United States Department of Transportation
i
An improved bridge safety index for narrow bridges.
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1983-08-01
[PDF-4.25 MB]
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Abstract:In this report, a new bridge safety index is developed based upon an extensive
statistical study of accident data on 78 bridges. A total of 655 accidents
were recorded at these bridges over the six-year period between 1974 and 1979.
Cluster analysis showed that 26 of the bridges could be grouped into a "less-safell
class and the remaining 52 bridges culstered into the Ilmore-safell group. Assigning
a score of 0 to the less-safe group and 1 to the more-safa group, logistic regression
analysis was used to develop an equation for the probability that a
bridge is safe. The R2 of the equation is 0.53 and i.ncludes the following variables
bridge wid~h, bridge length, ADT, vehicle speed, grade continuity, shoulder reduction,
and traffic mix. A sensitivity analysis shows that bridge safety may be
improved most by reducing vehicle speed and secondly by increasing the bridge
width. Recommendations are made for the data that should be collected and the number
of bridges that should be represented in future studies.
The Bridge Safety Index developed in this study i·s considerably better than all
that have been proposed previously since it is based objectively upon accident rates
and has a much better correlation with accident rates than any previously proposed
Bridge Safety Index. Consistent use of this index in setting funding priorities for
projects to improve bridge safety should result in a reduction of accident rates,
property damage, injuries and fatalities at bridges.
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