10.3 Traffic Crash Investigation

10.3 Traffic Crash Investigation

Traffic crash investigation is a critical law enforcement responsibility that combines criminal law enforcement, physical scene investigation, evidence collection, and detailed technical documentation. Police officers responding to motor vehicle crashes must secure the scene, provide emergency medical relief, identify physical evidence, determine causation, and enforce applicable provisions of the Pennsylvania Vehicle Code (Title 75).

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Duty to Stop & Render Aid: Statutory Offenses

Pennsylvania law imposes strict statutory duties on drivers involved in motor vehicle collisions. Drivers who flee the scene of a crash ("hit and run") face criminal prosecution under Title 75, Chapter 37, based on the severity of consequences.

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On-Scene Crash Investigation Methodology

When dispatched to a motor vehicle crash, responding officers must execute a systematic, prioritized operational sequence to manage hazards and preserve scene integrity.

                    On-Scene Crash Response Sequence
  ┌─────────────────────────────────────────────────────────────┐
  │ 1. Arrival & Scene Safety: Position cruiser, control traffic│
  │    mitigate fire/fuel hazards, establish inner/outer perimeter│
  ├─────────────────────────────────────────────────────────────┤
  │ 2. Emergency Medical Assistance: Render first aid, triage   │
  │    injured persons, coordinate EMS response & transport      │
  ├─────────────────────────────────────────────────────────────┤
  │ 3. Physical Evidence Preservation: Protect tire marks, gouges│
  │    debris fields, final rest positions; place evidence markers│
  ├─────────────────────────────────────────────────────────────┤
  │ 4. Driver & Witness Interviews: Separate drivers immediately│
  │    obtain independent statements, verify operator licenses   │
  └─────────────────────────────────────────────────────────────┘

Step 1: On-Scene Arrival & Hazard Mitigation

• Vehicle Positioning: Park emergency vehicles to shield the crash scene and emergency personnel from oncoming traffic. Use emergency lighting and traffic cones.
• Hazard Identification: Check for downed power lines, spilled fuel, hazardous materials (HAZMAT placarding), or fire risks.
• Perimeter Control: Establish a secure perimeter to prevent evidence destruction by bystanders or passing traffic.

Step 2: Emergency Medical Assistance

• Render immediate life-sustaining first aid until EMS arrives.
• Document the original positions of injured occupants before they are moved by medical personnel whenever possible.
• Identify hospital transport destinations for drivers and passengers.

Step 3: Securing Physical Evidence

Identify, flag, and preserve short-lived evidence (fluid trails, blood, tire marks, loose debris) before environmental conditions or emergency vehicles disrupt the scene.

Step 4: Driver & Witness Interviews

• Separate Parties Immediately: Prevent drivers and witnesses from conversing to avoid statement contamination.
• Obtain Detailed Statements: Ask open-ended questions regarding pre-crash vehicle movements, travel speed, traffic signals, and weather/visibility conditions.
• Check Credentials: Verify driver licenses, vehicle registrations, and proof of financial responsibility (insurance).

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Physical Evidence Identification & Reconstruction Concepts

Physical evidence on the roadway provides objective proof of pre-collision speed, braking, steering inputs, and collision impact points.

Types of Roadway Evidence

                          Roadway Evidence Types
  ┌─────────────────┬─────────────────┬─────────────────┬─────────────────┐
  │   Skid Marks    │    Yaw Marks    │ Gouges & Scraps │  Debris Field   │
  ├─────────────────┼─────────────────┼─────────────────┼─────────────────┤
  │ Locked wheel    │ Curved tire     │ Deep pavement   │ Scattered glass,│
  │ braking; shows  │ scrub from      │ cuts from metal │ vehicle parts,  │
  │ straight line   │ steering loss   │ vehicle parts   │ fluids; identifies│
  │ speed reduction │ of control      │ during impact   │ impact centroid │
  └─────────────────┴─────────────────┴─────────────────┴─────────────────┘

1. Skid Marks: Caused by a locked, sliding tire that is non-rotating during heavy braking. The sliding tire heats and melts asphalt oil, leaving a dark, straight line. Skid marks allow investigators to calculate minimum pre-braking speed.
2. Yaw Marks (Tire Scrub): Curved, diagonal striations caused by a tire that is rolling and sliding sideways simultaneously due to severe steering inputs during a loss of control.
3. Gouges & Scraps: Physical trenches or scratches cut into the roadway pavement by hard, metallic vehicle structural components (e.g., frame, axle) upon impact. Gouges frequently pinpoint the Area of Impact (AOI).
4. Debris Fields: The pattern of broken glass, plastic, paint chips, and vehicle fluids spread across the roadway. The point of maximum density (debris centroid) helps reconstruct the collision sequence.
5. Final Rest Positions: The locations where vehicles come to rest after impact. Investigators distinguish between controlled rest (driver steered vehicle to shoulder) and uncontrolled rest (post-impact momentum carried vehicle to rest).

Skid Mark Speed Calculation Formula

Investigators estimate minimum vehicle speed ($S$) prior to braking using the standard drag formula:

$S = \sqrt{30 \cdot d \cdot f}$

Where:

• $S$ = Minimum speed in miles per hour (MPH).
• $d$ = Total skid mark distance in feet.
• $f$ = Drag factor (coefficient of friction) of the road surface (e.g., 0.70 for dry asphalt).
• $30$ = Mathematical conversion constant.

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PennDOT AA-500 Crash Report Drafting

In Pennsylvania, officers must complete the official PennDOT Form AA-500 (Commonwealth of Pennsylvania Police Health and Safety Crash Report) for all reportable crashes.

Criteria for a Reportable Crash

Under 75 Pa.C.S. § 3746, a crash is reportable if it involves:

1. Injury to any person; OR
2. Death of any person; OR
3. Vehicle damage severe enough to require towing from the scene.

Key Components of Form AA-500

• Header & Event Data: Exact date, time, municipality, county, highway/route number, and GPS coordinates.
• Driver & Vehicle Information: License data, insurance policy details, VIN, vehicle make/model/year, and damage severity rating.
• Environmental Factors: Weather condition, lighting (daylight, dark-lighted), roadway surface condition (dry, wet, icy).
• Factual Narrative: A concise, chronological summary describing vehicle actions prior to, during, and after impact. Narrative must be strictly factual, avoiding unsupported opinions.
• Field Diagramming: A proportional scale diagram illustrating:
  • Roadway geometry, lanes, and North directional arrow.
  • Pre-impact travel direction arrows for all units.
  • Point of Impact (POI) / Area of Impact (AOI).
  • Physical evidence locations (skid marks, gouges).
  • Final rest positions of all vehicles and pedestrians.

By combining rigorous physical evidence collection with accurate PennDOT AA-500 reporting, officers preserve evidence vital for criminal prosecution, civil litigation, and highway safety improvements.