The Traffic Accident Reconstruction Origin -Article-

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iCAR- Intelligent Computer Assisted Reconstruction V 2.4

© Copyright 1996 James Perry All rights reserved

This is a development test release of iCAR. Use these programs at your own risk. Although they have been extensively tested, I take no responsibility for any losses.

iCAR is distributed for your personal use. You may not charge for distributing iCAR.

Please let me know of any problems, criticisms, or, especially, suggestions.

Thank you.

James Perry 299 West Cerritos Avenue, Anaheim, CA 92805

Table of Contents


"You can never be too rich, too thin, or have too much RAM and Disk storage." Ancient American proverb, ca. 1980

"The computer is no better than its program." Elting Elmore Morison, 1966

"The real problem is not whether machines think but whether men do." Burrhus Frederic Skinner, 1969

This manual describes the use and operation of iCAR, an extremely powerful collection of popular accident reconstruction programs and calculations assembled into an integrated, package. The manual includes installation instructions, a user's guide, and a reference section.

iCAR was developed with these goals in mind:

Getting Started

iCAR and Accident Reconstruction

Users of iCAR are assumed to have some accident investigation experience and training. In and of itself, accident reconstruction is as much an art as it is a science. The formulas and calculations used in iCAR are not only accepted as standards in the field, but are also firmly entrenched in solid scientific principles. The skills needed to obtain the necessary information to correctly use iCAR, however, vary widely. iCAR assumes it is being give good data and makes no judgements. As the saying goes -- Garbage In, Garbage Out.

A good reconstructionist has two traits: the ability to accurately gather facts and the talent to interpret them. iCAR helps in both regards, first, by showing what kind of information to gather, and second, by reducing the time and effort to distill the facts into useable information.

Some Hints

  1. Make use of iCAR's ability to record all calculations during a single session.
  2. Perform multiple iterations on questionable data using a variety of input.
  3. If possible, work with a single vehicle at time. This reduces the inherent bias that occurs when trying to fit data items to a specific scenario.
  4. Avoid pre-judging.
  5. Think of iCAR as a tool. No different than a calculator or a compass. It doesn't answer helps 'you' answer questions.
General Notes

  1. All data items are presented in a table format. The table allows for full-screen editing and complete cursor movement.
  2. There is no need to worry about setting the Caps Lock on or off, this is all handled internally.
  3. During data entry, "Note:" fields will usually appear. These fields contain important information about the item being entered, including: data ranges, types, and general information.
  4. All numerical fields use a "real" data type. I won't bore you with significant digits or byte size; the bottom line is that you can enter numbers at the level of precision you wish. A number like 3.12325674 is just fine.

The Menus

iCAR is entirely menu-driven. Simply move the cursor to your choice and strike the Enter key. On some systems, depending upon hardware, a key letter in the menu will be highlighted. Pressing this letter will have the same effect as using the cursor/Enter key combination.

There are six main menu selections. Each is explained below, as are any subsequent internal menus.

Conservation of Linear Momentum

Sounds serious, doesn't it? Here's the basic formula:

m1v1 + m2v2 = m1v1' + m2v2'

Momentum at impact = Momentum following impact

Simply stated, the combined momentum of the vehicles at impact must be the same after impact. There is no loss in momentum for the combined vehicles following impact because linear momentum is conserved. One vehicle may gain momentum as a result of the impact while the other loses some, but together they end up with the same amount after the impact as before. Okay, the hard stuff is out of the way. How much of this do you need to understand? Not much, really. Over 300 years ago, scientists introduced this concept and it is now accepted as a universal law of nature. It's right up there with gravity, folks. From an investigative point of view, four basic pieces of information are needed:

  1. the weights of the vehicles involved
  2. their post-impact heading angles
  3. their post-impact speeds
  4. their pre-impact heading angles.
The determination of vehicle weight can come from any number of sources, including: MVMA specifications, NATB Manuals, Branhams's Truck Index, etc. The pre- and post- impact heading angles are developed from scene evidence and taken from the diagram. The post-impact speeds are generally determined from some mutation of the speed-to-slide-to-stop formula.

IMPORTANT NOTE: Quite often, investigators confuse heading angles with magnetic headings or x/y plotting. Both of the latter are important in terms of accurate scene description and documentation, but only lend confusion when it comes to vehicle headings. Vehicle headings, especially in conjunction with momentum equations, should be vehicle relative. By this I mean one vehicle will always have a heading of 0 degrees at approach, and all other vehicle heading will be taken clockwise and relative to 0 degrees. A quick case in point. Assume there is a simple intersection-type collision. Vehicle 1 is heading east and is broadsided by Vehicle 2 which is heading north. Taking Vehicle 1 as the 0-axis vehicle will then make Vehicle 2's heading angle 270 since it is taken clockwise from the 0-axis. Similarly, if Vehicle 2 is taken as the 0-axis vehicle, then Vehicle 1's heading angle will be 90. So here's the rule: give one of the vehicles a 0 heading angle and determine all other angles from this point going clockwise. Trust me, it makes life easier. There are two options under this menu selection. A brief discussion of each follows:

Speed/Velocity Calculations

Generally speaking, speed and velocity mean the same thing: how far did something move during a given time. But the high-brows got involved and, even though there is a straight mathematical conversion between the two, many popular formulae require a differentiation. Speed is measured in miles and hours (MPH); velocity is measured in feet per second (FPS). There is a subtle difference between the two: speed connotes movement without regard to direction and velocity describes movement with regard to direction. If you need to convert one to the other, just use the Utilities option of iCAR.

Speed to Slide to Stop

: This is, without question, one of the pillars upon which accident reconstruction is based. It provides a sound, physical relationship which is wholly proveable. Considerations include: accurate assessment of the drag factor (including grade, if any), the actual slide- to-stop distance, and the percentage of braking. The results of this calculation allows the user to infer only one thing-the vehicle must have been going at least that fast to stop in the distance measured.

s = 5.5 SQR(dfb)

Speed to Slide to Stop - 2 surfaces

: This option performs the same function as its parent, only 2 surfaces can now be computed. Two surfaces were chosen since the chances of a vehicle cross more than that are fairly slim. The calculations for more than two surfaces is identical to the calculation for two.

Critical Curve

: Over the past couple of years, this calculation has fallen into some disrepute. There are some questions whether the calculation is over- or underestimating the speed.

s = 3.87 SQR(rf)

Free Fall

: The main consideration when using this option is to ensure that you get your signs right.

s = 2.74d/SQR(m (d+h))

Combined Speeds

: This option takes two speeds and combines them to provide a final speed. Caution should be exercised when it comes to combining speeds which are, in and of themselves, final speeds. A good example is a speed derived from a fall. That's the final speed, regardless of what happens after the fall.

Velocity - Known Distance, Known Time

: This is one of the basic formulae used in physics. It determines the velocity to travel a known distance during a known time.

v = d/t

Velocity Gained/Lost

: When the acceleration/deceleration rate is known and the time is known, this calculation will yield the velocity gained or lost.

Velocity At Any Time

: This option will provide the velocity at any time during an acceleration/deceleration when the initial velocity, acceleration/deceleration rate, and time are known.

Velocity At Any Distance

: This option will provide the velocity at any distance during an acceleration/deceleration when the acceleration/deceleration rate and distance are known.

iCRASH - Damage Only

The iCRASH subprogram computes speed changes experienced during a vehicle-to-vehicle or a vehicle-to-fixed object collision. It makes use of the locations and extent of structural crush and is based on the same energy calculations used in the CRASH III program. Users should be aware of CRASH III techniques and limitations before selecting this option. There are two options under this menu selection. A brief discussion of each follows:

New: Tells iCAR to reset all the numerical fields to 0 and clears all buffers. A fresh entry screen will then be presented.

Rerun: Retains all data which was entered during the most recent New or Rerun compilation and allows the user to make changes to any or all data items.

Showit: This option first prompts the user to determine if a printout is required, then generates a screen image showing vehicle damage, PDOF, and D. At present, the screen print utility is limited to use by Hewlett-Packard LaserJet products. A driver for dot-matrix printers is in the works.

Additional Notes: Whenever New is selected, the user will be given the option to create an ASCII file which will record the results from any number of runs. Also, during each run, the user is given the option to alter the vehicle stiffness constants. The decision to offer this option was based on the fact that the default constants, based primarily on the CRASH III model, have some age on them now. More up-to-date information is becoming available. If the user has this information, it should be used. When a larger body of this information becomes available to me, I will alter the internal constants to reflect this change.

Speed Estimates from Damage

Sometimes it is useful to compute a "rule of thumb" estimate of a vehicle's speed based on damage. There is one main menu and one follow-on menu as shown below.

Main Menu

Follow on Menu

Calculate Stiffness Values

Calculate stiffness values based on crash test data. The generated values would be used in the CRASH program.

Time/Distance Calculations

The following selections are used to determine where a particular vehicle was relative to another before a collision and to answer questions as to whether maneuvers by either party could have had a positive effect.


Radius of Curvature

: Uses the standard radius of curvature formula. The formula assumes that the curve being measured is a regular (constant) curve. A quick way to check this is to make two measurements in addition to the middle ordinate measurement. The two measurements must occur at like points (i.e., identical distances along the chord). If the measurements are the same, it's a regular curve; otherwise, it is an irregular curve and this calculation is invalid.

R = (cı / 8m) + (m / 2) where R = radius c = chord m = middle ordinate

Convert FPS to MPH

: Feet-per-second to miles-per-hour. mph = fps x 0.6818

Convert MPH to FPS

: Miles-per-hour to feet-per-second. fps = mph x 1.467

Conversion Formulae

: This option serves as a reference source. There are many conversions. Most of these are simple and not worth valuable computing time when a simple calculator will do the job. This option just provides a listing of input to get a desired output. No more, no less. Compute Drag Factor: For a vehicle with all wheels locked, the drag factor is the same as the coefficient of friction. The following three selections all determine acceleration and/or deceleration factors. They vary only what it known prior to the calculation.

Compute AD Factor

: Speed and Distance Known adf = sı / 30d

Compute AD Factor

: Velocity and Time Known adf = v / 32.2t

Compute AD Factor

: Distance and Time Known
adf = d / 16.1t^2

Compute Acceleration Factor

: Vi, Vf, and Time Known: Computes an acceleration factor when the initial velocity, final velocity, and time are known.
a = (vf - vo) / (32.2t)

Compute Deceleration Factor

: Vi, Vf, and Time Known: Computes a deceleration factor when the initial velocity, final velocity, and time are known.
a = (vo - vf) / (32.2t)

Compute AD Rate

: AD Factor Known: Once the factor is known, the rate is computed by multiplying the factor time the gravity (32.2 feet per second per second).

DOS Operations

iCAR provides the user with two important DOS-related operations. Both are important and necessary. Being able to escape into the DOS shell without leaving a program provides significant flexibility and a result recording operation which runs in the background allows the user to concentrate on the difficult task at hand without having to worry about the details of file manipulation.

Exit to DOS

: Temporarily suspends iCAR, clears the screen, and displays the DOS prompt, from which you can run other programs or DOS commands. You must remember, however, that iCAR is still resident, so your computer will not have as much memory as it would normally. To return from the shell, simply type EXIT at the DOS prompt.

Create an ASCII Session Record

: Creates an ASCII text file using a name of your choosing, activates an internal boolean variable, and will faithfully record calculations results for the duration of the session. Please remember that the file name chosen must follow normal DOS protocol, and that any identical file name will be overwritten. Choose a file name that means something to you (i.e.,

Appendix A - Vehicle Parameters

The appropriateness of the set of eight frontal stiffness coefficients used by the CRASH3 program were examined in an SAE paper entitled "A Comparison Between NHTSA Crash Test Data and CRASH3 Frontal Stiffness Coefficients". The authors (Messrs. Strother, Woolley, and James) generated a new set of stiffness categories which are shown in the following table.

Vehicle Type A(lbf/in) B(lbf/in^2) A(lbf/in) B(lbf/in^2)
Stiffness CatagoryCRASH3
Users Guide
Users Guide
et al
et al
1) Subcompact302.047.0 237.958.9
2) Compact259.043.0240.060.0
3) Intermediate317.056.0247.558.95
4) Full-size356.034.0236.751.5
5&6 Largest325.037.0247.257.9
7) Vans383.0126.0349.799.8
8) Pickups 480.050.0425.6 72.5
9) Front Wheel Drive373.038.0240.458.2

Appendix B - Vehicle Size Categories

          SIZE           WHEELBASE
          ----           -------------
          1               80.9 -  94.8
          2               94.8 - 101.6
          3              101.6 - 110.4
          4              110.4 - 117.5
          5              117.5 - 123.2
          6              123.2 - 150.0
          7              109.0 - 130.0 VANS
          8              PICKUPS [Select 1 to 6 based on wheelbase]
          9              JEEPS [Select 1 to 6 based on wheelbase]
          11             IMMOVABLE BARRIER

Appendix C - Vehicle Measurement Terms

James Perry is a Traffic Safety Investigator with Dynamic Science in Anaheim, California.
He holds an MS in Information Systems from Nova Southeastern University and a BGS in Psychology/Police Sciences from the Univerity of Nebraska-Omaha.
He can be reached at:
299 West Cerritos Avenue, Anaheim, CA 92805

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