On Scene Traffic Accident Investigation Levels I&II

Participant Guide Developed by Jay Hoekwater

On Scene Traffic Accident Investigation Levels I&II © 2008 State of Georgia. GPSTC. All Rights Reserved All rights reserved. No part of this product may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the publisher. First Edition The leader guide and participant material for this program was created using LeaderGuide Pro™ version 6.0.

Table of Contents

Overview ...... 2 Background and Purpose ...... 2 Measuring and Diagramming the Accident Scene ...... 4 Planning for Traffic Accident Investigation ...... 11 The Human Element in Traffic Accident Investigation ...... 15 The Roadway Element in Traffic Accident Investigation ...... 19 The Vehicle Element in Traffic Accident Investigation ...... 31 Traffic Accident Photography ...... 36 Measuring and Diagramming Accident Vehicles ...... 40

Overview

Why?

On-Scene Traffic Accident Investigation Level I focuses on the evidence collection process. Special emphasis is placed on evidence recognition and proper evidence collection techniques. The goal of this program is to train officers in evidence recognition and gathering techniques associated with serious vehicle collisions. The program is designed to aid officers’ understanding of prioritizing on scene and follow-up information gathering for the purpose of establishing vehicle-to-roadway and vehicle-to-vehicle relationships culminating in a fact based opinion of how the collision occurred.

Learning Objectives

 Identify six results of an accident that should be documented by the use of measurements  Explain two measuring techniques commonly used in traffic accident investigation  Describe the four step measuring process  Demonstrate the ability to measure and diagram traffic accident scenes by applying both measuring techniques  Identify six traits common to competent traffic accident investigators who plan and initiate effective investigations  Identify the five stages of planning and initiating traffic accident investigations  List three elements present in every traffic accident  List four kinds of information that must be obtained from each one of the three elements in a traffic accident  Identify three statement taking guidelines  Identify three pre-accident conditions associated with drivers and pedestrians  Recognize and analyze tire mark evidence  Recognize and analyze metal scar evidence  Recognize and analyze debris

 Recognize and analyze final positions of vehicles and bodies  Recognize and analyze signs a vehicle left the ground  Recognize and analyze six types of vehicle information that must be collected at the scene  Identify two means for documenting vehicle damage at the accident scene  Recognize two types of vehicle damage  List three reasons photographs are used in traffic accident investigation  Identify six things that should be photographed at the accident scene  List six measurements needed from accident vehicles  Demonstrate the process for measuring and diagramming accident vehicles

Background and Purpose

Prepare law enforcement personnel to conduct automobile collision investigations by:

 Recognizing evidence  Interpreting evidence  Collecting evidence

The success of any investigation is reliant on the quantity and quality of evidence collected.

What Is A Traffic Accident?

An occurrence that produces unintended:

 Death  Injury  Property Damage

What Is Traffic Accident Investigation?

Thorough examination of 3 factors:

 People  Roads  Vehicles

There are Five Levels of Traffic Accident Investigation.

 Accident reporting  On-Scene investigation  Technical investigation  Accident reconstruction  Cause analysis

Measuring and Diagramming the Accident Scene

Terminal Performance Objective

Given the need, students will correctly demonstrate the four step evidence collection process in accordance with the information presented in class.

Enabling Objectives

1. Describe the four step measuring process 2. Identify five results of an accident that should be documented by the use of measurements 3. Explain two measuring techniques commonly used in traffic accident investigation 4. Demonstrate the ability to measure and diagram traffic accident scenes by applying both measuring techniques

Causes for failed investigations

 No measurements  Sloppy/improper/inaccurate measurements

Measurements are used to establish the following:

 Vehicle to roadway relationships  Vehicle to vehicle relationships  Speed or speed loss in some cases

Measuring Process

The process of documenting an accident scene through the use of measurements involves (4) steps:

 Walk through  Field sketch  Creating a baseline  Measuring

Walk Through

Purpose:

 Locate evidence  Establish evidence collection priorities  Organize evidence collection procedure

Items needed for walk through:

 Camera  Chalk, lumber crayons, or spray paint  Pencil and measurement log form

Each item of evidence must be marked, photographed, listed and described on the measurement log form in a logical sequence.

The following six results of the accident should be located:

 Final positions  Tire marks (on and off road)  Metal scars (on and off road)  Debris  Signs a vehicle left the ground  Damage to Fixed Objects

Field Sketch

Purpose:

 Depicts scene  Evidence documentation  Baseline and scene orientation  Scale diagramming aid

Field sketch must include:

 Evidence location (accident results)  Roadway layout (view obstructions/traffic control devices)  North direction  Investigator name  Location/date/case number

Generally speaking measurements should not be recorded on a field sketch.

Establish A Baseline

A baseline is usually a tape measure anchored along the edge of the road provided road is straight. Baselines must be straight.

Measuring and Recording

Once the previous steps have been taken, the investigator can begin to measure. Each measurement must be recorded on the measurement log sheet as it is made.

Number of marked points needed:

 One point (marks less than 3 feet, small scrapes or dents in fixed objects, vehicle parts, liquid debris less than 3 feet)  Two points (final positions of bodies, vehicles, major vehicle parts, marks and liquid debris longer than 3 feet, long stretches of damage to fixed objects)  Three points (curved marks, offset skids, large fields of debris)

Two measurements are required for each marked point.

Measure and Record Measurements

Two measuring techniques:

 Coordinate (90 degrees from baseline)  Triangulation

Coordinate measuring disadvantages:

 Time consuming  Requires two people  As the distance from the baseline increases, accuracy decreases.

Triangulation advantages:

 Accuracy  Saves time  Can be done by one person

Note: Triangulation measurements do not have to be made from fixed objects.

Safety

 Wear a safety vest  Plan measuring with safety in mind  Do not turn your back to traffic if possible  Do not assume motorists will see you  Arrange to have help for traffic control

Diagramming

Necessities:

 IPTM Template  5mm mechanical pencil  Engineers triangle  Flex curve  Bow Compass

Bow Compass Uses:

 Drawing parallel lines  Plotting triangulation measurements

Diagramming hints:

 Use black ink  Make bold consistent lines  Center lines must be centered  Road edges must be parallel  Intersecting roads must be plotted at the correct angle  Scale bar must be exactly 1 inch in length  Scale bar must be labeled correctly  Construction lines must be made lightly  Diagram must be accurate  Curved roads must not be drawn straight and straight roads must not be drawn curved

Scale:

 1 inch = 10 feet; 1/10; 1/120  1 inch = 20 feet; 1/20; 1/240  1 inch = 2 feet; 1/2; 1/24

Planning for Traffic Accident Investigation

Terminal Performance Objective

Given the need, students will correctly list the five planning stages that contribute to effective traffic accident investigations in accordance with the information presented in class.

Enabling Objectives

1. Identify six traits common to competent traffic accident investigators who plan and initiate effective investigations. 2. Identify the five stages of planning and initiating traffic accident investigations.

This instructional unit is designed to help officers understand:

 Traits  Training  Equipment  Need for an investigative plan

Accident investigator traits:

 Aptitude for accident investigation  Basic understanding of accident causes, investigative techniques and a general knowledge of accident reconstruction principles  Methodical/Thorough  Hard working  Self study

Consider facts (avoid pre-determinations)

Equipment

 Pens, pencils, paper  Forms  Chalk or paint  Photography equipment  Measuring equipment  Basic hand tools

The five planning stages are:

 Learning of the accident  Arrival at the scene  When the emergency is under control  When urgent data collection is complete  When on-scene work is finished

Learning about the Accident

 Ask when and where the accident happened and severity  Decide urgency of response and best route to the scene  Find out if traffic is blocked and if other emergency vehicles are on the way  Arrange for additional help  Start for the scene

Consider:

 Vehicles leaving the scene  Low visibility  View obstructions  Traffic control devices

Arrival at the Scene

 Park safely (traffic flow)  Care for injured  Locate drivers and witnesses (hit and run)  Identify hazards  Warning devices  Traffic control  Protect evidence  Arrange to have damaged vehicles removed

When the Emergency is under Control

 Preliminary questions (driver condition)  Question witnesses  Observe vehicle conditions (lights, tires, etc.)  Make photos  Measure accident results on the roadway  Record where injured parties and vehicles are taken  Have the road cleared

When Urgent Data is Complete

 Make arrests or issue citations as necessary  Complete on-scene examination of vehicles  Make additional photographs (view obstructions, traffic control devices, etc.)  Make additional scene measurements  Have location cleaned up

When On-Scene Work is finished

 Notify relatives of dead and injured and owner of the vehicle  Inform other agencies of conditions needing attention  Organize notes  Complete factual data on report  Complete investigation and report

The Human Element in Traffic Accident Investigation

Terminal Performance Objective

Given the need, students will correctly identify on scene and follow up information gathering methods from principals and witnesses in accordance with the information presented in class.

Enabling Objectives

1. List three elements present in every traffic accident 2. List four kinds of information that must be obtained from each one of the three elements in a traffic accident 3. Identify three statement taking guidelines 4. Identify three pre-accident conditions associated with drivers and pedestrians

Obviously the human element is of major importance in any traffic accident investigation. The major contributing factor in automobile crashes is human error. Accident investigators must realize the importance of collecting all the information about the accident from those involved. This is no small task and quite often it is as difficult as collecting other information about the accident.

Three elements are involved in every traffic accident:

 People  Roads  Vehicles

Four Kinds of Information

 Identification  Description  Condition prior to the accident  Results of the accident on the element

Driver Identification

 Extremely important  Not to be taken for granted  Must prove by use of evidence

Investigative Obstacles

 Occupants (deceased or injured)  Survivor (blames deceased)  Occupants (false claims)  Assumptions (Vehicle owner may or may not be the driver)

Consider Physical evidence

MATCH DAMAGE!

Others Who Need to be Identified

 Witnesses  Pedestrians  Passengers  EMT’s  Tow Truck Operator

 First on the scene  Police officers  Anyone with info

Georgia Motor Vehicle Accident Report is sufficient for identification information

Two Levels of Inquiry

 On-Scene Inquiry  Follow-up

On-Scene

 Routine identification of drivers, passengers, and witnesses  General vehicle condition (blowout, brake failure, general vehicle condition)  Questions about events (speed, maneuvers, final positions)  Questions about tire marks (panic braking, etc)

Follow-up

Statement essentials:

 Find out what the person will or will not say about the accident  Reduce the statement to writing  Attempt to get the person to agree to what has been written

Guidelines

 When  Where

 How (objective, positive easily understood questions, do not argue or suggest, courtesy, understanding, tactful)

Consider Prior Condition

Questions about the trip (24 hours)

Also:

 Obvious physical handicaps  Temporary disabilities such as pre-accident injuries, fatigue, and alcohol or drug impairment  Indications the driver was pre-occupied or distracted  Contents of vehicles which suggest driver condition  Nature of the trip (may suggest fatigue or intoxication)

Accident Results

Questions about the results of the accident

 Tire marks  Vehicle condition  Driver condition  Occupant condition

Documenting Injuries

 Photograph injury patterns (deceased, include measuring device)  Obtain all injury reports (court order, subpoena)  Request autopsies (prosecutable, suspicious, etc.)

The Roadway Element in Traffic Accident Investigation

Terminal Performance Objective

Given the need students will correctly identify and characterize roadway evidence in accordance with the information presented in class.

Enabling Objectives

1. Recognize and analyze tire mark evidence 2. Recognize and analyze metal scar evidence 3. Recognize and analyze debris 4. Recognize and analyze final positions of vehicles and bodies 5. Recognize and analyze signs a vehicle left the ground

In some fatal accidents there may be no witness, driver, or pedestrian explanations. Investigators who rely on witnesses only, may disregard other evidence.

 Collision scene examination needs to be done ASAP.  Almost every collision leaves physical evidence.  Evidence may supplement witness statements, prove or disprove theories and guide the direction of the investigation.

Investigator responsibilities:

 Photographing  Measuring  Describing

Record the Four Kinds of information

 Identification of the accident location  Description of the road  Condition of the road at the time of the accident  Results of the accident on the road

Skid Marks

A skid mark is a friction mark caused by a tire that is locked and not free to rotate. Two things can cause skid marks

 Braking  Collision damage

Friction marks on bituminous pavement are caused when tar or asphalt is softened as the tire slides across it. On cement friction marks are usually abraded tire rubber. Burning or severe softening rarely occurs with ordinary tires

On asphalt friction marks appear as a black smear possibly showing parallel rib marks called striations (pre-impact).

Friction marks on cement are a nearly invisible whitish track (Occasionally faint black marks abraded rubber, short term evidence)

On soft material marks made by a sliding tire may cause a furrow.

Friction marks made on gravel or abrasive material may leave surface scratches.

On wet surfaces a friction mark may appear as an erasure or cleansing mark (removes dirt or water from the surface).

Six Factors Influence Skid mark life

 Kind of mark  Weather  Amount of traffic  Road repairs or construction  Surface  Type of tire

Skid Mark Characteristics

 Pre-collision skid mark characteristics  Straight (may swerve to road edge)  Usually less than 300 feet in length  Usually four marks (2,3,1)  Left and right equally dark and wide  Front usually more prominent than rear  Marks are the width of the tire at the beginning (may be wider, rarely narrower)  Mark can usually be located within a few feet of its beginning  Ends abruptly where the vehicle stops or the collision starts  Marks show striations which are parallel to the mark  Outer edges usually more prominent than the middle

There are Two Front and Rear Skid Mark Differences

 Length  Tire over deflection

Length

 Vehicle weight distribution (at rest or constant speed)  Pitch  Instability

Over deflection

 Under inflation  Cupping

Prominent edge marks aid in identifying front and rear tire marks. Occasionally rear tires make over deflected skid marks (typically over-loaded pickup trucks)

Skid Mark Irregularities

Gap skids

 10 feet or more in length  More common in pedestrian accidents

Skip or Bounce Skids (usually 3 feet or less)

Causes:

 Collisions  Road bumps or holes  Bouncing during braking

Collision scrubs (usually 10 feet or less)

 Caused by collision and not braking  Good indicator of area of impact

Broadside skids

 Not caused by braking  May result from loss of control  Collisions

Offset skids

 Similar to scrubs as they mark the position of a tire at the onset of the collision  Good indicator of the area of impact  End of pre and beginning of post collision skid

Swerves are slight deviations from straight skids

 May not be caused by collision  Skidding tires have no lateral traction  Side forces cause the swerve (cross slope unequal road friction, unequal brakes)

Collisions usually cause skid mark irregularities

 A tire which was not skidding before collision may be on a wheel locked by collision damage  A tire that was skidding before collision may leave no skid mark afterwards (driver no longer in control, damage to brakes, tire may leave the ground for a distance)

All skid marks should be photographed and measured regardless of whether they occur before or after collision.

The impending skid or shadow should be included as part of the overall measurement if it is visible.

Recording Skid Mark Observations

Once located the following should be recorded:

 Location of the beginning and end of each mark  Description of each mark (if you are sure it is a skid mark, be prepared to explain the characteristics that make it such; if you are unsure simply call it a friction mark or tire mark)  Direction of the skid (skid marks normally have a faint beginning and an abrupt end)  Measurements of skid marks (for placement not length)  Photograph all marks  Measure track width of marks and vehicle from center of the tread on one side to the center of the tread on the other

Accident Significance

 Vehicle to roadway relationships when the mark was made  Direction of travel prior to and after the collision  Area of impact  Mechanical or tire failure  Speed before and after impact  Identification of the vehicle (hit and run)

Yaw Marks

A yaw mark is a scuff mark made on a surface by a rotating tire which is slipping more or less parallel to its axis. Yaw marks are friction marks not caused by braking but by steering. The term yaw means the vehicle is rotating about its vertical axis as it moves along its path.

Appearance and Life

 Usually yaw marks are lighter than skid marks.  Yaw mark evidence does not last nearly as long as skid mark evidence.

Two Characteristics Of Yaw Marks

 Yaw marks are always curved because they result from steering.  Striations will also be present and they appear as oblique marks nearly crosswise of the mark.

Recording Yaw Mark Observations

 Measurements of the path of curvature including any crossover marks  Photograph marks so striations can be seen

Accident Significance

 Speed determination in some cases  Vehicle to roadway relationships

Acceleration Scuff Marks

Acceleration scuffs are caused when sufficient power is supplied to the driving wheels to make at least one of them spin on the road surface.

Appearance

 Appear similar to braking skids on pavement  On soft or loose material the spinning tire kicks the loose material backwards out of the furrow.

Accident Significance

 Not common  May indicate reckless driving

Flat Tire Scuff Marks (Flop Marks)

Flat tire scuff marks are made by an over deflected tire. Remember, over deflected means seriously under inflated or over-loaded.

Accident Significance

 If it leads up to the area of impact may indicate tire failure (assuming accident vehicle does have a corresponding flat)  Quite often drivers may claim a tire failure caused the accident (opposite direction collisions)

Tire Imprints

A tire imprint is a mark made on the road or other surface by a rolling tire that is not slipping.

Imprint Characteristics

 The imprint is usually equally clear for all tires and is the same width as the tire making the imprint.  They always start strong and usually end gradually.  If striations are present they will be parallel to the mark and the tread design may show.

Accident Significance

 Not normally as significant as other tire mark evidence (sometimes confused with skid marks)  They can be significant in showing where a vehicle ran off the pavement and came back on again as sometimes occurs prior to a head-on collision.

Road Scars

Marks made on the pavement by some metal part of an accident vehicle.

Scratches (narrow; usually indicate where a vehicle overturned)

Scrapes (wider than scratches; usually indicate are of impact)

NOTE: Be alert for metals scars caused by towing.

Gouges are places where the pavement has been dug out by strong metal parts

 Caused by collision forces  May be an indicator of area of impact

Chips are small deep gouges

 Caused by collision forces  Usually a good indicator of area of impact

Chops are broad shallow gouges

 Typically made by vehicle frames and rims  Generally occur at maximum engagement  May indicate direction

Grooves are long narrow gouges

 Made by projecting metal parts and wheels  May continue some distance beyond maximum engagement

Scars on fixed objects

 May be on guardrails, poles, signs or trees  Can be useful in determining the path of a vehicle and its final position

Accident Significance

 Vehicle roadway relationships (must match to vehicle part)  Speed in some cases (rare)

Debris

Debris is loose material scattered at the scene as a result of the accident.

Four Types of Debris

 Underbody  Vehicle liquids (flow of accident)  Vehicle parts (hit and run)  Vehicle contents (solid cargo)

Accident Significance

 It is usually not a good indicator of area of impact because of the way it scatters.  Debris can be useful in determining the flow of the accident.  Speed (in some cases)  Final positions (liquids)

Flips and Falls

In some accidents a vehicle may leave the ground.

Accident Significance

 Path of travel  Speed in some cases

Final Positions

A final position is the place where a vehicle, body, or some other object comes to rest after a collision.

Two Types of Final Positions

 Controlled (intentional)  Uncontrolled (unintentional, vehicle dynamics)

 Locate by measurements  Uncontrolled final positions of vehicles (major vehicle parts)  Ejected Bodies

Uncontrolled final positions must be located by measurements even if the vehicle has been moved (use witnesses and observe marks and debris). Controlled final positions may be noted but normally do not need to be measured.

The Vehicle Element in Traffic Accident Investigation

Terminal Performance Objective:

Given the need, students will correctly identify the on scene and follow up investigative techniques used to collect evidence from vehicles in accordance with the information presented in class.

Enabling Objectives:

1. Recognize and analyze six types of vehicle information that must be collected at the scene 2. Identify two means for documenting vehicle damage at the accident scene 3. Recognize two types of vehicle damage

The complete evaluation of accident vehicles is just as important as the complete evaluation of the drivers and roadway evidence. Often times, vehicle evaluations have been overlooked by those responsible for investigating accidents.

Proper vehicle examinations can assist in:

 Determination of vehicle to vehicle relationships  Occupant information and movement  Vehicle to roadway relationships  Mechanical defects  Speed in some cases

Vehicle inspection (scene, follow up)

Vehicles are evidence (protect to avoid tampering)

Vehicle Observations

 Damage  Information about the trip  Occupants  Pre-accident condition  Record  Photographs  Notes  Measurements

Record the Information

Identification of the vehicle

 Owner’s name and address  Vehicle identification number  License number

Description

 Size and general structure (motorcycle, sub-compact, full size)  Body style and appearance (2 dr sedan, 4 dr hard top, wagon, bus, etc)  User or service (private, rental, government)

Condition of the vehicle at the time of the accident

Results of the accident on the vehicle

On Scene Priorities

 Final Positions  Identification (locating for follow-up)  Damage survey (photos, notes)  Contents  Match damage (vehicle to roadway)  Locked wheels (observe towing)

Follow-up Data Collection

 Manufacturer recalls  Types of damage  Signs of ground contact  Tires and lamps  Injury sources  Running gear  Safety belts, gear position  Dash and speedometer  Doors  Fire

Two Types of Damage

 Contact  Induced

Contact damage is damage to any part of a motor vehicle by direct contact with some object which is not part of the vehicle.

There are four things which can cause contact damage.

 Vehicles  People  Roads  Fixed Objects

Contact damage can be internal or external

Characteristics

 Closely compacted crumpled body parts  Fine hard scratches  Paint smearing  Ragged tears or punctures (sheet metal)

Induced Damage

Induced damage is damage to any part of the motor vehicle caused by:

 Shock of the collision  Contact with some other part of the vehicle

Characteristics

 Folds  Bends  Creases  Wrinkles

Tires and Lamps

Follow-up tire examination focus:

 Tread depth  Air pressure  Description  Match to road marks

Lamps

Light switches should not be turned on for the purpose of examining lamps.

Measuring Vehicle Damage

 Speed estimates  Direction of thrust (principal force).

Traffic Accident Photography

Terminal Performance Objective

Given the need, students will correctly identify three reasons for photographing accident scenes in accordance with the information presented in class.

Enabling Objectives:

1. List three reasons photographs are used in traffic accident investigation 2. Identify six things that should be photographed at the accident scene

Photography is an important part of any investigation whether it be an accident or criminal investigation. Photographs are just one means by which information can be recorded. Photography is an indispensable means of recording certain kinds of accident information and a useful supplement in recording many other kinds.

Three Reasons To Use Photographs

 Reminder  Credibility  Record Evidence

Who Should Make the Photos?

 On-Scene Investigator  ID Technician  Professional

When Should Photos be Made?

ASAP

Dynamic Evidence (final positions, some debris, certain tire marks etc.)

What Should be Photographed?

 Final positions (vehicles, bodies)  Road evidence  Off road marks  Landmarks  Driver approach view (100, 300, 500 ft)  All four sides of involved vehicles

Photographing Road Evidence

 Photographs must show clear detail  Close-up photographs are sometimes required  Photograph all relevant marks  When in doubt take a picture  Include a measuring device

Photographing Vehicle Damage

Photographs of vehicle damage should be taken at the scene whenever possible (subsequent damage). More detailed photographs are part of the follow up and should include:

 Contact Damage  Induced Damage

Photographs of contact damage should include:

 Imprints  Friction or abrasion marks  Lamps  Damage to loads  Injury sources  Damaged tires and wheels

Special Photographs

 Matching vehicle damage (road contact, clothing imprints, vehicle to vehicle, soles of shoes, occupant contact)  Road surface conditions  Weather  Traffic control  View obstructions

Photo Techniques

 Practice  Charting  Fill flash  Bracketing  Panoramic (overlapping)

Photograph Identification

 Numbering  Technical data  Camera and lens type  Filters  Film type  Processing  Date, time, photographer  Chain of custody

Measuring and Diagramming Accident Vehicles

Terminal Performance Objective:

Given the need, students will correctly demonstrate the four step process for recording vehicle damage through the use of measurements and diagrams in accordance with the information presented in class.

Enabling Objectives:

1. List six measurements needed from accident vehicles 2. Demonstrate the process for measuring and diagramming accident vehicles

Measuring and diagramming accident vehicles is becoming an important part of accident investigation. The development of sophisticated accident reconstruction computer programs has increased the need and importance of measuring damaged vehicles.

The following measurements are needed:

 Contact and induced damage  Wheelbase  Front and rear overhang  Overall length  Front and rear track  Vehicle width at the middle

Measuring Techniques

 Record the measurements on the measurement log sheet  Record additional measurements such as wheelbase, overhang, etc on an appropriate form

Diagramming Techniques

Once the measuring process is complete a scale diagram can be made which will show an aerial view of the damage. A scale of 1 inch equals 2 feet is a good scale to use. Diagramming vehicles requires patience. The investigator must believe in his measurements and plot points on the paper which have been measured. Those points are then connected with straight lines.

Table of Contents

Overview 1 Series of Events 2 Math/Calculator Review 9 Drag Factor/Coefficient of Friction 14 Speed Estimates 21

Overview

Why?

On-Scene Traffic Accident Investigation Level II, introduces students to traffic accident reconstruction principles that will help them establish speed loss from certain types of skid mark evidence. This is the first course in a series of traffic accident reconstruction courses that are intended to enhance a student’s ability to thoroughly investigate serious traffic accidents.

Learning Objectives

 List four environmental factors that can influence perception delay.  Identify four different kinds of reaction times generally recognized in traffic accident investigation.  Apply the order of operations to mathematical problems.  Apply the rules for using positive and negative numbers to mathematical problems.  Find the square and square root of numbers in mathematical problems.  Assess principles of friction and the role of frictional and gravitational forces as they apply to stopping a vehicle.  Define coefficient of friction and drag factor.  Evaluate and estimate coefficient of friction and drag factor.  Recognize and analyze skid mark evidence for speed determination.  Define grade of the road.  Define percent of braking.  Demonstrate an ability to estimate vehicle speed.  Demonstrate an ability to combine speeds.

Terminal Performance Objective

Given the need, students will correctly identify a series of events and their relevance to a traffic accident in accordance with the information presented in class.

Enabling Objectives

1. List four environmental factors that can influence perception delay. 2. Identify four different kinds of reaction times generally recognized in traffic accident investigation.

Pre-Scene Series of Events

Pre-trip events are those events that occur before and include situations that exist before the trip is started. The following are examples of driver pre-trip events or situations:

 Driver experience  Driver training  Intelligence  Alertness  Reaction  Habits  General health condition and age  Fatigue  Stress  Pre-occupation  Distractions  Attendance at a party  Limited sleep or no sleep

Vehicle Pre-Trip Events

 Defective equipment  Overloaded

Driver Trip Events

The following are examples of driver trip events or situations:

 Stopping for a meal or coffee  Fatigue, illness or depression  Consumption of alcohol or drugs  Erratic or other unsafe driving

Vehicle Trip Events

The following are examples of vehicle trip events or situations:

 Tire blowout  Brake, headlight or steering failure  Other mechanical failure  View obstruction, e.g., dirty windshield, or load transfer  Load falling off vehicle

Point of Possible Perception

The point of possible perception is the place and time at which a normal person could perceive a hazard.

Perception delay is the time from the point of possible perception to actual perception. The point of possible perception and actual perception may be influenced by many driver and environmental factors.

 Perception Delay Factors  Experience  Intelligence  Judgment  Alertness  Natural senses (age must be considered)  Knowledge of area  Distractions

Environmental Factors Influencing Perception Delay

 Weather and light conditions  Load on vehicle and protrusions  Location of traffic-control devices  View obstructions  Point of Perception

The point of perception or actual perception is the point where a situation such as a hazard is comprehended or perceived as a hazard.

Reaction

Reaction is a person's voluntary or involuntary response to a hazard or to situations that have been perceived. Reaction time is the length of time from when a person perceives a given situation as being a hazard to when he reacts to his perception. Four different kinds of reaction time are recognized. They are as follows:

Reflex reactions are mostly instinctive and require the shortest time because they involve no thought.

 Simple Reaction

Simple reactions are the most common kind in driving because the stimulus is expected and the driver has already decided what he will do when the event occurs (putting on the brakes when a signal turns yellow).

 Complex Reaction

Complex reactions call for a choice among several possible responses. The decision has not been made in advance. For example, a pedestrian unexpectedly steps into the roadway ahead and the driver has to decide whether to turn right, left, or slow down. Complex reactions are slower than simple reactions.

The length of time to react depends on how complex the stimulus is, how many choices there are for reaction, and how often the driver has been in a similar situation. Normally, from half a second to two seconds may be required. Most driving is done by habitual complex reactions.

 Discriminative Reaction

Discriminative reaction occurs when a driver is required to make a choice between two or more actions which are not habitual or practiced, for example deciding whether to turn to the right or the left of a vehicle which is straddling two lanes. This is the slowest of all the reactions, and may require as much as a minute if the situation is complicated and the urgency slight.

The action point follows reaction and is the place where a person puts into action his decision based on his perception of a hazard, such as braking or steering. The action point will be influenced by the driver's:

 Operating skills and habits  Ability to control the vehicle  Freedom of movement  Knowledge of vehicle  Reaction time

Evasive Action

Evasive action is an action or combination of actions such as steering or braking taken by a traffic unit with intention to avoid a collision or other hazardous situation.

Evasive Action Distance

Evasive action distance is the distance traveled after the action point to where the traffic unit stops by itself or otherwise avoids a collision or to the area of impact.

True Area

The true area is that area leading up to the point of no escape during which evasive action could be initiated in order to avoid a collision.

Point of No Escape

The point of no escape is the place and time after or beyond which the accident cannot be prevented by a particular traffic unit. Because of committed motion and laws of physics, no action will avoid the collision completely at this point, although

© 2008 State of Georgia. GPSTC. All Rights Reserved On Scene Traffic Accident Investigation Level I Rev. Date 2/20/2008 6 action such as braking or steering may reduce the seriousness of injury or damage. The following are factors which influence the point of no escape:

 Visibility of hazard  Roadway alignment  Positioning of traffic-control devices  Driver distractions  Weather and light conditions  Condition of roadway surface  Type, size and condition of vehicle being operated  Cargo being carried

Encroachment

Encroachment is when a traffic unit intrudes or enters into the rightful path or area of another traffic unit.

Area of Impact

The area where a traffic unit strikes another traffic unit or some other object or it overturns.

Maximum Engagement

Maximum engagement is the point of maximum penetration or engagement by one object into another such as the maximum penetration of one traffic unit into another traffic unit or object during collision.

Disengagement

Disengagement is the separation of two objects, for example, traffic units, after maximum engagement.

The final position of a vehicle or body is the location where it comes to rest after collision. The final position of a vehicle may be controlled or uncontrolled, that is, it may be forced to that location as a result of the collision or it may be steered to that position. In some cases, a vehicle may not have a final position, as in situations where a driver accelerates his vehicle and drives it to a location quite some distance from the area of impact.

Terminal Performance Objective

Given mathematical problems related to speed estimates on a written examination, students will accurately resolve those problems in accordance with the information presented in class.

Enabling Objectives

1. Apply the order of operations to mathematical problems. 2. Apply the rules for using positive and negative numbers to mathematical problems. 3. Find the square and square root of numbers in mathematical problems.

Operations used for arithmetic are commonly known.

 A plus sign (+) is used for addition.  A minus sign (-) is used for subtraction.  A division sign (÷) or (/) is used for division.  A multiplication sign (x), parentheses ( ), or dot () is used for multiplication.

Order of Operations

The order of operations is always multiplication, division, addition and subtraction. If parentheses are used, the operations inside the parentheses are always done first. If there are several terms in the numerator and/or the denominator, these operations are done first.

Positive and Negative Numbers

Positive and negative (signed) numbers are part of arithmetic that often results in accident investigation problems. The rules for addition, subtraction, multiplication and division of signed numbers are simple.

Adding numbers with like signs

For adding numbers with like signs, their absolute values are simply added and the same sign is assigned to the total.

(+7) + (+6) = + 13 (-7) + (-6) = - 13

Adding numbers with unlike signs

For adding numbers with unlike signs, their absolute values are subtracted and the sign of the larger number is assigned to the difference.

(+7) + (-6) = +1 (-7) + (+6) = -1

Subtracting numbers with like signs

For subtraction of a positive number from a positive number, the opposite negative is added:

(+9) - (+6) = (+9) + (-6) = + 3

(+8) - (+4) =

To subtract a negative number, add its opposite positive:

(+7) - (-6) = (+7) + (+6) = + 13

(+4) - (-3) = (+4) + (+3) = + 7

Multiplying positive and negative numbers

The rule for multiplication of signed numbers is simple. If the signs of the two numbers are the same, the resulting product (answer) is positive; if the signs of the two numbers are different, the resulting product is negative.

(+7)(+3) = + 21 (-7)(-3) = + 21 (+7)(-3) = - 21 (-7)(+3) = - 21

Dividing positive and negative numbers

The rule for division of signed numbers is essentially the same as that for multiplication. If the two numbers have the same sign, the quotient (answer) is positive; if the two numbers have different signs, the quotient is negative.

(+6)/(+3) = + 2 (-6)/(-3) = + 2 (-6)/(+3) = - 2 (+6)/(-3) = - 2

Grouping positive and negative numbers

If more than two numbers are multiplied, do the multiplication by grouping in order to get the correct sign.

(-3)(+2)(-4) = (-6)(-4) = + 24

(+4)(-2)(+2) = (-8)(+2) = - 16

Square Roots and Exponents

The square root of a positive number is a number that, multiplied by it, gives the positive number. Thus the square root of 9 would be + 3 or -3. This is indicated by the radical:

√ 9 = + 3 or - 3

In more complicated expressions, the operations are done under the radical before the square root is taken.

Exponents are numbers written to the right of and above positive or negative numbers. For example:

3 ²

In this case the exponent (or square of the number) signifies the number must be multiplied by itself.

Percentages

3/4 = .75 1/2= .50 2/3= .66

Percentages can be added, subtracted, multiplied and divided like whole numbers.

.70 + .02 = .72 .70 - .02 = .68 .70 X .80 = .56 .80/.20 = 4

Drag Factor/Coefficient of Friction

Terminal Performance Objective

Given the need, students will correctly estimate, adjust, and apply drag factor in accordance with the information presented in class.

Enabling Objectives

1. Assess principles of friction and the role of frictional and gravitational forces as they apply to stopping a vehicle. 2. Define coefficient of friction and drag factor. 3. Evaluate and estimate coefficient of friction and drag factor.

Drag factor is a component of the speed loss equation. If the investigator cannot define and explain drag factor he has little hope of giving testimony regarding a speed estimate.

Determining the rate of deceleration can be the most difficult problem in the speed estimate process. A vehicle's deceleration is related to its drag factor, f, and the coefficient of friction, μ (Greek letter mu). Drag factor is related to acceleration by the following equation:

 a = fg  f = drag factor (no units)  g = the acceleration of gravity (32.2 fps ²)

Drag factor and coefficient of friction are related but they are not the same.

Friction

© 2008 State of Georgia. GPSTC. All Rights Reserved On Scene Traffic Accident Investigation Level I Rev. Date 2/20/2008 14 Friction can be thought of as the resisting force to motion between two surfaces at their interface (contact).

Types of Friction

There are three kinds of friction that affect objects which are in contact with one another. They are as follows:

 Abrasion - The process of rubbing away or wearing down by friction.  Adhesion - The physical attraction or joining of two substances; stickiness.  Visco Elasticity - Highly resistant to flow and flexible with the property of being able to return to its original shape following deformation.

Each of these three kinds of friction has two modes.

 Static - No motion between two objects in contact.  Dynamic - Motion between two objects in contact.

Abrasion

Static - The push before the tire breaks loose. The effects of abrasion are not seen in the static mode. They become evident once the motion begins.

Dynamic - Tire and pavement grindings occur when there is motion between the tire and pavement. This is evidence of the role abrasion plays in the dynamic mode.

Adhesion

Static - Tires are designed to be sticky to improve a vehicle's stopping ability and traction. Their stickiness in the static mode is evident any time a car is parked on a hill.

© 2008 State of Georgia. GPSTC. All Rights Reserved On Scene Traffic Accident Investigation Level I Rev. Date 2/20/2008 15 Dynamic - In the dynamic mode the stickiness between the tire and road surface is increased because of the heat. The heat created as a braked tire slides across the pavement also increases friction and causes the vehicle to stop in a shorter distance.

Visco Elasticity

Static - A motionless tire is somewhat deformed where it touches the pavement due to the weight on the tire. As torque is applied to the tire it begins to flex and is resistant to the impending motion. This resistance gives the vehicle the push needed to begin its movement.

Dynamic - A tire sliding on pavement tends to flex or deform. The tire material is pressed into the road surface irregularities. This aids it in its ability to stop by increasing the friction.

Principles of Friction

For sliding friction, several conditions are generally always expected. With a rubber tire sliding on pavement, however, some of these conditions might not be present. The five general principles of friction are as follows:

 If the sliding surface is horizontal, the horizontal force required to slide the object is proportional to the object's weight.  Dynamic friction is lower than static friction.  The friction force does not depend on how much area of the sliding object is in contact with the surface.  The friction force does not change when velocity changes.  The friction force does not change when the temperature changes.

Coefficient of Friction

Simply defined coefficient of friction (μ) is the horizontal force required to slide an object across a level surface divided by the weight of the object. It can be expressed mathematically as:

Drag Factor

Drag factor is given the symbol, f. It is defined as the force required for acceleration in the direction of the acceleration divided by the object's weight.

 f = F/w.

Acceleration is defined as a rate change of velocity with respect to time and has units of ft/sec/sec.

The following equations also express drag factor:

 f = μ N+G (This equation assumes all wheels are sliding)  f = Drag factor (no units)  μ = Coefficient of friction  N = Percentage of braking (since this equation assumes 100% braking, N equals 1)  G = Grade

 f = (μ N+G)p (This equation assumes less than 100% braking and is used to calculate the drag factor for each wheel)  f = Drag factor (no units)  μ = Coefficient of friction  N = Percentage of braking (a percentage between 0 and 1, normally the percentage cannot be measured so 0 is used if the brake completely malfunctions)  p = Percentage from the weight supported by an individual wheel divided by the total weight (Assume the drag factor of the left front wheel is being calculated. Typically each front wheel retards approximately 30% of the vehicle's total weight. Therefore, p, would equal .30).

Differences between Coefficient of Friction and Drag Factor

Coefficient of friction means the object must be sliding across a level surface. Drag factor does not specify sliding and level.

Drag factor and coefficient of friction will be equal only in cases where all wheels are locked and sliding on a level surface.

Drag Factor Related to Gravity

 a = fg

This equation shows the relationship between acceleration, drag factor, and the acceleration of gravity which is equal to 32.2 fps ². Recalling the relationship between acceleration, gravity, and drag factor, multiplying drag factor times gravity will give us the rate of acceleration.

Determining Drag Factor/Coefficient of Friction

When investigating an actual accident you may need to know the actual drag factor (coefficient of friction). There are several methods for determining this:

 Test-skid the accident vehicle or an exemplar vehicle.  Slide a drag sled to obtain a friction coefficient.  Use existing highway department skid numbers for the road in question.  Use a chart on friction coefficients and apply the appropriate adjustments to the case at hand.

Drag Sleds

 Wet roads  Soft surfaces  Loose surfaces (gravel or sand)

When a drag sled is used the following equation applies:

 f = F/w  f = Drag factor  F = Force in pounds needed to slide the drag sled  w = Weight of the drag sled

Determining Drag Factor from Test Skids

Test skids can be performed with or without special equipment. In some cases accelerometers are used or pavement spotters bumper guns). Most police departments do not have access to this type of equipment. Their best alternative is to use the accident vehicle or one just like it. When test skids are made the following procedures should be followed:

 Test skids should never be made if there is danger of causing another accident.  The accident conditions should be duplicated as nearly as possible i.e. place, surface condition, vehicle load, similar vehicle, temperature, etc. Next to safety this is the most important factor.  Travel in a straight path at a constant 2 - 3 mph above the desired test speed. Decelerate to the test speed, apply the brakes as hard as possible and let the vehicle slide to a complete stop.  When making test skids, always make at least two sets of test skids. Measure the longest test skid from each set. Compare the longest test skid from each set. They should be within +/-5 % of each other. If they are not within 5 % keep making test skids until two are within 5 % . Then choose the longest to be used in the equation.

 f = S²/ 30d  f = Drag factor  S² = The square of the test vehicle's speed  30 = Math constant (representing gravity)  d = Distance of the longest test skid within 5 % of the next longest from another set.

Coefficient Of Friction Charts

Typical Drag Factors

 Dry Asphalt .6-.8  Wet Asphalt .45-.7  Loose Gravel .4-.7  Ice, Snow .10-.25

Speed Estimates

Terminal Performance Objective

Given the need, students will correctly estimate vehicle speed by using mathematical formulae and analyzing skid mark evidence in accordance with the information presented in class.

Enabling Objectives

1. Recognize and analyze skid mark evidence for speed determination. 2. Define grade of the road. 3. Define percent of braking. 4. Demonstrate an ability to estimate vehicle speed. 5. Demonstrate an ability to combine speeds.

Speed Estimates from Skid Marks

Two factors are important in estimating vehicle speed from skid mark evidence:

 the distance the vehicle skidded  the slipperiness (coefficient of friction) or drag factor of the surface on which the skidding took place

To calculate the speed of the vehicle based on the observations of marks on the road, the following steps must be taken:

The officer must be sure the marks are true skid marks made by tires on wheels which were not rotating. Usually skid marks:

 Are nearly straight, or at least do not clearly swerve  Are visible for all wheels, or at least one on each side  Are nearly equally distinct for left and right tires  Begin and end at fairly distinct points rather than gradually appearing and disappearing  Are equally distinct for both edges of the tire  Have striations of rib marks, if any, parallel to the marks  Decide whether all wheels locked or were nearly locked.

Consider the vehicle as fully skidding unless:

 There are definite signs that the vehicle was rotating or swerving.  There is a definite reason to believe that some brakes were not functioning.

Determine how far the vehicle skidded with braking equivalent to that of having all the wheels locked. When determining skid distance consider the following:

 Measure each skid mark separately if possible.  If all wheels leave marks, use the length of each mark in the equation

Determine how much drag the tires and the road surface produced to slow the vehicle with all wheels locked.

Once the necessary data has been determined an equation be used to calculate the vehicle's speed loss. The following equation can be used:

 V=√ 2ad

This equation represents the velocity to slide to a stop. If the slide is not to a stop but ends in a collision, rollover, or fall without ground contact, the starting speed will be higher.

Speed and Velocity Defined

Velocity is defined as the rate change of distance with respect to time. It is measured in units of feet per second. Speed is defined as the rate change of distance with respect to time. It is measured in units of miles per hour.

 V = d/t distance equals feet and time equals seconds  S = d/t distance equals miles and time equals hours

Converting Velocity and Speed

To convert velocity to speed or speed to velocity the number of feet in a mile (5280) is divided by the number of seconds in an hour (3600).

To convert speed to velocity multiply 22/15 times speed.

To convert velocity to speed multiply 15/22 times velocity.

Minimum Speed Equation

 S = 5.47 √ df Minimum Speed Equation

Adjusting for Grade

Percent grade is the change in elevation in feet per 100 feet. Grade is easily measured using a carpenter's level, the longer the better. Place the level on the road surface in the direction of travel. If the bubble is not showing level, raise the level until the bubble shows level. Then measure the distance the level is raised from the road surface (measure to the bottom of the level). The percent grade is the distance from the road to the level divided by the length of the level.

The formula for calculating percent of grade is:

 G = V/H  G = % grade  V = Vertical measurement from road surface to the bottom of the level  H = Horizontal measurement (length of the level)

Unequal Braking

Passenger cars do not have the same load on each axle when they are at rest or moving at a constant velocity. During hard braking, load is shifted to the front axle. Because of this load shift, no correction needs to be made when all wheels are locked.

If, however, one axle has a drag factor different from that of the other axle, a braking capability of 100 % can not be assumed. A vehicle which is braking experiences weight shift (pitch) to the front axle. Therefore, on most passenger cars the front axle retards approximately 60 % (30 % for each front wheel) of the vehicle's total weight. The rear axle retards the remaining 40 % of the vehicle's weight (20 % for each rear wheel).

Multi-Surface Skids

In some accidents a vehicle will skid from one surface to one or more other types of surfaces. On occasion the wheels on one side skid on a different type of surface than the wheels on the other side. When this happens the distance the vehicle skidded and the coefficient of friction for each surface must be determined before the speed can be calculated.

In those traffic accidents where a vehicle has had a continuous dissipation of energy, the combined speed equation must be used to calculate the vehicle’s initial minimum speed. An example of this would be a vehicle skidding and then colliding with an object. The combined speed equation

 Sc = √ S1² + S2² + S3² ......  Sc = Combined speed  S1² = Speed 1  S2² = Speed 2  S3² = Speed 3

This equation can be used to combine as many speeds as necessary. Speeds must be added in this manner (algebraically) not arithmetically.

Damage Speed Estimates

In some instances accident investigator's estimate (guess) vehicle impact speeds based on vehicle damage. This is not a science, it is an estimation based on the investigator's experience. When this is done a range of speeds should be used. For example, if the investigator estimates an impact speed of 25 mph, he should develop a range of between 20-30 mph. These speeds could then be combined with the skid speed (if any) leading up to impact. A rule for combining speeds is that all speeds from energy dissipation can be combined provided the energy dissipation is continuous.