Collision Investigation

Collision reconstruction applies physics — work-energy, momentum conservation, kinematics — to the evidence at a crash scene. The standard analytical battery includes skid-mark speed, yaw-mark speed, pedestrian-throw distance, EDR data, conservation of momentum for multi-vehicle collisions, AEB analysis, and rollover stability via the Static Stability Factor.

8.1Skid-Mark Speed

Drag factor reference values

Worked examples (dry asphalt μ = 0.7)

For ABS-equipped vehicles, the formula does NOT apply directly — ABS produces intermittent dotted marks rather than full-wheel-lock skid. Use EDR data instead.

8.2Yaw-Mark Speed

Example: r = 40 m, μ = 0.7 → v = √(9.81 × 40 × 0.7) ≈ 16.6 m/s ≈ 60 km/h. Yaw marks are curved with striations across the tyre (rotating but slid sideways), often wider than skid marks.

8.3Pedestrian Throw Distance

Multiple empirical equations relate pedestrian-throw distance to vehicle impact speed:

• Searle: v = √(2gd × μ / (1 + μ²))
• Han: v ≈ 4.06 + 0.89 × √d (m units)
• Wood: v ≈ 0.0257d + 4 (variable form)

For an 18 m throw, equations give 8–14 m/s (28–50 km/h). The forensic engineer reports a range with the equation used and method assumptions.

8.4Conservation of Momentum

Example: Vehicle A (1400 kg, 60 km/h) + Vehicle B (1100 kg, 80 km/h) lock together: v_f = (1400 × 60 + 1100 × 80) / 2500 = 68.8 km/h.

8.5Event Data Recorder (EDR / "Black Box")

Modern passenger vehicles record pre-crash data per SAE J1698 — typically integrated into the airbag-control or engine-control module. Captures the last 5–15 seconds before triggering event:

• Vehicle speed
• Engine RPM, throttle position
• Brake-pedal application percentage
• Steering-wheel angle
• ABS, traction-control, stability-control activation
• Seat-belt status
• Delta-v at impact

For ABS-equipped vehicles, the EDR is the primary source of pre-crash speed, replacing the skid-distance formula.

8.6AEB Analysis

AEB performance combines time-to-collision (TTC), reaction time, and maximum deceleration:

1. TTC at detection = distance / closing speed
2. Reaction distance = reaction time × speed
3. Available braking distance = total distance − reaction distance
4. Required braking distance = v² / (2a)

If required < available: AEB stops in time; collision preventable. Worked example: 50 km/h vehicle, pedestrian at 25 m, AEB reaction 0.4 s, max deceleration 7 m/s² → required 13.78 m vs available 19.4 m → AEB should prevent collision.

Sensor degradation in fog

• Camera — most affected by fog (Mie scattering)
• Lidar — moderately affected
• Radar (77 GHz) — least affected (radio wavelength >> fog droplet)

8.7Static Stability Factor (Rollover)

8.8Airbag + Seatbelt — Complementary

Airbags and seatbelts are designed to work together, not as alternatives. With seatbelt: belt restrains occupant + airbag inflates, occupant arrives at airbag at low speed and gently decelerates. Without seatbelt: occupant continues at full vehicle speed; inflating airbag impacts face / chest at high speed → injury from inflation force itself.

An injury pattern of facial trauma + chest impact in an airbag-deployed unbelted occupant is consistent with the inflating-airbag-as-injury-source mechanism.

8.9Hit-and-Run Examination

1. Bodywork — fresh dents, paint cracking, recent re-paint (UV photography)
2. Paint / pigment transfer — SEM-EDX, FTIR, PDQ database matching
3. Glass — refractive index (GRIM), composition (LIBS / ICP-MS)
4. Biological — blood, tissue, hair on grille / bumper / undercarriage; DNA-typed
5. Tyre tread — comparing suspect tyre against impressions and shed rubber
6. Mechanical — engine compartment, suspension for under-vehicle contact debris