Traffic Crash Investigation

Traffic Crash Investigation

Definition of Motor Vehicle Traffic Crash

• A motor vehicle crash that occurs on a trafficway or in which a harmful event occurs on a trafficway
• Includes motor vehicle crashes and motor vehicle traffic crashes
• To be classified as a motor vehicle crash, the following elements must be present:
  • Involvement of at least one motor vehicle in transport
  • At least one harmful event
  • The harmful event is the result of an unintentional act
  • The harmful event is not the direct result of a cataclysm
  • The crash was not initiated by an action of an aircraft or watercraft
  • The crash does not include any harmful event involving a railway vehicle in transport prior to involvement of a motor vehicle in transport

Unstabilized Event and Stabilized Event

• Unstabilized event: an event or set of events that originates when control is lost and terminates when control is regained
• Stabilized event: marks the end of an unstabilized event, usually when persons and property come to final rest

Chain of Events

• A series of events that make up a traffic crash
• Events may not always occur in the same order
• Each traffic unit involved in the crash has its own chain of events
• The chains of events for two traffic units involved in the same crash will always have one or more links or events in common

Investigating a Traffic Crash

• Upon arrival at the scene, make the scene safe to prevent further damage or injury
• Identify and locate drivers, passengers, witnesses, and other people who may have a connection with the collision
• Obtain information from people involved in the crash while they are still at the scene
• Handle evidence and valuable belongings properly
• Take photographs and measurements of the scene before clearing the roadway

Interviewing Witnesses and Drivers

• Explain who you are and what your interest is in the collision
• Be objective, positive, and diplomatic
• Ask specific and clear questions
• Do not suggest answers or argue with the person being questioned
• Be adaptable and practice good listening skills

Tire Marks

• Tire marks can be classified into two types: imprints and friction marks
• Imprints are marks made on a surface by a rolling tire without sliding
• Friction marks are made by a tire that is sliding or both rotating and slipping on a surface
• Skid marks are friction marks made by a tire that is sliding without rotation
• Scuffmarks are friction marks made by a tire that is both rotating and slipping### Types of Skidmarks
• Gap skidmarks are created when brakes are pumped or the driver's foot slips off the brake pedal, resulting in short spaces between the skidmarks.
• Skip or bounce skidmarks are caused by bouncing during braking, road bumps or holes, or collisions, and are characterized by short distances between interruptions.
• Yawmarks are scuffmarks made by a rotating tire that is slipping parallel to its axis, also known as centrifugal skidmarks, critical speed scuffmarks, or sideslip marks.
• Acceleration scuff is a scuffmark made when sufficient power is supplied to the driving wheels to make at least one spin or slip on the road surface.
• Flat tire mark is a scuffmark made by an over-deflected tire with too little air pressure.

Measuring Skidmarks

• When measuring the length of gap skidmarks, only measure the marks themselves, excluding the spaces between them.
• When measuring skip skids, treat them as if there were no skips, and measure the entire length of the skid from beginning to end.

Investigating Skidmarks

• The exact point where skidding begins is difficult to determine due to tires sliding without leaving a visible mark and the time required for the wheels to stop rotating.
• The length of the transitional period depends on how quickly brakes are being applied.
• Any speed calculated from skidmarks is referred to as a minimum speed.
• The investigator must be careful in observing the evidence in the indefinite area where the brakes are applied, known as the shadow, impending skidmarks, or incipient skidmarks.

Analyzing Skidmarks

• The investigating officer can determine the minimum speed of a motor vehicle prior to the collision using basic speed analysis and math calculations.
• The steps to determine minimum speed include locating and measuring skidmarks, finding the average skidding distance, determining the coefficient of friction, and finding the minimum speed.

Determining Coefficient of Friction

• There are numerous methods of determining the coefficient of friction, including published data on typical values for different road surfaces.
• Test skids can be conducted to obtain a specific value for the surface in question, duplicating conditions at the time of the collision.
• The mathematical formula to obtain a coefficient of friction value is: 30 x D, where f is the drag factor, S is the test speed, and D is the longest skid length.
• The drag tire or drag sled method can also be used to obtain a coefficient of friction value.

Determining Minimum Speed

• Once the coefficient of friction and skid distance are known, the minimum speed of the vehicle at the beginning of the skid can be determined using the nomograph or mathematical formula.
• The nomograph can be used to determine the coefficient of friction value or crash vehicle speed once the variables are known.

Introduction to Traffic Radar

• RADAR stands for Radio Detecting and Ranging, a device used to detect and measure the speed of distant objects by analyzing radio waves reflected from their surfaces.
• The first traffic law was passed in 1652, prohibiting the riding or driving of horses at a gallop within city limits.
• The first automobile speed regulation was enacted in 1912, limiting automobile speeds to 12 mph in the country and 8 mph within city limits.

Frequency and Wave Length

• Frequency is the number of waves transmitted in one second, usually measured in cycles per second.
• Wavelength is the distance from the beginning of the peak to the end of the valley, with most radars operating within the X-band (10.525 billion waves per second) or K-band (24.15 billion waves per second).
• Radar signals travel at the speed of light, 186,000 miles per second.

Doppler Principle

• The Doppler Principle states that relative motion causes a signal's frequency to change.
• When objects are moving closer together, the frequency increases, and when they are moving farther apart, the frequency decreases.
• The amount of frequency change is determined by the speed of the relative motion.

Factors Affecting Radar Accuracy

• Angle Effect: The angle of the radar antenna can cause an improper target reading.
• Reflective capability: The radar unit's operation is affected by the reflective capability of targets, their position, and their actual speed.
• Batching: The radar adds a fraction of the patrol vehicle's speed to the violator's speed.
• Scanning Effect: The rapid swinging past a fixed object of a stationary radar antenna can produce a speed reading.
• Panning Effect: Can occur with two antenna units when the antennas are pointed at each other, causing feedback.
• Turn on Power Surge: Turning on the power to a unit can cause a speed reading due to the sudden surge of voltage.
• Mirror Switching Effect: Can occur on Speed gun Eights if the operator points the unit out the back window and uses the mirror to read the display without activating the mirror switching device.
• Patrol Speed Shadow Effect: Occurs if the beam of the radar unit attaches onto a moving object instead of the terrain.

Basic Operations of Moving Radar

• Target Speed = Closing Speed - Patrol Speed
• High Doppler and Low Doppler frequencies are used to calculate the target speed.
• Testing the unit involves activating the high and low tuning forks and holding them in front of the unit to give a target speed.

Fundamentals of Case Law

• The officer must prove that a particular speed law was violated, the defendant was the driver, and the offense occurred on a public thoroughfare.
• The officer must also prove that the correct vehicle was stopped, and that the vehicle in question appeared to be traveling at the speed indicated on the RADAR equipment.
• Special requirements for moving radar include:
  • Operator having sufficient training and experience
  • The radar instrument must have been in proper working condition (internal test)
  • The device was used where road conditions would distort the readings as little as possible (e.g., not raining)
  • The patrol car's speed was verified (speedometer)
  • The instrument's accuracy was tested (tuning forks)