10.2 Traffic Collision Investigation Principles & Physical Evidence

10.2 Traffic Collision Investigation Principles & Physical Evidence

AZPOST Comprehensive Examination Focus: Recruits must demonstrate proficiency in collision scene management, prioritizing responder and scene safety, medical triage, traffic control, and enforcement of Arizona hit-and-run statutes under A.R.S. §§ 28-661 through 28-665. Furthermore, candidates must master the forensic identification, classification, and interpretation of roadway physical evidence—including skid, yaw, scuff, and imprint tire marks, contact versus induced vehicle damage, and debris patterns—and apply mathematical formulas to calculate minimum vehicle speed from physical evidence pursuant to Ariz. Admin. Code R13-4-116(E)(1)(d)(iii).

Traffic collision investigation requires peace officers to operate simultaneously as emergency scene managers, forensic evidence collectors, and statutory investigators. Documenting physical roadway evidence accurately is vital to reconstructing collision mechanics, determining fault, and supporting criminal prosecutions for vehicular homicide or aggravated assault.

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1. Collision Scene Management Priorities & Safety Protocols

When responding to a traffic collision, an officer must adhere to a strict sequence of operational priorities. Tactical discipline and scene safety must always precede evidence documentation:

[ Phase 1: Safe Arrival ] ---> [ Phase 2: Scene Protection ] ---> [ Phase 3: Triage & Care ] ---> [ Phase 4: Investigation ]
  Emergency driving with         Cruiser fend-off angle;          START triage protocol;          Perimeter security; mark
  due regard; spot hazards       wear ANSI retro vest             airway & tourniquets            evidence & diagram scene

Operational Priority Breakdown

1. Safe Arrival & Vehicle Positioning: Operate the patrol cruiser with due regard under A.R.S. § 28-624. Upon arrival, position the patrol vehicle in a fend-off position (angled 30 to 45 degrees across travel lanes, front wheels turned away from the work zone). This redirects oncoming traffic away from responders and creates a physical barrier that deflects errant vehicles away from the scene if struck. Officers must don an ANSI/ISEA 107 Class 2 or 3 high-visibility retroreflective safety vest.
2. Scene Protection & Hazard Assessment: Identify catastrophic scene hazards: downed power lines (maintain a minimum 30-foot perimeter; never touch vehicles in contact with wires), ruptured fuel tanks, hazardous materials placards, and structural instability of crushed vehicles.
3. Triage and Medical Care: Conduct initial casualty triage using the START protocol (Simple Triage and Rapid Treatment). Check responsiveness, open airways, apply immediate arterial tourniquets or pressure bandages to control massive hemorrhage, and direct responding EMS units to critical victims.
4. Traffic Control & Perimeter Containment: Deploy flares, reflective cones, and warning signs upstream from the collision to provide advance warning. If a fatal or serious physical injury collision has occurred, establish an extensive crime scene perimeter, cordoning off all roadway evidence with yellow barrier tape.

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2. Arizona Hit-and-Run Statutory Architecture (A.R.S. §§ 28-661 through 28-665)

Arizona statutes establish strict criminal accountability for drivers who fail to remain at collision scenes and fulfill statutory duties of identification and assistance.

Hit-and-Run Investigative Procedures

• Canvassing for Physical Clues: Search the impact zone for vehicle trim, paint transfer, shattered glass, and broken headlamp/taillight lens fragments. Automotive lens fragments often contain stamped SAE/DOT numbers, mold numbers, or manufacturer part codes that identify the exact year, make, and model of the suspect vehicle.
• Paint Sample Recovery: Recover foreign paint transfers and paint chips using clean forensic bindles. Laboratory micro-spectrophotometry can match multilayer automotive primer, basecoat, and clearcoat systems against the FBI Paint Data Query (PDQ) database.
• Broadcast & Technology Integration: Immediately broadcast a comprehensive BOLO (Be On the Lookout) alert containing vehicle make, model, color, direction of flight, and specific impact damage patterns. Query local Automated License Plate Readers (ALPR), intersection red-light cameras, commercial CCTV surveillance footage, and check area auto body repair shops and glass replacement facilities.

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3. Physical Roadway Evidence: Tire Mark Classifications

Tire marks on road surfaces provide primary objective evidence of vehicle dynamics, driver braking behavior, steering inputs, and speed prior to impact.

                                  [ Roadway Tire Mark Classifications ]
                                                   |
       +-------------------+-------------------+---+-------------------+-------------------+
       |                   |                   |                   |                   |
[ Skid Marks ]      [ Yaw Marks ]       [ Scuff Marks ]     [ Flat Tire Marks ]  [ Imprint Marks ]
Locked sliding tire; Critical speed     Rolling & sliding   Deflated / shredded  Tire rolling through
straight, shoulder  sideslip; curved,   simultaneously      tire; wavy, rim      liquid or soft dirt;
pavement marks      oblique striations                      gouges, sidewall     no sliding action

The Five Standardized Tire Mark Categories

1. Skid Marks:
   • Mechanism: Caused by a locked, sliding tire that is not rotating. Friction heats both the tire rubber and the roadway asphalt, melting the tire oils and bituminous binders to leave a dark deposit of molten rubber and road tar.
   • Tire Loading & Appearance: During heavy braking, dynamic weight transfer shifts the vehicle's normal force onto the front axle. This forces the front tires to compress against the asphalt, leaving darker, heavier marks along the outer shoulders (edges) of the tire path. Rear tires experience reduced normal force, leaving lighter, more uniform or center-weighted skid marks.
   • Gap Skids: Intermittent locked-wheel skid marks separated by spaces of 10 feet or greater. Created when a driver repeatedly applies, releases, and re-applies the brakes (manual pumping or reflex panic release). Each locked segment must be measured and calculated separately.
   • Skip Skids: Intermittent skid marks separated by short gaps of less than 3 to 5 feet. Caused by locked wheels bouncing across bumps, washboards, or dips in the road surface. The entire length, including the short air gaps, is measured as one continuous skid mark.
   • ABS Braking Marks: Vehicles equipped with Anti-Lock Braking Systems (ABS) prevent total wheel lockup, modulating brake caliper pressure multiple times per second. ABS marks appear as faint, shadowy cyclic tire smudges. Document and measure from the earliest observable shadow to final rest.
2. Scuff Marks:
   • Made by a tire that is rolling and sliding simultaneously (e.g., during rapid acceleration, swerving, or partial wheel lockup).
3. Yaw Marks (Critical Speed Scuffs / Sideslip Marks):
   • Mechanism: Produced when a vehicle traverses a curve at a speed exceeding the tire-road friction limit, causing the vehicle to slide outward laterally under centrifugal force while the tires continue to rotate.
   • Defining Characteristics:
     • The mark path is distinctly curved.
     • Shows distinct striations (scrub lines or grooves) that run diagonally or obliquely across the tire mark in the direction of the lateral slide.
     • The rear tires track outside the front tires (out-tracking).
     • The outside front tire produces the most pronounced, defining mark because cornering weight transfers to the outside front suspension.
4. Flat Tire Marks:
   • Produced by a deflated, punctured, or blown-out tire. Characterized by scalloped, wavy marks, two thin parallel tracks from the tire sidewalls being compressed between the wheel rim and pavement, and jagged rim gouges in the asphalt.
5. Imprint Marks:
   • Two-dimensional stamp left when a rolling tire passes through a fluid (water, wet paint, oil, blood) or a three-dimensional impression in soft earth (mud, sand, wet snow). Imprints capture the precise tread design without sliding.

Comparative Tire Mark Diagnostic Matrix

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4. Vehicle Damage Analysis & Area of Impact (AOI) Determination

[ Contact Damage ]                                   [ Induced Damage ]
Direct physical contact with foreign object          Remote structural deformation from stress transmission
- Crushed grills, sheared metal, bumper crumple      - Buckled roof pillars, warped quarter panels
- Paint transfer, foreign rubber scrubs, striations  - Shattered glass with NO direct contact point

Contact Damage vs. Induced Damage

• Contact Damage: Damage resulting from direct physical contact between a vehicle and another vehicle, pedestrian, or fixed obstacle. Characterized by crushed bumpers, sheared metal, paint transfer, external rubber rub marks, punctures, and scratches that match the shape of the striking object.
• Induced Damage: Secondary structural deformation or distortion caused by the transmission of impact shock waves through the unibody or vehicle frame. Examples include roof panel buckling, rippled fenders, bent chassis rails, or door frame pinching that occurs far from the initial point of physical impact.

Area of Impact (AOI) Determination

The Area of Impact (AOI) (or Point of Impact) is the precise spatial location on the roadway where the vehicles first collided. Reconstructing the AOI relies on specific physical roadway evidence:

1. Underbody Debris: Dried mud, rust, road dirt, and gravel dislodged from the vehicle's underside at the instant of maximum collision impact and structural deceleration. Underbody debris falls directly downward and serves as one of the most reliable indicators of the Area of Impact.
2. Liquid Spatter: The sudden explosive burst of engine fluids (radiator coolant, motor oil, battery acid) radiating outward in a starburst pattern upon impact, distinct from slow trailing drainage (liquid dribble) that leads to final rest.
3. Gouge Marks and Undercarriage Scrapes: Deep pavement gouges, chips, and scrapes carved into the asphalt when collision forces drive structural undercarriage parts (suspension arms, wheel rims, frame crossmembers) downward into the road surface.
4. Skid Mark Deflections (Offsets or "Crooks"): An abrupt, sharp angular deflection or offset in a locked-wheel skid mark occurring when an external collision force laterally shoves the sliding vehicle at impact, establishing the exact point of impact.

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5. Speed Estimation from Skid Marks: The Minimum Speed Formula

When a vehicle slides with locked wheels to a halt, work is done against friction to convert the vehicle's kinetic energy into thermal energy. Applying the work-energy theorem allows investigators to calculate the minimum pre-braking speed from skid mark length and roadway friction.

The Fundamental Skid Formula

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

Where:

• $S$ = Minimum pre-braking vehicle speed in miles per hour (mph).
• $30$ = Mathematical conversion constant derived from gravitational acceleration ($2 \times g = 2 \times 32.2 = 64.4\text{ ft/s}^2$) converted to units of miles per hour ($30.075$).
• $d$ = Total skid distance in feet (the average length of all four locked-wheel skid marks, or the longest skid marks when all four wheels are locked).
• $f$ = Drag factor (coefficient of friction) of the road surface.

Determining the Drag Factor ($f$)

The drag factor represents the frictional resistance between the vehicle tires and the pavement. It is measured on-scene using a calibrated drag sled ($f = \frac{\text{Pull Force}}{\text{Sled Weight}}$) or by conducting an instrumented test skid at a known speed.

| Roadway Surface Condition | Typical Drag Factor Range ($f$) | |:---|:---|:---| | Dry Asphalt / Concrete (Good) | 0.70 to 0.85 | | Wet Asphalt / Concrete | 0.50 to 0.65 | | Packed Gravel | 0.40 to 0.60 | | Wet Grass / Dirt | 0.30 to 0.45 | | Ice / Hard-Packed Snow | 0.10 to 0.20 |

• Grade Adjustment ($f' = f \pm G$): If the roadway has an uphill or downhill slope, the drag factor must be adjusted. For an uphill grade, add the decimal grade ($f + G$); for a downhill grade, subtract the decimal grade ($f - G$). (For example, a 4% downhill grade on 0.75 asphalt: $f' = 0.75 - 0.04 = 0.71$).
• Combined Speed Formula: If a vehicle skids across multiple surfaces (e.g., 60 feet on asphalt and 40 feet on gravel), or skids to a collision and then post-impact skids, calculate the speed dissipated on each segment and combine using: $S_{\text{total}} = \sqrt{S_1^2 + S_2^2}$

Perception-Reaction Time (PRT) and Stopping Distance

Total stopping distance encompasses both the distance traversed while the driver perceives and reacts to a hazard, plus the mechanical braking distance:

1. Perception-Reaction Time (PRT): The standard accepted forensic baseline is 1.5 seconds (comprising 0.75 seconds perception time to identify the hazard and 0.75 seconds reaction time to physically transfer the foot to the brake pedal).
2. Perception-Reaction Distance: The vehicle continues at constant velocity during PRT: $d_{\text{pr}} = 1.466 \times V \times t_{\text{pr}}$ (Where $1.466$ converts mph to feet per second, $V$ is speed in mph, and $t_{\text{pr}} = 1.5$ seconds). At 60 mph, a vehicle travels $1.466 \times 60 \times 1.5 = 132$ feet before braking begins!
3. Total Stopping Distance: $d_{\text{total}} = d_{\text{pr}} + d_{\text{skid}} = (1.466 \times V \times 1.5) + \frac{V^2}{30 \times f}$

Operational Calculation Example

Problem: A sedan leaves four locked-wheel skid marks averaging 120 feet on a level, dry asphalt roadway with a verified drag factor of $f = 0.75$. Calculate the vehicle's minimum speed at the start of the skid.

$S = \sqrt{30 \times 120 \times 0.75} = \sqrt{2700} \approx 51.96\text{ mph}$

[!WARNING] AZPOST High-Yield Exam Point: Minimum vs. Actual Speed: The skid formula calculates MINIMUM speed, never maximum or exact speed. This calculation accounts solely for energy dissipated while sliding with wheels locked. It ignores perception-reaction distance, pre-skid rolling braking, and residual kinetic energy absorbed in vehicle crush damage or post-impact travel. The vehicle was unquestionably traveling faster than 51.96 mph before braking.

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6. High-Yield Exam Traps & Operational Scenario

High-Yield Exam Traps for Section 10.2

1. Yaw vs. Skid Striation Geometry: Yaw marks have diagonal/oblique striations and rear-wheel out-tracking; skid marks have striations parallel to the tire path.
2. Measuring Gap vs. Skip Skids: Gap skids (driver released brakes, gaps > 10 ft) are measured as separate segments. Skip skids (bouncing wheels, gaps < 3-5 ft) are measured as one continuous mark.
3. Contact vs. Induced Damage: Contact damage involves direct physical impact, crushing, or paint transfer. Induced damage is remote deformation (buckled roof, warped panels) from stress transmission.
4. AOI Identifiers: Underbody debris, gouge marks, and skid mark offsets (crooks) establish the Area of Impact.
5. Hit-and-Run Felony Tiers: Fleeing an accident involving death or serious injury where the driver CAUSED the crash is a Class 2 Felony under A.R.S. § 28-661. If the driver did NOT cause the crash, it is a Class 3 Felony.

Operational Application Scenario

Scenario: Officer Gomez investigates a two-vehicle broadside collision at an urban intersection. Vehicle 1 (pickup truck) entered the intersection against a red light and struck Vehicle 2 (sedan) on the driver side. Vehicle 1 fled the scene. Vehicle 2's driver sustained multiple compound fractures and internal bleeding (serious physical injury). On the roadway, Officer Gomez documents 90 feet of locked-wheel skid marks left by Vehicle 2. At 75 feet into the skid, the skid marks exhibit a sharp 40-degree lateral deflection to the left, accompanied by a heavy concentration of dry underbody mud, deep asphalt gouges, and radiating radiator fluid spatter.

At the point of impact, Officer Gomez recovers a fractured bumper guard and amber turn signal lens stamped with a manufacturer code matching a late-model commercial pickup. An ALPR hit two miles away leads officers to the fleeing vehicle and driver.

• Forensic Analysis: The skid mark deflection, underbody mud, and gouges at 75 feet identify the exact Area of Impact (AOI).
• Speed Reconstruction: With a road drag factor of $f = 0.70$ and 90 feet of pre-impact and post-impact sliding, Vehicle 2's minimum speed was $S = \sqrt{30 \times 90 \times 0.70} = \sqrt{1890} \approx 43.47$ mph.
• Statutory Enforcement: Because the fleeing pickup driver caused the collision resulting in serious physical injury, the driver is charged with a Class 2 Felony under A.R.S. § 28-661(A)(1) and (B).