The Traffic Accident Reconstruction Origin -Article-

Accident Investigation Manual for Patrol Officers

Introduction

This Traffic Collision Investigation Manual for Patrol Officers is a replacement for a short pamphlet written by the author in 1985 for use at the Central Arizona Regional Law Officers Training Academy (CARLOTA). The original booklet was also used with success for in-service training of police officers.

The new and updated second edition has been expanded to include additional information on vehicle dynamics, speed calculations and equations, and diagraming methods. It is a helpful reference text to any police recruit collision investigation course. The manual is also useful as an entry level introduction to collision investigation for non-police personnel, such as attorneys and insurance investigators.

About the Author

Luis Martinez is a police officer with a central Arizona police department and has been in law enforcement since 1979. He started teaching collision investigation at CARLOTA in 1982. Sgt. Martinez is also an adjunct faculty member at Central Arizona College, teaching criminal justice courses. He has completed courses in collision investigation and reconstruction from The Traffic Institute (Northwestern University) and the Institute of Police Technology and Management (University of North Florida).

He is an accredited reconstructionist by the Accreditation Commission on Traffic Accident Reconstruction (ACTAR), and has testified as an expert in accident reconstruction in municipal and superior courts. Luis holds a bachelor of arts degree in management from the University of Phoenix and is currently attending graduate school (Northern Arizona University), working on a Master of Education degree.

SECOND EDITION Copyright 1994 by Luis Martinez

PERMISSION IS HEREBY GRANTED TO REPRODUCE THIS MANUAL FOR TRAINING PURPOSES ONLY, SO LONG AS THE COPYRIGHT NOTICE IS RETAINED. REPRODUCTION FOR COMMERCIAL PURPOSES IS NOT AUTHORIZED.

                   TABLE OF CONTENTS

                                                         Page
Introduction................................................  i

Chapter 1: Answering the assignment call....................  1

               Patrol vehicle operation..................  1

               Collision scene management................  3

               Traffic cones/flares......................  7

               Interviewing drivers & witnesses..........  11

Chapter 2: What the road shows..............................  16

               Tire marks................................  18

               Skid marks................................  20

               Skip skids................................  23

               Gap skids.................................  24

               Acceleration marks........................  25

               Tire tread print..........................  25

               Scuff marks...............................  26

               Metal scars...............................  27

               Debris....................................  31

               Other signs of impact.....................  32

Chapter 3: What the vehicle shows...........................  36

               Contact and induced damage................  36

               Exterior damage assessment................  38

               Vehicle impact orientation................  40

               Rollovers.................................  42

               Vehicle interior examination..............  43

Chapter 4: Measuring and diagraming the collision scene.....  46

               What to mark and measure..................  46

               Diagraming the scene......................  51

               Coordinates...............................  52

               Triangulation.............................  56

               Drawing the scale diagram.................  59

Chapter 5: Calculating speed from tire marks................  76

               Speed from skid marks.....................  76

               Test skids................................  80

               Drag factors from published tables........  83

               The template nomograph....................  84

               Critical speed (yaw) marks................  85

               Combined speeds...........................  88

Chapter 6: The collision report narrative...................  92

               Example #1................................  93

               Example #2................................  95

Appendix

     Equations for the traffic collision investigator......  99

     Traffic collision terminology......................... 102

The orange cones are a useful item to carry in the cruiser's trunk for daylight use, however, their size and bulk limit the number of them that can be carried to no more than 8 to 10 average size cones. Flares, on the other hand, are compact and the average police car can carry 2 boxes with room to spare.

CAUTION- Before igniting any flares the officer must insure that no gasoline leaks or other flammable materials are present. Many tractor-trailers involved in interstate commercial transportation carry flammable and/or toxic materials, exposure to fire could be catastrophic.

When utilizing cones or flares to establish a traffic control pattern, the key thing to remember is: how does the traffic control devices appear to an approaching motorist ? Officers sometimes throw a few scattered flares on the roadway and expect oncoming drivers to somehow interpret a pattern and follow a proper path, where there is none. A lane closure requires the placing of flares at evenly spaced distances in a pattern that conveys a message to the motorist, mainly which direction are motorists expected to go...or not to go. This is accomplished by placing the flares lined at an angle leading away from the roadway edge, and in the direction traffic is expected to flow.

Figure 2 shows a recommended pattern to use in redirecting traffic flow (A) when the collision is confined to one lane. This will require another officer to control the flow of traffic in each direction. If the complete roadway is obstructed (B) then the officer must close the entire roadway width.

Officers must keep in mind that the distance gap between the first flare or cone positioning and the collision scene is dictated by the speed at which the majority of the traffic is traveling. Motorists

Table 1

 
Speed Limit  1st Cone/Flare

25 MPH            68 Feet
35 MPH           112 Feet
45 MPH           167 Feet
55 MPH           227 Feet
65 MPH           301 Feet

A distance gap of 60 to 70 feet (from first warning device to collision scene) might be adequate in a slow speed zone (25 MPH), 300 to 400 feet may be required on a major state highway or under severe weather conditions. Officers must utilize extreme caution when on a traffic way and always remain alert and facing traffic whenever possible.

If the roadway is wet or iced over, the safe distances above will not be sufficient and a traffic control point manned by an officer 1/4 to 1/2 mile ahead will be necessary. Nighttime collisions, highway curves, and hills present added safety problems during scene management and the officer must consider the visibility problems they create.

The method described above is really only applicable when the colliding vehicles are confined to one lane of traffic on a multi-lane (2 or more lanes) roadway. If the collision scene occupies the entire width of the roadway, channelization will not be effective as there is no place to channel traffic to. In such a case it would be better (and safer) to completely close the roadway to all traffic until the collision scene has been cleared.

Scuff marks, also known as yaw marks or critical speed scuffs, are tire marks left on the roadway by wheels that are sliding and rolling simultaneously. The wheel is rolling and slipping sideways at the same time.

When a vehicle "spins out" or "slips out" while cornering, or is oriented in a direction different from its direction of travel, scuff marks will be deposited. Often they are in the form of light parallel grooves, referred to as striations or hash marks, which run straight but are diagonal to the outline of the continuous scuff mark. They are made by the sidewall or rib of the tire.

One of the most important bits of information one can obtain from scuff marks is that the vehicle is taking a turn at a critical cornering speed. Critical cornering speed is the speed at which the vehicle is on the threshold of spinning out or slipping laterally.

Tire scuff marks which occur under these circumstances are critical speed scuffs made by tires sliding as the vehicle traverses the curve, and are made by the outside edges of the tires. The scuff mark left by the rear tire will fall outside the scuff mark made by the front tire for that side of the vehicle tending to slip off the roadway as a result of centrifugal force.

It is important to remember that scuff marks are made by steering, or rather over-steering, as opposed to skid marks which are made by braking. Two important characteristics to look for when examining scuff marks are their curved path and the striations (see pages 28 & 29).

This document was written in Word Perfect. It is offered here, including graphics, in its Word Perfect format.

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Luis Martinez is a Sergeant with the Eloy (AZ) Police Department and teaches accident investigation and criminal justice courses at Central Arizona College. He holds a B.A. in management and is nearing completion of a Master Degree in Education at Northern Arizona University. Luis holds ACTAR accreditation number 38. Sgt. Luis Martinez can be reached at [email protected]

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