Watters, P. and O'Mahony, M, The relationship between geometric design consistency and safety on rural single carriageways: [in] proceedings of the European Transport Conference 2007, Leeuwenhorst, The Netherlands, October 17th-19th 2007, Association for European Transport, 2007, pp[1-15]
Rural single carriageways in Ireland tend to fall below current design
standards and have higher accidents rates (per vehicle kilometre) than roads
designed to modern standards. Resources are not available to improve all
rural single carriageways; therfore sections need to be identified as priority
sections for improvement. Geometric design consistency studies can be used
to identify inconsistent sections on highways, which can then be targeted for
improvement. No geometric data exists for rural single carriageways in
Ireland. A method of estimating geometric data from digital maps was
implemented on some 70km of highways. 19 curves and 19 tangents were
then selected to represent the overall geometric makeup of the highway.
Numerous geometric indices were measured on site. A spot speed survey
was conducted at the midpoint of each tangent and curve and operating
speed was calculated for each site. The spot speed survey is used to estimate
the operating speed on straights (tangents). An operating speed model is
used to estimate the operating speed on curves. Using these estimated
operating speeds a sample geometric design consistency evaluation is carried
out. Elements were classified as good, fair or poor using a design evaluation
criterion. An accident database for the N52 was obtained. Accidents that
occurred on the N52 and were possibly caused by the road alignment were
extracted from the database. 53 accidents at 40 locations were observed. 19
of these accidents occurred in locations that were classified good, 8 accidents
occurred in locations classified fair and 13 accidents occurred in locations
classified as poor. A relationship exists between geometric design consistency
and safety. Of the 40 locations which had accidents over the 8 year period
from 1999 – 2005, 13 of these locations were detected as needing realignment
by the geometric design consistency evaluation. A geometric
design evaluation can be used to pin point locations on highways where
accidents could conceivable be higher. Improvement works and resources
can therefore be concentrated on these sections and hence rural single
carriageways can be made safer.
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