TRAFFIC INVESTIGATION
ANSWERING THE ASSIGNMENT CALL
An officer's responsibility to conduct a motor
vehicle collision investigation begins at the time he
or she is directed to the collision scene. It is at that
precise moment that the officer should begin to
obtain as much information as it is available from
the radio operator, such as : type of collision,
location, extent of injuries, traffic condition, etc1 .
TRAFFIC INVESTIGATION
SCENE MANAGEMENT
Upon arriving at the collision scene a primary concern should be
preventing the collision from getting worse, that is, effective scene
management. Traffic channelization should be used to avoid second
and third collisions. Motorists are strongly attracted to collision scenes
and "gawking" is the primary cause of a great many secondary
collisions. At the early stages of the process we refer to as scene
management, the officer's primary tool to control traffic and to protect
those persons injured is the patrol vehicle. It should be parked on the
roadway and in such a way as to provide a shield between the crashed
vehicles, the injured, the officer, and traffic. Obviously the emergency
overhead red-blue lights should be operating and visible to oncoming
traffic.
TRAFFIC INVESTIGATION
Figure 1 on the preceding page shows the proper positioning of the
patrol vehicle at the collision scene in order to provide the best
protection to the officer and civilians alike. The police vehicle is depicted
in black, the collision vehicles are outlined in white.
TRAFFIC INVESTIGATION
Night-time traffic collision scene management presents an
added set of safety factors not present during daylight
hours. Visibility is severely restricted and the large number
of emergency vehicles at the scene, with overhead lights on,
tend to confuse and "hypnotize" approaching drivers. This
common phenomena causes the driver visual attention to
"home in" on the lights, and thus fail to see other things on
the roadway directly ahead (such as police officers diving
out of the way). Whenever possible the officer should work
while facing the oncoming traffic.
TRAFFIC INVESTIGATION
The use of reflectorized safety vests is an absolute must
when working nighttime collisions, particularly if an officer's
duty uniform is of the dark or black variety. If the law
enforcement agency does not issue this type of vest, it is a
worthwhile investment for any patrol officer to obtain one
of these vests.
TRAFFIC INVESTIGATION
TRAFFIC CONES/FLARES
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.
TRAFFIC INVESTIGATION
TRAFFIC INVESTIGATION
INTERVIEWING DRIVERS AND WITNESSES
The immediate purpose of any traffic collision investigation is
to obtain the relevant facts about the incident. The level of
effectiveness in a criminal investigation is largely dependent
upon the investigator's ability to obtain information. In an
collision investigation this ability is no less critical. It is
important to differentiate, at this point, interviews from
interrogations. The balance of this discussion will address the
process of interviewing drivers and witnesses in a traffic
collision.
TRAFFIC INVESTIGATION
INTERVIEWING DRIVERS AND WITNESSES
The immediate purpose of any traffic collision investigation is
to obtain the relevant facts about the incident. The level of
effectiveness in a criminal investigation is largely dependent
upon the investigator's ability to obtain information. In an
collision investigation this ability is no less critical. It is
important to differentiate, at this point, interviews from
interrogations. The balance of this discussion will address the
process of interviewing drivers and witnesses in a traffic
collision.
TRAFFIC INVESTIGATION
Driver interviews must be conducted individually, with each
driver given the opportunity to state what happened away
from the other driver. This will prevent argumentative
behavior, and it will keep the second driver from
"developing" defenses while listening to the first
driver.During the interview it is important to closely observe
each driver for signs of alcohol or drug use, injury, or shock.
TRAFFIC INVESTIGATION
Disinterested parties (bystanders) who were not
involved in the collision are generally the most
effective and reliable eyewitnesses. They should be
contacted as soon as possible by the investigating
officer, or any other available officer. It should be
remembered that bystanders are attracted to the
excitement and atmosphere of the scene.
TRAFFIC INVESTIGATION
TIRE MARKS
Tire marks and roadway evidence are extremely important in determining
how vehicles moved in to the impact point, and from impact to final
resting locations. In collision investigation tire marks are the equivalent of
a fingerprint in a criminal investigation, and just as important. When
brakes are applied the vehicle has a tendency to shift forward, this results
in a weight transfer from the rear wheels to the front wheels. Typically,
55% - 60% of the total weight of the vehicle rests on the front wheels
during braking, leaving 45% - 40% weight applied to the rear wheels.
TRAFFIC INVESTIGATION
SKID MARKS
This skid mark is created by a tire
that is locked, that is, sliding and not
rolling. Skid marks tend to be
straight, although they can exhibit
some curvature due to asymmetrical
braking (not all brake pads locking
simultaneously) or due to the crown
of the road. This can make the
vehicle depart from a straight-ahead
path. Front tire skid marks tend to be
darker than rear tire marks
TRAFFIC INVESTIGATION
Skid marks are an extremely
important piece of physical
evidence to the collision
investigator. They can be used
to determine speed and to
establish the path of the
vehicle while skidding.
Unfortunately, tire mark
evidence has a life span. They
are affected by weather,
sunlight, and traffic.
TRAFFIC INVESTIGATION
When looking for skid marks it is of
extreme importance to determine
the point where the skid mark
begins. This beginning point of the
skid mark is a relatively faint mark,
as compared to the rest of the dark
tire mark. During initial brake
application there is a short time
delay between the time the braking
system/tire combination locks the
wheel, and the point at which the
tire heats up sufficiently to begin
leaving a mark.
TRAFFIC INVESTIGATION
This faint beginning is called the skid mark shadow. To
locate this shadow the investigator must kneel or bend
down to the roadway level, 20 to 30 feet ahead of the
mark, and look towards the apparent beginning of the skid
mark.
TRAFFIC INVESTIGATION
Curved skid marks indicate that the vehicle
that made them is rotating while
simultaneously skidding. When this
happens all four tire marks can be
observed. This rotation during a skid can
be initiated by the driver beginning a
turning maneuver as wheels are locked in
braking, the vehicle then continues in
rotation. Curved skid marks can also be
indicators of unequal braking. If left side
tires are braking with greater force than
right side tires, the vehicle will tend to
rotate counterclockwise in the direction
offering the higher resistance.
TRAFFIC INVESTIGATION
SKIP SKIDS When skid marks are not
continuous but are intermittent,
they may have been made by a
vehicle bouncing along on the
roadway. In this situation the length
of the skid mark and the length of
the space between them is uniform
and consistent, and less than 3 to 4
feet apart. This condition can result
when the wheel strikes a pothole,
or bump on the roadway, which
starts the vehicle bouncing.
TRAFFIC INVESTIGATION
GAP SKIDS Many times skid marks are observed in which there is a gap between the
termination of the skid marks on the roadway and a re-initiation of the skid mark
some distance down the roadway. This is the result of the driver applying the brakes,
and subsequent release and re-application. Sometime the driver may momentarily
release the brakes because he/she believes that the collision conflict situation
ahead, in which a collision appears imminent, has passed, only to reapply them
again when he/she realizes that the initial judgement was incorrect.
TRAFFIC INVESTIGATION
ACCELERATION MARKS Often, tire
marks that look like skid marks are
really acceleration marks. These marks
are created when a vehicle is
accelerated rapidly from a stopped
position or when moving at a slow
speed, so as to make dark tire marks.
These acceleration marks closely
approximate skid marks in their
appearance. They begin as heavy dark
marks and slowly disappear as the
rotational velocity of the tire begins to
approach the linear velocity if the
vehicle.
TRAFFIC INVESTIGATION
TIRE TREAD PRINT An imprint of the tire tread pattern indicates that the wheel was
rolling and not skidding. The effect thus created is much like an ink stamp, in which the
pattern of rubber is imprinted on a flat surface without smearing. The print may be the
result of loose matter picked up by the tire as it rolled on the roadway. Tire prints are
different from skid marks in that they convey the tire tread pattern of the tire without
any of the slick or smoothly worn features characteristic of a skid mark. In addition, the
print pattern is uniform in contrast and noticeably similar to other print marks left by
tires on other wheels of the same vehicle.
TRAFFIC INVESTIGATION
SCUFF MARKS
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.
TRAFFIC INVESTIGATION
TRAFFIC INVESTIGATION
METAL SCARS
When a moving vehicle is damaged in such a way that
metal parts come in contact with the roadway surface,
scars or scratches are left. Scars are helpful in indicating
the direction of movement of the vehicle on impact.
When they are correlated with the parts of the vehicle
which made the scars they can also confirm the
position of the vehicle on the roadway at impact.
TRAFFIC INVESTIGATION
WHAT THE VEHICLE SHOWS Now that the road markings
and the information they can reveal have been
discussed, we can turn our attention to the technical
examination of the vehicle. Final position of the vehicles
is usually the most important observation to make on all
collisions. A quick glance at all traffic units involved upon
the officer's arrival can give a general idea as to final
positions. This, of course, should be followed by efforts
directed at locating the vehicles by measurement.
TRAFFIC INVESTIGATION
TRAFFIC INVESTIGATION
If photos are to be taken, the basic four view
sequence of photographs should be taken before
allowing vehicle removal. Additional photos should
be obtained showing closeup or detailed areas of
damage, matching damage parts of two vehicle
showing how they came in contact, and any other
item on the vehicle that can help explain the
collision.
