's mass and stance, allows a rough speed estimate using one of the Searle or Wood equations. Glass embedment in the victim's clothing, paint smear at the contact point, and tyre tread on the legs or torso of a run-over victim are the high-yield items.

On a divided highway, a head-on collision is almost always a wrong-way driver or a lane-departure event; reconstruction hinges on which vehicle crossed the centre line first. A T-bone at an urban intersection is almost always a signal-violation case; reconstruction hinges on the signal-phase log if the junction is CCTV-monitored.

The standard skid-mark speed formula in Indian and US accident-reconstruction practice is:

S = √(30 · d · μ · n)

Where S is speed in miles per hour, d is the skid distance in feet, μ is the drag factor (effective coefficient of friction) and n is the braking efficiency, a fraction between 0 and 1 that accounts for how many of the four wheels were locking. The metric equivalent, with S in km/h and d in metres, is S = √(254 · d · μ · n), which most Indian SFSL reports now use as the primary form.

As a worked example: a 30-metre skid on dry asphalt (μ = 0.75) with all four wheels locked (n = 1) gives S = √(254 · 30 · 0.75 · 1) = √5715 ≈ 76 km/h. That is the minimum speed at the start of the skid. The impact speed will be lower, sometimes much lower, because the vehicle continued to decelerate over the residual distance to impact.

The drag-factor table every candidate should memorise:

• Dry asphalt or concrete: 0.70 to 0.80. Pune Highway Safety Patrol uses 0.75 as the default value in working reports.
• Wet asphalt: 0.40 to 0.50. A monsoon-day NH-48 reconstruction lives in this band.
• Gravel or compacted earth: 0.50 to 0.60. Most Indian semi-urban roads.
• Loose sand: 0.30 to 0.40.
• Snow or ice: 0.10 to 0.30. Relevant in Ladakh, Himachal and J&K casework.
• Tar with diesel spill: as low as 0.20. A common trap on truck-route stretches.

Braking efficiency n is frequently omitted, which introduces significant error. If only the front wheels lock, n drops to roughly 0.65 because the rear wheels still contribute residual rolling-tyre friction that the locked-wheel formula does not capture. If one wheel locks and three roll, n falls further. A modern ABS-equipped vehicle, by design, never produces a fully locked-wheel skid; it produces a series of intermittent scuff marks, and the formula above does not apply at all. ABS cases need an EDR pull instead.

Correct mark identification is critical to reconstruction. Conflating a yaw mark with a braking mark is the most common error in Indian SFSL hit-and-run reports, and defence counsel regularly use this error to challenge the prosecution's speed estimate.

1. Braking mark
   Continuous dark skid produced by a fully locked wheel sliding on the road. Even width, straight or gently curved, sometimes shows tyre-tread imprint at the start before heat erases it. Length is the input to the speed formula. Use μ for the surface, n for how many wheels locked.
2. Scuff mark
   Partial slide where the wheel is rotating slower than the vehicle's translational speed. Patchy, mottled appearance. Common with ABS-equipped vehicles. Does not give a clean speed estimate on its own; an EDR pull or an in-depth crush-energy analysis becomes necessary.
3. Yaw mark
   Curved striated mark left when a vehicle is cornering at the limit of lateral grip and slides sideways while still rolling. The striations run obliquely across the mark, not along its length, which is the diagnostic. Speed-at-yaw is calculated from chord and middle ordinate via S = √(127 · R · μ) in km/h, where R is the radius of the path.
4. Acceleration mark
   A skid produced when the driven wheels spin under hard acceleration before grip is established. Tapered, with the wider end at the start. Usually short. Relevant in racing-related and ramming cases under BNS provisions on rash driving.
5. Gouge mark
   A scrape or trench cut into the pavement when a metal undercarriage component (frame rail, axle, exhaust, broken suspension) contacts the road during or after impact. Locates the vehicle at maximum collision force with much greater precision than the resting position.

The yaw mark is the primary evidence source in a single-vehicle loss-of-control case and one of the few situations where speed can be estimated without a straight braking skid. The vehicle is mid-corner, the driver's input has exceeded available grip, and the car slides outward in an arc. Measure the chord of the arc, measure the middle ordinate, compute the radius, plug into S = √(127·R·μ), and you have an estimate that is independent of braking distance.

Once a candidate vehicle is recovered, whether at the scene or weeks later through ANPR work as set out in Speed Detection Devices, the examination protocol has six anchor checks. Each one is documented separately in the SFSL report.

• Paint examination. Multi-layer paint samples are taken from every damage zone and matched against transfer paint on the struck object or victim. Indian SFSLs use FTIR-microscopy and pyrolysis GC-MS as the workhorse methods; layer sequence from primer outward to topcoat is the signature. See Paint Evidence, Layers and Instrumental Analysis for the layer-by-layer method.
• Glass embedment. Tyres, undercarriage and bumper fascia are checked for embedded glass shards. The shard refractive index and density are matched against glass at the scene; a match places the vehicle at the impact location with high probative weight.
• Headlight and turn-signal filament examination. A filament that was hot at the moment of impact stretches and oxidises in a characteristic way; a filament that was cold (lamp off) shatters cleanly. The microscopic appearance tells you whether the lamp was on or off, which is decisive in night-time hit-and-run cases where the defence is "the lights were on, I never saw the pedestrian."
• Tyre damage and tread. Cuts, bead damage and embedded debris on the tyre face. The tread pattern is cast for comparison with any tyre-impression evidence at the scene.
• Undercarriage inspection. Hair, fibre, blood and tissue can be transferred to the undercarriage in a pedestrian run-over case. The vehicle is put on a hoist and photographed under a UV/ALS hand lamp; high-yield zones are the rear axle, the exhaust shield and the chassis cross-members.
• EDR download. Vehicles sold in India after AIS-184 (April 2024) carry an Event Data Recorder. The IO recovers pre-crash speed, throttle position, brake application, steering wheel angle and seatbelt status for the seconds before airbag deployment. The download requires the vehicle's CAN-bus interface and the manufacturer's CDR tool; the resulting log is admitted under section 63 BSA as an electronic record.

A hit-and-run case in India runs through a defined sequence: scene response, victim recovery, evidence collection, vehicle search via ANPR plus FASTag data, vehicle examination at SFSL, paint and glass comparison, and the file submission within the BNSS time-bound investigation window. BNS 106 prescribes up to five years for causing death by negligence in a hit-and-run where the driver does not report; BNSS 2023 requires the investigation to be completed within 90 days for offences punishable up to 10 years.

The high-yield evidence carriers in a hit-and-run, in priority order:

• Victim clothing. Paint smears at contact zones (front of legs for pedestrians, lower torso for cyclists), embedded glass, tyre-tread impressions. The clothing is preserved per the packaging protocols set out in Processing Physical Evidence at the Scene.
• Scene-recovered paint chips. Multi-layer flakes lifted from the road surface or the struck object. The layer count and layer sequence narrow the candidate-vehicle make and model before any plate work begins; SFSL maintains a paint-database query against the National Forensic Sciences University's reference set.
• Glass at the scene. Headlight glass, windscreen fragments, side-window cubes (toughened glass shatters into characteristic cubes). Refractive index, density and elemental profile match the recovered vehicle's intact glass.
• Tyre tracks. Cast in plaster of Paris or dental stone where the surface allows. Tread pattern, wear pattern and any cut signatures narrow the suspect vehicle.
• CCTV and ANPR pulls. As detailed in Speed Detection Devices, the timestamped vehicle-location data is the fastest path from scene to candidate plate.
• Chain of custody. Every item, every transfer, every seal. The packet protocol and Form-95 entries follow Chain of Custody without compromise; CoC breaks are the single most common reason hit-and-run prosecutions collapse on appeal.

The common errors that recur in SFSL hit-and-run files reviewed by NFSU faculty: treating skid length as full stopping distance (under-estimates speed); failing to record road gradient (a 6 % downhill grade changes effective μ); conflating yaw with skid in single-vehicle loss-of-control cases (wrong formula, wrong answer); and reporting the EDR speed without authenticating the 65B/63 certificate at the time of seizure.