10.3 Practical Collision Investigation, Field Sketches & Reconstruction

10.3 Practical Collision Investigation, Field Sketches & Reconstruction

AZPOST Comprehensive Examination Focus: Recruits must demonstrate the ability to conduct practical traffic collision investigations, comply with Arizona Crash Report statutory mandates under A.R.S. § 28-667, accurately execute field measurements using both coordinate and triangulation methods tied to tangible reference points, distinguish rough field sketches from finished scale diagrams, apply hit-and-run and commercial motor vehicle (CMV)/Hazmat cargo protocols, analyze pedestrian collision dynamics and impact trajectories, and perform forensic lamp filament examinations (hot shock versus cold shock) pursuant to Ariz. Admin. Code R13-4-116(E)(1)(d)(iv).

Accurate collision scene measurement and diagramming convert raw physical evidence into an objective, permanent geometric record. In serious injury and fatal collisions, this documentation provides the foundational basis for forensic collision reconstruction, courtroom exhibits, and civil or criminal adjudication.

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1. Arizona Crash Report Mandates & Investigative Protocol (A.R.S. § 28-667)

Under A.R.S. § 28-667, a law enforcement officer who investigates a motor vehicle collision resulting in bodily injury, death, or total property damage to an apparent extent of $2,000 or more must complete an Arizona Crash Report within 24 hours of completing the investigation.

                               [ Arizona Crash Report Mandates ]
                                               |
       +---------------------------------------+---------------------------------------+
       |                                       |                                       |
[ Statutory Triggers ]                 [ Mandatory Report Fields ]             [ Statutory Deadline ]
- Any Injury or Death                  - Environmental conditions, lighting    - Completed within 24 Hours
- Total Property Damage >= $2,000      - Complete driver/vehicle credentials   - Transmitted to ADOT
- Occurs on Public Highway             - Physical evidence & sequence          - Public record per Title 28

Mandatory Report Elements

1. Administrative & Spatial Metadata: Date, time, day of week, jurisdiction, primary street, nearest intersecting cross street or highway milepost marker, and department incident report number.
2. Environmental & Roadway Profile: Ambient lighting (daylight, dusk, dawn, dark-lighted, dark-unlighted), weather (clear, rain, blowing dust, snow, severe crosswinds), road surface conditions (dry, wet, ice, loose gravel, construction residue), and roadway design (divided highway, multi-lane, one-way, two-way center turn lane).
3. Vehicle and Occupant Data: Full vehicle identification number (VIN), license plate state and number, year, make, model, registered owner, insurance carrier and policy number; driver license number and classification; seating positions of all occupants, restraint system usage, airbag deployment status, and suspected impairment indicators.
4. Sequence of Events & Contributing Factors: First harmful event, most harmful event, vehicle maneuvers prior to crash, point of initial contact, driver actions (speeding, failing to yield, red light violation, distracted driving), officer narrative, and scene diagram.

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2. Roadway Measuring Systems: Coordinate vs. Triangulation Methods

To preserve physical evidence before roadway clearance, investigators must utilize standardized measuring techniques tied to permanent geographic anchors:

     [ COORDINATE METHOD ]                           [ TRIANGULATION METHOD ]
     
        | (Perpendicular Distance)                        RP 1               RP 2
        |                                                   \                 /
        |----[Evidence Point]                                \               /
        |                                                     \             /
        |                                                      [Evidence Point]
  ======+===================== Baseline                ================================
       RP   (Distance along Baseline)                         (Base Distance RP1-RP2)

The Coordinate Method (Baseline and Perpendiculars)

• Procedure: The investigator establishes a straight or smoothly curved physical baseline along the roadway (such as a raised concrete curb, painted fog line, or edge of pavement). A permanent point along this line is designated as the Reference Point (RP) (the zero point).
• Measurements: Every evidence point is located using two measurements:
  1. Station Distance: The distance measured along the baseline from the zero reference point.
  2. Perpendicular Distance: The distance measured outward at a strict 90-degree right angle from the baseline to the item of evidence.
• Best Suited For: Long, straight roadways, uniform divided corridors, and standard rectilinear intersections with defined curbs.

The Triangulation Method

• Procedure: The investigator selects two permanent, immovable Reference Points (RP1 and RP2) on or near the roadway. The straight-line distance between RP1 and RP2 is carefully measured and recorded as the baseline of the triangle.
• Measurements: For each evidence point, straight-line distance measurements are taken directly from RP1 to the evidence point, and from RP2 to the evidence point, forming a geometric triangle.
• Geometric Constraint: To prevent severe geometric distortion and plotting error, the angle formed at the evidence item (the apex angle) should ideally be between 30 degrees and 150 degrees. Angles less than 30 degrees (too narrow) or greater than 150 degrees (too flat) dramatically magnify measurement errors.
• Best Suited For: Irregular, curved, or winding roadways, wide intersections lacking straight curbs, off-road rollover sites, and desert or rural dirt surfaces.

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3. Reference Points: Tangible vs. Intangible

Every measurement on a collision diagram must originate from a reliable reference point to ensure evidence positions can be re-established years later in court:

[!IMPORTANT] Mandatory Rule for Intangible Points: Every intangible reference point MUST be tied back to at least two permanent, tangible reference points with precise baseline or triangulation measurements. An unanchored intangible point is legally useless in accident reconstruction.

Measuring Vehicle Final Rest Positions

A motor vehicle is a rigid, rectangular three-dimensional object. Measuring a single point (such as the center of the vehicle) is insufficient because it fails to record the vehicle's angular orientation or heading. Officers must document at least TWO points on each vehicle (typically the centers of the front and rear wheel hubs on the same side, or two opposite diagonal corners).

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4. Field Sketching vs. Finished Scale Diagramming

[ Rough Field Sketch ]                              [ Finished Scale Diagram ]
- Drawn at the scene during investigation           - Prepared later using CAD or drafting tools
- NOT to scale (labeled "Not to Scale")             - Drawn to precise scale (e.g., 1" = 10' or 20')
- Contains raw measurements & dimension lines       - Clean, uncluttered; measurements in tables
- Contemporaneous field note (permanent evidence)   - Formal courtroom trial exhibit for jury

Comparison of Diagram Formats

Mandatory Elements of a Field Sketch

Every field sketch must include:

1. North Arrow: Clearly indicating magnetic or true North.
2. Roadway Geometry: Edges of pavement, curbs, lane lines, medians, shoulders, crosswalks.
3. Traffic Control Devices: Stop signs, traffic signals, turn arrows, speed limit signs.
4. Physical Roadway Evidence: Skid marks (labeled with lengths), yaw marks, gouge marks, scrapes, fluid spills, debris fields.
5. Vehicle Final Rest Positions: Differentiating uncontrolled final rest (vehicles coming to a halt solely from collision dynamics) from controlled final rest (vehicles intentionally driven or moved by operators after impact).
6. Reference Points: Clearly labeled RP1, RP2, or Baseline with physical description.
7. Legend & Symbol Key: Identifying vehicles (V1, V2), pedestrians (P1), and evidence items (G1 = gouge, D1 = debris).
8. Administrative Metadata: Agency case number, date, time, location, and officer's name and badge number.

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5. Hit-and-Run Investigation Protocols & Forensic Canvassing (A.R.S. §§ 28-661 through 28-665)

Hit-and-run collisions demand immediate, aggressive forensic processing before physical trace evidence is obliterated by vehicular traffic or weather:

• Impact Zone Evidence Sweep: Conduct an organized grid search of the Area of Impact. Collect fractured lamp lenses, grille fragments, trim pieces, side mirror housings, and underbody components.
• Decoding Automotive Lens Markings: Headlight, turn signal, and taillight lenses are manufactured under strict Federal Motor Vehicle Safety Standards (FMVSS 108). Lenses carry molded markings including SAE/DOT codes, optical mold numbers, manufacturer trade logos, and date stamps. Forensic databases and dealership parts catalogs can translate these codes to pinpoint the exact vehicle make, model, and year span.
• Multilayer Paint Fragment Recovery: Recover paint chips from the roadway or paint transfer scraped onto the victim vehicle. Store paint flakes in rigid containers (druggist folds/bindles) to preserve layer structure. Microscopic Fourier-transform infrared spectroscopy (FTIR) compares the multilayer structure (electrocoat, primer-surfacer, basecoat, clearcoat) against the Royal Canadian Mounted Police / FBI Paint Data Query (PDQ) database to identify vehicle make, model, and plant of manufacture.
• Digital & Community Canvassing: Query local Automated License Plate Readers (ALPR) along flight corridors; examine commercial CCTV, intersection red-light cameras, transit bus exterior cameras, and private video doorbells; notify local body repair shops, scrap yards, and glass replacement companies.

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6. Commercial Motor Vehicle (CMV) & Hazardous Materials Collision Protocols

Collisions involving commercial motor vehicles (CMVs) and hazardous materials (Hazmat) cargo present extreme life-safety risks and complex regulatory frameworks under the Federal Motor Carrier Safety Regulations (FMCSR, 49 CFR Parts 350–399) and Arizona law (A.R.S. Title 28, Chapters 14 & 19).

[ Initial Hazmat Approach ] ---> [ Identify Placard & UN Number ] ---> [ Consult ERG ] ---> [ Establish Isolation Perimeter ]
  Upwind, uphill, upstream;        4-digit UN/NA ID number;             Orange guide;         Safe standoff distance;
  binoculars from distance         diamond hazard placard               protective actions    evacuate downwind zone

Hazardous Materials Response Protocols

1. Tactical Approach: Always approach a CMV crash scene from upwind, uphill, and upstream. Position cruisers at a safe standoff distance and utilize binoculars to assess damage before approaching.
2. Placard Identification & UN Numbers: Identify DOT diamond placards and 4-digit UN/NA identification numbers displayed on cargo tanks or trailers. The 9 DOT hazard classes encompass:
   • Class 1: Explosives
   • Class 2: Gases (Flammable, Non-flammable, Toxic)
   • Class 3: Flammable and Combustible Liquids (gasoline, diesel)
   • Class 4: Flammable Solids
   • Class 5: Oxidizers and Organic Peroxides
   • Class 6: Toxic Materials and Infectious Substances
   • Class 7: Radioactive Materials
   • Class 8: Corrosive Substances (acids, bases)
   • Class 9: Miscellaneous Hazardous Materials (lithium batteries, dry ice)
3. Utilizing the Emergency Response Guidebook (ERG):
   • Yellow Pages: Index of substances by 4-digit UN identification number.
   • Blue Pages: Alphabetical index of substances by chemical name.
   • Orange Pages: Primary safety guides detailing public safety, fire response, spill containment, and evacuation distances.
   • Green Pages: Table of Initial Isolation and Protective Action Distances for toxic inhalation hazard (TIH) materials.
4. Cargo Tank Transport Types: Recognize common cargo tank designs: DOT 406 (non-pressure liquid tank, oval cross-section, transports gasoline and fuel oil); DOT 407 (low-pressure chemical tanker, circular cross-section, often insulated); DOT 412 (corrosive liquid tanker, narrow circular cross-section with external stiffening rings); MC 331 (high-pressure gas tank, rounded bullet-shaped ends, transports propane and anhydrous ammonia); MC 338 (cryogenic liquid tank, vacuum-insulated double wall).
5. Post-Crash CMV Mechanical Inspections: In fatal and serious CMV collisions, a certified Commercial Vehicle Inspector conducts a comprehensive post-crash inspection under 49 CFR Part 396. This includes measuring pushrod travel on air brake chambers (identifying out-of-adjustment brakes), steering lash, coupling fifth-wheel integrity, tire load ratings, downloading electronic engine control module (ECM) data, and auditing driver Electronic Logging Devices (ELD) for Hours of Service (HOS) violations.

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7. Pedestrian Collision Dynamics & Impact Trajectories

Pedestrian collision dynamics depend on the height of the striking vehicle's front profile relative to the pedestrian's Center of Gravity (CG). In an adult, the center of gravity is located approximately at the mid-pelvis (near the navel); in a small child, the center of gravity is significantly higher relative to height (near the sternum/chest).

                                  [ Pedestrian Collision Trajectories ]
                                                    |
       +--------------------+-----------------------+-----------------------+--------------------+
       |                    |                       |                       |                    |
[ Wrap Trajectory ]  [ Forward Projection ]  [ Roof Vault ]          [ Fender Vault ]     [ Slide and Drag ]
Low bumper / hood    High front-end (SUV)    High speed passenger    Vehicle turning or   Pedestrian overrun;
strikes adult legs;  strikes above CG;       car; pedestrian clears  pedestrian glances   dragged beneath
upper body rotates   pedestrian thrown       windshield and roof     off front fender     vehicle undercarriage
onto hood / cowl     forward on roadway      completely to the rear  to the side          along pavement

The Five Primary Pedestrian Trajectories

1. Wrap Trajectory:
   • Mechanism: Typical when a vehicle with a low bumper and aerodynamic sloping hood (standard passenger car) strikes an adult pedestrian.
   • Kinematics: The bumper strikes the pedestrian's lower legs below their center of gravity, sweeping the legs forward. The upper torso rotates downward onto the hood. The pedestrian's head strikes the windshield, cowl, or A-pillar. As the vehicle decelerates, the pedestrian is thrown forward off the hood onto the roadway.
2. Forward Projection Trajectory:
   • Mechanism: Typical when a vehicle with a high front-end profile (full-size SUV, pickup truck, transit bus) strikes an adult, OR when a standard passenger car strikes a small child whose center of gravity is below the bumper line.
   • Kinematics: The vehicle strikes the pedestrian at or above their center of gravity. The pedestrian is knocked violently forward in the direction of vehicle travel, impacts the pavement ahead of the vehicle, and slides/rolls to final rest. Carries high risk of secondary run-over.
3. Roof Vault Trajectory:
   • Mechanism: Occurs in high-speed collisions (typically 40+ mph) involving passenger cars with low frontal profiles.
   • Kinematics: The pedestrian is struck below the center of gravity at high velocity, vaults completely over the hood and roof of the vehicle, and lands on the pavement behind the vehicle.
4. Fender Vault Trajectory:
   • Mechanism: Occurs when a vehicle strikes a pedestrian near the outer corner of the bumper or while swerving.
   • Kinematics: The pedestrian glances off the vehicle fender, vaults over or around the fender to the side, and comes to rest on the roadway shoulder.
5. Slide and Drag (Under-ride) Trajectory:
   • Mechanism: Occurs when a pedestrian is knocked down or already lying prone on the roadway and is overrun by a vehicle, becoming entangled in the undercarriage and dragged along the pavement.

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8. Lamp Filament Examination: Hot Shock vs. Cold Shock

In nighttime and low-light collisions, determining whether a vehicle's headlights, taillights, or turn signals were ON or OFF at the instant of impact is frequently contested. Forensic examination of incandescent tungsten filaments provides definitive physical proof:

[ HOT SHOCK ]                                        [ COLD SHOCK ]
Filament was ON / Illuminated at Impact              Filament was OFF / Unlit at Impact
- Tungsten heated to incandescent 2,500°C            - Tungsten at ambient temperature (brittle)
- Soft, ductile metal stretches, uncoils, arcs       - Clean, jagged, brittle fracture without stretching
- Bulb breaks -> Tungsten Oxide (WO3) yellow-white   - Bulb breaks -> NO oxidation powder on posts
  powder deposits on glass shards & posts            - Coil geometry remains tight and regular

Physical Characteristics of Filament Shock

• Hot Shock (Lights ON at Impact):
  • When a light is illuminated, electrical current heats the tungsten coil to an incandescent temperature of 2,200°C to 2,500°C (4,000°F to 4,500°F), rendering the tungsten ductile and pliable.
  • Upon severe collision impact, inertia causes the softened, glowing tungsten to stretch, sag, uncoil, and twist like warm wire.
  • Adjoining coils may touch, producing electrical arcing and molten metal fusing.
  • Tungsten Oxide Residue ($WO_3$): If the glass bulb envelope breaks during the impact, atmospheric oxygen enters the chamber. The white-hot tungsten reacts instantly with oxygen, producing a yellowish-white smoke and powdery deposit of tungsten oxide ($WO_3$) that coats the inside of the glass fragments and filament support posts.
• Cold Shock (Lights OFF at Impact):
  • When unlit, the tungsten filament is at ambient temperature and is brittle.
  • Upon severe collision impact, the brittle filament snaps cleanly with sharp, jagged fractures.
  • The broken pieces retain their original tight, uniform coil spacing without any uncoiling, sagging, or plastic stretching.
  • Even if the glass envelope shatters, no tungsten oxide powder forms because the cold metal cannot oxidize.
• Modern LED and HID Lighting: Light Emitting Diodes (LED) and High-Intensity Discharge (HID) xenon lights do not utilize tungsten filaments. To determine illumination status in modern vehicles, investigators inspect light switch physical positions, analyze event data recorder (EDR) records, and evaluate Body Control Module (BCM) crash telemetry.

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9. High-Yield Exam Traps & Operational Scenario

High-Yield Exam Traps for Section 10.3

1. Crash Report Damage Threshold: A.R.S. § 28-667 requires a written report if total property damage is $2,000 or more, or if any person is injured or killed, submitted within 24 hours.
2. Triangulation Angle Limits: The apex angle at the evidence item in triangulation must be between 30° and 150° to prevent geometric distortion.
3. Intangible Reference Anchoring: Every intangible reference point (curb extension, centerline intersection) must be tied back to at least two tangible reference points.
4. Pedestrian Trajectory Triggers: High-profile vehicles (SUVs, trucks) or small children produce forward projection; low-profile passenger sedans striking adult pedestrians produce wrap trajectories.
5. Hot vs. Cold Shock Indicators: Hot shock produces stretched, uncoiled filaments and yellow-white tungsten oxide powder; cold shock produces clean brittle fractures without uncoiling.

Operational Application Scenario

Scenario: Officer Vance responds to a nighttime collision at an unlighted intersection. Vehicle 1 (sedan) turned left across oncoming traffic, colliding head-on with Vehicle 2 (pickup truck). Driver 1 claims Driver 2 was driving without headlights, making the pickup invisible. Driver 2 claims headlights were fully illuminated on low-beam. Both vehicles sustained heavy front-end crushing. An adult pedestrian walking along the roadway edge was struck by the spinning sedan and sustained fractured lower legs and head trauma from striking the sedan's hood and windshield.

Officer Vance recovers the broken headlight assemblies from Vehicle 2. Laboratory microscopic examination of Vehicle 2's low-beam headlight bulb reveals that the outer glass envelope shattered upon impact, the tungsten filament coils are severely stretched, uncoiled, and fused to the support post, and the interior bulb surface is coated with a distinct yellowish-white powdery residue of tungsten oxide ($WO_3$).

• Forensic Lamp Analysis: The uncoiled ductile stretching and tungsten oxide powder constitute textbook hot shock, scientifically proving Vehicle 2's headlights were illuminated and glowing hot at the exact instant of impact, directly refuting Driver 1's claim.
• Pedestrian Dynamic Analysis: The adult pedestrian's lower leg bumper fractures and subsequent hood/windshield head impact confirm a classic wrap trajectory caused by the low-profile passenger sedan.
• Crash Report Statutory Compliance: Because personal injury occurred and property damage exceeded $2,000, Officer Vance completes and submits the Arizona Crash Report within the mandatory 24-hour statutory deadline under A.R.S. § 28-667.