GIS and analysis of traffic accidents
March 30th, 2005 by Tom Johnson

The CrimeMap listserv
is one of the best around for nuts-and-bolts tips.  Most of its
contributors are professional crime analysts (unlike journalists who,
it must be said, are semi-pros at best).  To subscribe, check out

One of the folks on Crimemapping made a fine contribution today filled
with “heads-up” tips when it comes to crime mapping. 
Lopez, a crime analyst for the
Connecticut State Police, suggests….

        “The analysis of traffic accidents is an
interesting one.  My perspective is looking at, for the most part,
.  State Police Departments as a general rule have jurisdiction for
the enforcement of crime, traffic accidents, and motor vehicle enforcement on
interstate highways.  In Connecticut, we are able to pick up a good “X” and “Y”
for every accident.  We then take the data and run it through Spatial Analyst. 
In looking at motor vehicle accidents on interstate highways you may
Some traffic accident hotspots occur in very dangerous areas, ones that do not
lend themselves to offering a great deal of motor vehicle enforcement.  That is
to say, if you use high visibility enforcement you might be in a very congested
area with high traffic volume.  The area is already hazardous and might not
offer any safe area for ticketing.  Enforcement in these areas might actually
contribute to accidents.  I have been told by some veteran officers the “trick”
is to get people in and out of large metropolitan areas as fast as
2.      The design of the highways and surrounding
areas might have a greater effect on accidents than enforcement.  To effectively
lower accidents on interstates you have to have long range plans that include
all agencies that govern the highway.  I believe you will find many of the
traffic accident hotspots on interstates fall right in the middle of where two
or more interstate highways converge.  In Hartford for example, you have I-84
and I-91 intersecting in the middle of the city.  People have to make decisions
on going north and south/east and west.  Of course in the afternoon, you have
that sun in your eyes.  These types of decisions have to be made in seconds, and
for those people not familiar with the area are at even more of a disadvantage. 
Highway design plays a huge factor in highway accidents.
3.      In doing time studies of motor vehicle
accidents, many are occurring at changes of shifts, and at peak times where
police services are at a premium.  On interstates, especially around
metropolitan areas you will find most of the accidents are occurring
around 07:00 to 09:00 and then at 15:00 to 18:00.  This is the precise time
people are coming home from work and children getting out from school; a time
that has high demand for all police services.
It is very interesting to plot DWI Accidents, meaning those accidents in which
DWI was the cause of the accident.  You may find that your highest
concentrations of accidents occur near routes going to major universities, large
entertainment complexes, or perhaps to out of state jurisdictions where the
drinking age might be lower or where bars stay open longer.  In these
circumstances select enforcement can be effective.
Careful consideration should be given to areas of the highway that are
undergoing construction.  Police visibility should be present to get people to
slow down. Strict enforcement of speed limits in these areas may yield to a
lower accident rate.
closing, I believe you will find the most accidents are occurring on interstates
where there are very high volume of cars, little or no highway shoulders, areas
that could benefit from a better design and where interstate highways meet.  The
answer to a reduction to these accidents is a difficult one to find.  Giving out
more tickets is not necessarily the answer.” 
Analysis Unit
Connecticut State Police

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