final demonstrative evidence & visual trial theory · 11/18/2013 1 investigation &...

11/18/2013

1

Investigation & Prosecution of

Vehicular Homicides

Warren Diepraam

Montgomery County District Attorney’s Office

Demonstrative Evidence & Visual Trial Theory

Got Cars?

Vehicular Crimes Cases • High Profile

• Highly Emotional

• Highly Likeable Defendants

• High Degree of Expertise

• Highly Complicated

Question?

The Crime Scene

• Location?

• Move Along?

• Cleared Scene?

• Police Attitudes?

• Evidence?

What Evidence?• Photos

• Maps

• Diagrams

• Charts

• Reconstruction

• SFSTs

• Blood Evidence

• Videos

• Cell Phones

• Social Media

• Doctor’s Records

• 911 Calls

• Medical Records

• Autopsy

• Insurance Records

• Receipts

• GPS Data

• Dispatch Recordings

• OnStar / AAA Roadside

• Civil Documents

• Internet Posting

• Statements

No, Really! What Evidence?

• Photos

• Measurements

• Cars

• Phones

• Blood

• Social Media

11/18/2013

2

Got the Evidence – Now What?

• Demonstrative Evidence vs. Exhibits

• Predicates

• Rules of Evidence

• Publishing & Presenting

–Trial Fusion (www.trialfusion.net)–PowerPoint

• Consider . . .

Learning Types• Kinetic Learners = 5%

• Auditory Learners = 30%

• Visual Learners = 65%

Visual65%

Auditory30%

Kinetic5%

Visual Learners Introduction ‐ Visual Facts

You MUST Make It Visual! Sounds Fun, But . . .• We Don’t Have the Equipment.

– Forfeiture Funds?

–Grants?

–MADD or Other Organizations

–Beg, Borrow, or Buy Your Own

–Go “old school” and blow it up!

= WINNING!

11/18/2013

3

When You Can – Bring It In! Photos – Key Points

• Quantity?

• Quality?

• Get there Fast!

• All the Evidence.

• Think Outside the Box.

• Consider the Defenses.

Good Photo of Damage Better Photo of Damage

Good Photo of Scene Good Photo of Scene

11/18/2013

4

Better Photo of Scene Better Photo of Scene

Some Beer… Wine Cooler…

11/18/2013

5

Still Cold… Here’s Why

DNA On the driver’s seat . . .

. . . and the wheel. It’s my Car’s Fault…

11/18/2013

6

Looks fine to me? Just Another Excuse

Are You? Quantity vs. Quality

Quantity vs. Quality Quantity vs. Quality

11/18/2013

7

Quantity vs. Quality Quantity vs. Quality

Move To Better LightingVICTIM

DEFENDANT

Drinking Locations Drinking Locations

11/18/2013

8

Don’t Forget These….

Follow Up

Booking PhotosGOOD. BETTER. BEST.

Maps – Key Points• Consider the Source.

• Careful with Formatting.

• Do not take the place of photos.

• Add to them for enhanced effects.

Where to find them?• The internet

• Map Making Companies

• Law Enforcement & Gov’t Agencies

–Tax Assessor’s Office

–Appraisal & Flood Control Districts

Maps

www.maps.google.com

11/18/2013

9

Maps

www.terraserver.com

Maps

Last Resort?

Pole Impact

Debris Field

Nguyen’s House

Smith’s House

Mitchell’s House

State’s Exhibit 26 A

State’s Exhibit 27

Path of Travel / Speed Calculations

Toll Plaza #1 @ 2:45:30 a.m.

Toll Exit to Bellaire @ 2:52:30 a.m. & Crash

Drive Time at Speed Limit = 10 Min

Defendant’s Drive Time = 90 MPH

11/18/2013

10

Maps – Tyler Crash Location

Bar

Diagrams and Charts

• Don’t Make More Complicated than Data.

• Keep very simple.

• Easy to Read.

• Use to Summarize.

Wrong Way Sign

Do Not Enter Sign

State’sExhibit96

Claren DamasoBricyn Afong

Stopped for red light

Arakawa ran red light

HOT = BEND

HOT = BEND

SCHOOL ST

Old 105 H

WY

11/18/2013

11

Crash Data Retrieval

Throttle position in %, shown wide open‐70%‐

closed (idle)

Vehicle speed shown at

57‐65‐62‐55‐47 MPH

MPHEngine speed shown 4032‐4160‐2304‐1088‐

896 RPM

Brake switch status shown from off to on

State’sExhibit

2

DPS Speed 1: 117DPS Speed 2: 93

Total Station (Bring it in)

249 North

249 South

WalgreensChevron

HardinStore Road

Presenting DNA

11/18/2013

12

D8S1179

D21S11

D7S820

CSF1PO

D3S1358

TH01

D13S317

D16S539

D2S1338

D19S433

vWA

D5S818

FGA

Amelogenin

D18S51

TPOX

D8S1179

D3S1358

TH01

D19S433

vWA

TPOX

Amelogenin

D5S818

1 IN 2,298,000 Caucasians

Clay Abbot

State’s Exhibit 5

Swab from wheel

D8S1179

D21S11

D7S820

CSF1PO

D3S1358

TH01

D13S317

D16S539

D2S1338

D19S433

vWA

D5S818

FGA

Amelogenin

D18S51

TPOX

D8S1179

D3S1358

TH01

D19S433

vWA

TPOX

Amelogenin

D5S818

1 IN 3,123 Caucasians

D21S11

D13S317

D16S539

FGA

Clay Abbot

State’s Exhibit 5

Swab from driver’s seat

Reconstruction Evidence – Key Points

• Can Get Complicated.

• Keep It Simple.

• Show the basics.

• Give some background.

• Make It VISUAL!

Skid MarksSkid Marks: an elongated stain in the rear of one's underwear caused by poopy pants

Types of SkidsSkid Marks – marks made by tires that are not rotating.

Skip Skid – short choppy, usually made by a bouncing tire.

Gap Skid – segmented with gaps between segments.

Curved Skids – continuous but slightly curved.

Overlapping Skids – rear tire marks over front marks.

Skuff Marks – tires that are still rotating (sideslip, impact, acceleration scuffs).

Yaw Marks – rolling tire is sliding sideways.

Squeegee Marks – tire as it wipes moisture from road.

Erasing – removal of dirt or debris by wiper action of skidding tires.

Shadow – faint tire mark preceding the darker skid mark.

Tire Print – imprint in soft dirt or sand made by a rolling tire.

Furrows – mounding of soft material (dirt, snow, grass) made by skidding tire.

Jog, Offset – irregular mark caused by sudden change in direction (usually impact).

Scallop – irregular mark caused by flat tire.

Characteristics of Skid Marks

Skid Marks

Wheel Motion Slide, No Roll

Operation Braking

Number From 4‐Tired Vehicle Mostly 4, Also 1, 2, 3

Right & Left Tires Equally Strong

Front & Rear Tires Front Stronger

Width If Straight, Same As Tire

Beginning Usually Abrupt

End Usually Abrupt

Striations Always Parallel to Mark

Other Details Outer Edges of Ten Stronger

Length 1 to 500 Feet

11/18/2013

13

Anticipate Defenses and Rebut Them Before the Defense

Defendant’s Tire Marks Flat Tire Marks

Shadow Skid

Impending Skid

Length: 188 feet

Use Diagrams to Explain

Put It All Together

Length: 188 feet

Both Tires Locked=.80

SQUARE ROOT OF:

32 x 188 x 0.80

Gravity=32

= 69MPH

Diagrams for Difficult Concepts

2000 lbs

4000 lbs

50.0lbs 4000

lbs 2000

Weight

Force HorizontalFactor Drag

w

Ff

Direction of Force

State’s Exhibit182

11/18/2013

14

TibiaDirection of Force


ROADWAY SURFACE

DIRECTION OF FORCE


ROADWAY SURFACE

DIRECTION OF FORCE

FRICTIONAL FORCES


Collinear Collisions

Opposite Direction

Same Direction

11/18/2013

15

MINIMUMPRE‐IMPACT SPEED OF COBALT = 93 MPH

Weight = 3105

( x?

)

+ (Weight= 4952

xPre‐ImpactSpeed = 13

x.276

)

=( (

)

)xxMinimumPost‐Impact Speed= 41 .927

+Weight= 4952

x x

.988

MinimumPost‐Impact Speed= 39

Weight = 3105

SOLVE FORApproachAngle

DepartAngle

DepartAngle

State’sExhibit75

Vericom

State’sExhibit

5

State’sExhibit

5

11/18/2013

16

SFSTs – Key Points

• Help to Understand.

• They Need to See It.

• In Court Demos.

• What’s the Purpose?

Horizontal Gaze Nystagmus

Hawk‐Eye Video HGN Demonstratives

HGNLeft Eye Right Eye

(1)________ ________ Lack of smooth pursuit

(2)________ ________ Distinct & Sustained at Maximum Deviation

(3)________ ________ Onset Prior to 45 Degrees

Number of Clues Indicating Intoxication: ______

TOTAL CLUES: ______

4

6

How does HGN apply to driving?

11/18/2013

17

HGN= Here Goes Nothing Walk & Turn(1)_____ Can’t balance during instructional phase(2)_____ Starts too soon(3)_____ Stops while walking(4)_____ Steps off line(5)_____ Doesn’t touch heel to toe(6)_____ Uses arms to balance(7)_____ Wrong number of steps(8)_____ Improper turn (or loses balance on turn)

Number of Clues Indicating Intoxication: ________TOTAL CLUES: ________

2

5

11/18/2013

18

One Leg Stand(1)_____ Sways

(2)_____ Hops

(3)_____ Puts Foot Down

(4)_____ Raises Arms for Balance

Number of Clues Indicating Intoxication: ________

TOTAL CLUES: ________

2

3

11/18/2013

19

Put it all together.

Beyond AReasonable Doubt

Probable Cause

Reasonable Suspicion

Signs of Intoxication

- Blood .15

- Lying ‐ “No” Open Containers & “ 2 Beers”

- Lost Control of Vehicle

- Inappropriate Actions

- Refuses Portable Breath Test

- Refuses One Leg Stand

- Refuses Walk & Turn

- HGN 6/6 Clues

- Swaying

- Admits to Drinking “A LOT”

- Attempts to Hide Poor Balance

- Cannot Follow Instructions

- Slowed Ability to Comprehend & Answer Questions

- Slowed Movements & Reactions

- Bloodshot, Glassy Eyes

- Strong Odor of Alcohol Beverage on Breath

- Slow, Hesitated, Deliberate Steps

- Speeding & Not Using Blinker

11/18/2013

20

Blood & Breath Evidence –Key Points

• Scrutinized by Defense Attorneys.

• Boring to Listen to.

• Difficult to understand.

• Solution =

• Make it Visual!

Breath Results

-- 0.122 -- 0.119

Blood Evidence –Start to Finish.

• Blood Draw Video & Photos.

• Lab Photos.

• Chain of Custody Chart.

• Records.

The Blood Kit.• Two blood tubes

• Needle and holder

• Consent forms

• Blood‐collection report

• Four blood‐type labels

• Providone‐Iodine prep pad

• Four coded security seals

• Absorbent materials

• 4‐mil plastic Zipr‐Top Bag

• Mailing carton

• Instructions

Submission Form ‐ The Beginning

11/18/2013

21

Lab Intake. • Open & Review Contents.

• Assign Lab Number.

• Mark Contents.

• Enter into System.

• Assign Technician.

• Take to Evidence Storage.

Evidence Storage The Toxicology Lab

Instrument #1: Screening Instrument

Instrument #2: Gas Chromatography Flame Ionization Detector

11/18/2013

22

Instrument #3: Gas Chromatography‐Mass Spectroscopy

Lab Reports – The End.

Carisoprodol (Soma) 4 mg/L

Diazepam (Valium) 200 mg/L

Hydrocodone (Vicodin)

Chain of Custody.

The Lab Science

The GCMS Testing Process

Blood Draw Lab Admits Aliquot Prep

ELISA / EMITAliquot PrepAuto‐sampler

Oven Travel Time Detection

11/18/2013

23


Blood Draw


Blood Draw Lab Admits





ELISA or EMIT


ELISA or EMIT

Aliquot PrepAuto-sampler


Analytical Column / Oven

Mix SeparationTravel Time

Retention TimeDetected

11/18/2013

24


Analytical Column / Oven

Mix SeparationTravel Time

Retention TimeDetected


Components Elute

BombardmentFragmentation

Sorted andCounted

The GCFID Testing Process


Auto‐samplerOvenTravel Time

Flame IonDetection





Videos – Key Points.• Great to Capture entire Crash Scene.

• Don’t Make Too Long.

• Be careful with what you hear.

• Be careful with what you see.

• Where to Get Them?

–Make Your Own

–Witnesses / Spectators

–News Coverage

11/18/2013

25

Reenactment DRE Evaluation

Blood Draw Crash Scene

Aircraft? Social Media & Cell Phones

11/18/2013

26

Social Media – Key Points.• YouTube

• Myspace

• Facebook

• Twitter

• Special Interest Websites

• Blogs

• Newspapers & TV

• www.search.org

Photos, Photos, Photos . . .

Statements & Admissions Punishment Evidence

But Wait – A Solution? Cell Phones AAA CALL

11/18/2013

27

Cell Phones7:51 PM U drunk?8:25 PM Yea8:27 PM CRASH

Cell Phone Tracking

Claims to be home: 2:45am

Relay Tower 1 at 2:42 AM

Relay Tower 2 at 2:44 AM

Crash at 2:45 AM

Receipts & Records – Key Points

• Confirm what you already know.

• Establish a timeline & fill in the gaps.

• Prove your elements.

• Open doors to other stuff?

Bar ReceiptsPer the Waiter in Grand Jury:

Defendant drank 1 Martini at 8 PM

Defendant drank 1 Martini at 8:15 PM

Started drinking the wine at 8:30 PM

Got “friendly pours” because he was a regular customer at Truluck’s

His friend had one glass and he finished the bottle

CRASH HAPPENED AT 9:45 PM

Bar Receipts

Defendant left the bar at 7:04 PM.

Witness called 911 & reported crash at 7:20 PM.

Officers arrived to scene at 7:24 PM.

Insurance Records

11/18/2013

28

Insurance Records Insurance Records

Insurance Records Medical Records

Admits to Alcohol Use

Admits to “2” Drinks

Admits to being Driver

Est. Speed 65 MPH

Weather – Clear

Call Rec’d 2:44 AM

2 Days Prior to Crash

Soma

Valium

Hydrocodone

Norco

11/18/2013

29

Medical Records Autopsy Report

TABC Investigation

Presenting the Victim?

• Prior to Crash.

• After the Crash.

• Don’t Commit Error.

• Day in the Life.

• Try to Get Them There.

Carlo Lujan

11/18/2013

30

Consider these . . .

Digital Evidence Course

Any Questions ?

[email protected]

A. Tire Marks: When a tire leaves a mark on the roadway it is not always a skid mark. It should be

described and identified as a particular type of mark based on its characteristics.

B. Skid Marks: This is a tire friction mark that is made by a tire that is sliding without rotation on the

road or other surface. We usually associate a skid mark with the results of a driver applying enough

brake pedal pressure to lock the wheels so that they are sliding and no longer rotating. A mark or

even the distance traveled by a vehicle, which leaves no mark, can be considered and measured as a

skid mark also. If a vehicle is traveling sideways as a result of being broadsided, the wheels are not

rotating forward or backward and as a result the tires are sliding just as if the brakes were locked.

Another example would be vehicles tires that are locked due to damage from the collision. If the

metal is forced back into the tires so that they do not rotate the vehicle is considered skidding. A

good investigator will check this restriction by having the vehicle raised and attempting to rotate the

tire.

A skid mark will leave certain characteristics on the roadway. They are relatively straight. A slight

swerve could be the result of the lower edge of the road or a driver able to steer with ABS. They

may be any length up to about 300’. There may be anywhere from 1‐4 marks from a four‐tired

vehicle. Marks made by left and right tires are generally equally dark and equally wide. Front tire

marks are usually more prominent than the rear tire skid marks due to weight shift. The width of a

mark is the width of the tire tread, sometimes it is a little wider. Skid marks end abruptly and they

often show striations that are parallel to the mark. The outer edges of skid marks from the front

tires are sometimes more prominent whereas the middle portion of a skid mark from the rear tires

is more prominent. Striations parallel to the mark are the best indication that the tire is not rotating

and the mark is a skid mark.

C. Skip Skid Mark: This mark is typically made by a vehicle bouncing during braking, usually a

tractor‐trailer. They are usually seen with unloaded semi‐trailers. During braking, the drag on

the sliding tires cause the axle to twist, thus compressing the springs and boosting the semi‐

trailer body. This lift increases the load on the tire and road producing a darker mark. The

momentum then carries the spring and wheel upward, relieving the weight on the tire and

reducing the friction which causes a light mark if any. This process is repeated leaving a series of

intermittent marks. They are to be considered and measured as one skid mark including the

gaps in between them.

D. Collision Scrubs: These are marks caused by the damage to the vehicle as opposed to braking

which locks a wheel. They are usually less than 10’ long unless the tire remains locked. These marks

are very distinct due to the force exerted by the tire on the pavement being greatly increased during

this short time.

E. Crooks: An abrupt change of direction cause by an external force which deflects the vehicle from

its forward motion. A crook is a very good evidence of the area of impact. By placing the vehicle

back over the skid mark at the crook will give you a very good indication of the vehicles point of

impact after accounting for the front overhang.

F. Impending Skids: This is also referred to as “shadow”. When brakes are applied and the wheel is

rotating less and less rapidly. Even after the wheel stops we do not always immediately see a skid

mark. The friction on the tire may not generate enough heat to immediately leave a mark. The

mark will start very light and darken. The skid mark should be measured from the first point it is

visible. Actually the greater friction is just prior to the wheel locking up. So one should not discount

this impending skid. In support of this is the ABS system. A vehicle equipped with ABS will generate

120% braking efficiency compared to a non‐ABS vehicle. ABS brakes continuously pulsate and are at

peak friction just prior to lock‐up.

G. Yaw Marks: These are marks that are made by a tire which is rotating and slipping on the

roadway. A yaw is made by a rotating tire that is slipping more or less parallel to its axis. In a

normal turn made at a moderate speed, the rear wheels track inside of the front ones. But when a

turn is made so fast that traction is lost, the rear wheels track outside of the front ones. This is due

to the centrifugal force tending to keep the vehicle going straight ahead becoming greater than the

friction between the tires and the road surface. The tires sideslip and the mark made is a yaw mark.

Characteristics of a yaw mark are that they are always curved because they result from steering.

Any striations that do appear are nearly crosswise to the mark at the beginning and changing to

oblique marks as the yaw progresses. We can determine a vehicle’s speed based on the radius of

the yaw mark and the drag factor of the roadway. Only pre‐impact yaw marks can be used in this

formula. If a vehicle goes into a yaw after impacting a vehicle or object, that yaw mark cannot be

used to determine speed. A yaw mark can start out as narrow as 1” at the beginning and then

widens as it progresses.

H. Acceleration Scuffs: A scuff mark made when sufficient power is supplied to the drive wheels to

make at least one of them spin on the road surface. These marks have the appearance of a skid

mark. The main difference is that an acceleration scuff will start very dark because the tire is

spinning and generating a lot of heat and will end with the mark gradually fading out. This would be

significant in an investigation if we needed to explain these marks in a photograph which we

determined were not part of the crash being investigated.

I. Flat Tire Marks: A mark made by a tire that is seriously under inflated or overloaded. With a

completely flat tire, the tire bunches up and the sidewalls of the tire also touch the road. The mark

made is scalloped. This is a very significant mark to be able to identify. It is a common claim of

defendants and even witnesses that a tire “blew out” and caused the drive to lose control when that

is very rarely the case. If there is no evidence of a flat tire mark prior to the impact with another

vehicle or object, then it would disprove a claim of a flat tire. But if a flat tire mark clearly leads to

the impact and that vehicle had a flat tire, you have good evidence that the tire was disabled.

J. Imprints: A mark on a road or other surface made without sliding by a rolling tire. If the imprint is

made in dirt, clear tire prints will be left which are made by a rolling wheel. A rolling tire will leave

ruts as opposed to furrows made by a sliding tire. On grass if the surface is uprooted or torn off in

the bottom of the furrow and the surface of the ground disturbed, the wheel was sliding. With

strong well‐rooted grass both sliding and rolling tires bend the grass forward and push it down. A

visible path remains, but whether it was made by a rolling or sliding tire may be uncertain.

K. Scratches and Scrapes: These marks are made with little pressure. They show where sheet

metal parts dragged across the surface or stronger metal parts moved lightly over the area. If the

marks are narrow they are considered to be scratches. If they are broad, they represent scraping.

Scratches are helpful in showing where a vehicle overturned and the path that it took after the

collision. Scrape marks will often help in locating the point of maximum engagement.

L. Gouges: Places where pavement material has been dug out by strong metal parts such as frames,

transmission housings and control arms that have been forced down on the road. With this mark

you should actually be able to reach down and feel the depression in the road. Gouge marks are

very good indicators of the area of impact.

M. Vehicles off the ground:

a. Fall: A downward and onward movement in the air under the force of gravity after forward

momentum carries an object beyond its supporting surface. A vehicle in a fall will almost always

land right side up, as rotation is minimal.

b. Flip: A sudden upward and onward movement off the ground when an object horizontal

movement is obstructed below its center of mass. The vehicle is rotating about its X axis and will flip

over on its side. Rotation during a flip is rapid and the vehicle usually lands upside down. A flip will

usually follow side‐slipping where a vehicle hits a curb or furrows in the dirt building up enough dirt

against the tires to act as a tripping mechanism. There will be an absence of marks between takeoff

and landing.

c. Vaults: End over end flips. They occur when the front tires of a vehicle are stopped by an

obstacle high enough so that the wheels do not roll over it. The vehicle will land upside down and

usually stays right where it lands.

N. Drag Factor: A number representing the acceleration or deceleration of a vehicle or other body

as a decimal fraction of the acceleration of gravity. The horizontal force needed to produce

acceleration in the same direction divided by the weight of the body to which the force is applied.

Then a vehicle slides with all wheels locked, on a level surface, the coefficient of friction and drag

factor have the same value. The drag factor for the vehicle in question on that surface is needed in

almost every calculation used to determine speed. It is the resistance to sliding between the tire

and roadway surface and tells us the rate that the vehicle will decelerate. The main things that

influence the drag factor are the slipperiness of the roadway and the hardness of the rubber of the

tires. Things such as the vehicles weight, temperature, tire tread or tire width are insignificant in

determining the drag factor.

II

METHODS OF DETERMING VEHICLE SPEEDS

There are several methods which can be used to determine the speed of a vehicle. The investigator

must first determine which method(s) is most appropriate based on the evidence and dynamics of

the crash. Some methods will only provide a minimum speed where others will give an impact

speed. Whenever possible a second method can be used to act as a check to help support the

indicated speed. Also sensitivity studys can be performed to show how little or great the speed

changes by changing the data in small increments. I will list the common methods used in

calculating speed and the date required for that calculation. Demonstrations of the formula used

and example crashes will be discussed in the classroom. It should also be noted that there are

formulas which solve for speed and formulas that solve for velocity. The result is the same but

which formula an agency or investigator uses may vary depending on his experience and training.

Therefore I will provide you with all the formulas which may be used.

A. Minimum Speed or Initial Velocity Formula: This formula will tell us based on the length of the

skid marks and the drag factor of the roadway, what the speed of the vehicle was. If the skid mark

distance precedes an impact, the skid distance will only provide the minimum speed which the

vehicle had to have been traveling to leave that length of skid marks. Obviously the vehicle still had

kinetic energy at the time it struck the other vehicle and would have continued skidding had it not

struck the vehicle. Since the skid marks do not account for the energy lost in damaging the cars, the

result is only a minimum speed. If the skid marks are post impact, they again are only a minimum

speed but can provide us with a vehicle after impact speed which can be used to determine a

vehicles impact speed.

B. Combined Speed Formula: We can combine minimum speeds obtained as the vehicle slides over

different surfaces (different drag factors). For example, if a vehicle skidded on cement, then onto

asphalt, and then onto a grass surface, we could start at the end of the crash and work the formula

three times using the appropriate skid distances and drag factors and end up with a speed at the

beginning of the skid marks. This will also be a minimum speed unless the vehicle skidded to a stop

without striking another vehicle or object.

C. Sideslip or Critical Speed Velocity (Yaws): As stated earlier, a yaw mark is the result of steering

input when a driver attempts to turn the vehicle too sharply for the vehicle’s speed. It is often seen

when a driver over‐corrects after the tires leave the road or when attempting to avoid another

vehicle or object in the road. After it is verified that this is a yaw mark, its radius is measured and

then used in this formula with the drag factor to determine the vehicle’s speed at the beginning of

the yaw mark. To determine the radius of a yaw mark an investigator would measure a chord and

middle ordinate off of the mark. These values are used in a radius formula to determine the radius

of the mark. That radius is then used to determine the vehicle speed.

D. Co‐linear Momentum: This formula will provide a vehicle’s velocity at impact. It can be used in a

crash where the vehicles come together and go off in the same direction. The most typical crash this

is used in would be a rear‐end collision. In order to use this equation the investigator must first be

able to determine five pieces of data. You must first know the impact speed of one of the two

vehicles involved. So if a vehicle was stopped at a red light and rear ended you would know his

speed at impact was 0 and could use this equation. You must also have the weights of the two

vehicles. The last two pieces of information needed are the post collision velocities of the vehicles.

This is the velocity that each vehicle was traveling immediately after the impact and is based on post

impact travel distance and the drag factor for that vehicle over that distance. This formula is limited

in that it must be an in‐line crash and you must know the speed at impact of one of the two vehicles.

It can be used in a rear‐end or head‐on crash as long as the above data can be determined.

E. Conservation of Momentum (360 degree angular): Crashes that involve an angular collision can

be reconstructed using this method. The law of conservation of momentum states that the sum of

the total momentum just prior to the crash is equal to the sum of the total momentum after the

crash. Therefore if we are able to determine the total momentum after the crash from our

investigation, then we can determine pre‐crash momentum that will give us the vehicle impact

speeds. We must have four pieces of data for each vehicle to be able to use these equations. Just

like in‐line momentum we need the vehicle weights and post collision velocities. We obtain the

weights by obtaining the curb weight from the Expert Auto Stats program and adding the weight for

occupants, fluids, and cargo. In some cases we have the vehicles weighed with certified scales. This

is most important with vehicles which have been modified as far as bumpers, tool boxes, etc. We

also need the angle which the vehicles approached each other. The last piece of information needed

is the angle which each vehicle departed the collision. The best evidence for determining an

approach angle is a pre‐impact skid mark. With no pre‐impact tire marks a damage analysis will

assist in placing the vehicles in relation to their first contact positions. The departure angles are

determined from post impact skid marks, collision scrubs, scrapes, scratches, fluid trail, etc.

Oftentimes the vehicles will experience rotation when departing a collision. The most important

data to use in determining the departure angle is the vehicles first 10‐20’ of travel in this case. A

common error which results in an incorrect speed would be to obtain a departure angle by drawing a

straight line from the vehicles impact position to its point of rest when the vehicle rotated and did

not travel in a straight path.

F. Conservation of Energy/Momentum: We use this formula whenever reconstructing a crash which

is considered a head‐on. Typically those where the vehicles approach each other at less than 10

degrees. In these cases the approach angle is very sensitive and a couple of degrees could mean a

difference of 10‐20 mph. Also the vehicles usually do not travel very far afterwards which results in a

low after impact speed. Not good cases for momentum alone. This combines energy and

momentum to determine the vehicle speeds at impact. We obviously need the same 8 pieces of

information we obtained to work a conservation of momentum problem. In addition to that we have

to obtain the vehicle Barrier Equivalent Velocity through crush as stated above. We then obtain the

energy dissipated for each vehicle and the force on each vehicle. The forces acting on each vehicle

must be equal. Therefore if they are not, we must balance the forces before continuing. This is done

with a calculation which provides adjusted A and B stiffness values for the vehicle you need to

balance. Once the forces are balanced we use the vehicle weights, after impact velocities, and the

barrier equivalent velocities in the conservation of energy formula. The momentum formula is also

used in combination with the Energy formula to solve the two vehicle impact speeds.

Forensic Toxicology Definitions for Court*

By Warren Diepraam

Montgomery County District Attorney’s Office

• Additive: two substances combine to affect the same areas of the central

nervous system • Aliquot: sample • Antagonistic: interference in the physiological action of a chemical

substance by another chemical substance • ARIDE: Advanced Roadside Impaired Driving Enforcement is a 16 hour

course that trains officers in roadside detection of drug impaired drivers • CNS: Central Nervous System is the body’s electrical and chemical

communication system • DEC: Drug Evaluation and Classification • DRE: Drug Recognition Expert is a highly specialized officer who has

completed hundreds of hours of training and experience in detecting drug impaired drivers, medical rule‐outs, and the types of drugs affecting the subject

• DRE Reconstruction: when no DRE evaluation was done, a review of facts and information by an experienced DRE to form an opinion on drug pharmacokinetics can be done in many cases by reviewing driving, medical, and jail information

• EIA: See ELISA • ELISA: Enzyme Linked Immuno‐Sorbent Assay is a fast and efficient

screening technique that uses antibodies and colors to identify a substance for subsequent GCMS, LCMS, or GCFID confirmation

• Elute: exit (eluent is a substance that has exited) • EMIT: Enzyme Multiplied Immunoassay Technique is a similar screening

method to ELISA which uses antibodies to bind to certain specified chemicals for quick screening and detection

• GCFID: Gas Chromatography with Flame Ionization Detection is the principle method for determining the type and quantity of volatiles in fluids, most notably alcohol types: ethanol, isopropanol, etc.

• GCMS: Gas Chromatography Mass Spectroscopy is the principle method for determining the type and quantity of substances in fluids for drugged driving cases

• Headspace: the closed gaseous space of matter above liquids or solids • LCMS: Liquid Chromatography Mass Spectroscopy • Metabolite: chemical by‐product produced by the body in response to the

introduction of a foreign substance (metabolic process is the body’s mechanism for changing chemical compounds)

• Active: has impairing qualities • Inactive: does not have impairing qualities

• Neurotransmitters: the body’s chemical and electrical pathways used to transmit messages about body functions

• NHTSA: National Highway Traffic Safety Administration is the primary federal government agency responsible for highway safety, impaired driving enforcement, and grants

• Parasympathetic Nerves: neurotransmitters that control the body’s vital functions, i.e. organ functions

• Pipette: device used to extract a small and accurate amount of fluid from a vial or tube (pipetting is a verb)

• Retention Time: the amount of time measured between injection of a substance into the column and detection at the end

• SPE: Solid Phase Extraction is a separation process whereby certain compounds are concentrated and separated from other compounds for use in screening with ELISA, EIA, EMIT or others

• Sympathetic Nerves: neurotransmitters that control the body’s fight or flight responses (i.e. motor skills)

• Synergistic: the opposite of antagonistic responses where the chemical effects of two or more substances combine to affect different functions

*The terms and acronyms contained in this document are intended as a general

guide for law enforcement and prosecutors, not as a scientific article.