United States Department of Transportation
i
Bridge rail and approach railing for low-volume roads in Iowa : final report, March 2010.
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2010-03-01
[PDF-21.65 MB]
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Edition:Final report
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Abstract:Bridge rail and approach guardrails provide safety to drivers by shielding more hazardous objects and redirecting vehicles to the
roadway. However, guardrail can increase both the initial cost and maintenance cost of a bridge, while adding another object that may
be struck by vehicles. Most existing low volume road (LVR) bridges in the state of Iowa are currently indicated to not possess bridge
rail meeting “current acceptable standards”. The primary objective of the research summarized in this report was to provide the nations
bridge and approach rail state of practice and perform a state wide crash analysis on bridge rails and approach guardrails on LVR
bridges in Iowa. In support of this objective, the criteria and guidelines used by other bridge owners were investigated, non-standard and
innovative bridge and approach guardrails for LVR’s were investigated, and descriptive, statistical and economical analyses were
performed on a state wide crash analysis.
The state wide crash analysis found the overall number of crashes at/on the more than 17,000+ inventoried and non-inventoried LVR
bridges in Iowa was fewer than 350 crashes over an eight year period, representing less than 0.1% of the statewide reportable crashes. In
other words, LVR bridge crashes are fairly rare events. The majority of these crashes occurred on bridges with a traffic volume less than
100 vpd and width less than 24 ft. Similarly, the majority of the LVR bridges possess similar characteristics.
Crash rates were highest for bridges with lower traffic volumes, narrower widths, and negative relative bridge widths (relative bridge
width is defined as: bridge width minus roadway width). Crash rate did not appear to be effected by bridge length. Statistical analysis
confirmed that the frequency of vehicle crashes was higher on bridges with a lower width compared to the roadway width.
The frequency of crashes appeared to not be impacted by weather conditions, but crashes may be over represented at night or in dark
conditions. Statistical analysis revealed that crashes that occurred on dark roadways were more likely to result in major injury or fatality.
These findings potentially highlight the importance of appropriate delineation and signing.
System wide, benefit-cost (B/C) analyses yielded very low B/C ratios for statewide bridge rail improvements. This finding is consistent
with the aforementioned recommendation to address specific sites where safety concerns exist.
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