This is a brief summary of Maggie Harthoorn's
findings as outlined in her recently com-
pleted master's thesis from the University
of Nebraska-Lincoln toward her degree
in Community and Regional Planning.
W
alkable and livable streets are safer
streets than fast-moving, car-oriented
systems, and street trees play a role in this
paradigm. The impacts of trees on roadway
user safety must be understood in order
to improve traffic safety. To the untrained
eye one may assume that crashes occur
more frequently where trees are present.
However, with the aid of SPSS (statistical anal-
ysis software) we found that the presence of
trees along streets results in an increase in
crash severity but not in crash frequency. This
finding supports the hypothesis that trees do
not cause traffic accidents. Crashes with trees
are, however, more likely to be more severe
than other types of crashes, possibly because
trees provide little buffer upon impact with
a vehicle. While these findings are useful in
understanding the impacts of trees on traffic
safety, further analysis of other variables is
necessary for any further conclusions to occur.
One such variable is the health of the trees on
our streets. Poor quality street tree planting
stock can lead to poor quality adult trees.
When this problem is combined with dwin-
dling funds for post-planting maintenance,
poor quality trees are increasingly unlikely
to be corrected after planting, leading to
hazardous situations for roadway users.
Observation of recently planted stock can
provide an idea of future condition and health
of the tree canopy once mature. A healthy
planting stock that is properly pruned and
maintained will produce a strong, healthy,
and productive urban canopy, while a pop-
ulation of young trees exhibiting structural
defects may require additional resources and
maintenance to produce the same results.
Based on the Florida Grades and Standards
model developed in 2015, 73% of trees in
Des Moines, Iowa are in good standing but
may require minor correction of incorrect
existing pruning cuts, pruning of adventitious
Examples of minor pruning required for trees
in Des Moines, Iowa. Examples include trees
in need of stub pruning (left) and adventitious
sprout pruning (right).
Trees & Traffic
Safety, and the
Role of Tree
Maintenance in
a Des Moines,
Iowa Case Study
Story and photos by Maggie
Harthoorn, Urban Forest Coordinator,
Friends of Grand Rapids Parks,
Grand Rapids, Michigan
www.urban-forestry.com 31
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