Instrumentation & Measurement Magazine 24-1 - 42

Problems Related to the Planimetric
Survey of Road Accidents
Edoardo Riva

B

y its nature, the scene of a road accident is a particularly difficult scenario to deal with from the point of
view of its topographical reconstruction since a high
rate of risk of alteration and loss of evidence is accompanied
by a high rate of irregularity of both the place where the event
may happen and the elements present in it. These two types of
risk create two different types of problems. The first concerns
the difficulty of promptly and completely identifying all of the
pieces of evidence, while the second one concerns the difficulty of fixing their position inside the planimetry as precisely
as possible through measurement operations.
Obviously, these two aspects are intimately linked since if
you do not precisely identify an object or a track, you will not
be able to make acceptable measurements, and conversely, if
an object or a track is precisely identified but acceptable measurements are not carried out on it, the evidence may lose its
value in whole or in part or even cause a distortion of the reconstruction of the dynamics of the events. To give an example,
if a tire track is not correctly identified in its completeness, the
measurement of it can only provide a partial indication. But
there is more: when I speak of a high rate of irregularity, I am
referring in particular to the infinite variety of forms that can
be assumed by a tire track, a pool of blood, a gasoline stain, a
car headlight fragment and even the body of a victim.
When it is necessary to locate this type of irregular tracks, it
is inevitable to select only a few points on which to carry out the
measurement without losing descriptive precision. In this article
I start with the description of problems related to the nature of
the scene of a road accident that may make it difficult, fallacious
or misleading to carry out measurements. Then I will move to
those problems related to the subjectivity of the operator and the
nature of the measurement tools normally used by Italian road
police and local police in routine cases (i.e., letters and numbers,
graduated rod, sheets of graph paper, chalk, spray paint, plastic
metric tape). The situation under consideration for the purposes
of this article is that of a road accident in which operators carry
out only topographical surveys (therefore not photographic
surveys); this is a situation which currently occurs in a large
number of accidents with minor consequences.

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As is well known, the methods based on direct measurement of distances used to make a planimetry of a road accident
site are essentially two: the Cartesian coordinates and the triangulation. In the first, the points are localized using a system
of orthogonal axes that are fixed starting from cornerstones
and is particularly suitable for straight road sections; in the
second, a base is fixed and defined by two points, and then the
distances of these extremes from the points of interest on the
road are measured, thus determining for each point a triangle
of which we know the length of the three sides.

Public Nature of the Scene
In the vast majority of cases, a road accident is a public event,
and its scene is accessible to an indefinite number of people until it is effectively delimited and interdicted. For this reason, the
place and the evidence present in it may run the risk of alterations due to the action of the same persons involved as well
as those of people unrelated to the facts. In addition, the same
stretch of road often presents tracks and elements attributable
to other accidents that had previously occurred.
In this case, measurement problems are related to: the risk
of measuring a track or an element from previous accidents; the
risk of measuring a track or an element attributable to actions
subsequent to the accident carried out by people unrelated to
the facts; and the risk of measuring a track or an element attributable to the accident, but partially altered or deleted after the
event has occurred. Some of these risks can be limited by minimizing the intervention time of the operators so as to have a fast
delimitation of the scene and collecting information from all of
the people involved in the accident and from all those who find
themselves on or near the scene (e.g., rescue personnel, witnesses, etc.) with regard to any actions they have carried out
(e.g., moving objects or unintentional erasure of tracks).

Exposure of the Accident Scene to
Natural Elements and Atmospheric
Agents
Just as the scene of a road accident is exposed to the action
of people (whether or not they are involved in the accident),

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February 2021
Instrumentation & Measurement Magazine 24-1

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