Impact of Shoulder Width and Median Width on Safety (2009) / Chapter Skim
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Pages 1-6
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Therefore, in an investigation to determine the safety impacts of design flexibility on rural multilane highways, the NCHRP project panel recommended that the second phase of the research focus on three geometric elements: lane width, shoulder width, and median type and width. This decision allowed the development of useful models compatible with the current efforts in the development of the Highway Safety Manual (HSM)
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Data from three states were used to develop prediction models that could be used for this purpose, with an emphasis on developing crash prediction models and Accident Modification Factors (AMFs) for multilane rural roads with respect to lane width, shoulder width, and median width and type.
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. Based on the project team's review of past literature, the recommended values for the HSM, and the AMF from NCHRP Project 15-27, the presence of shoulders appears to influence crash occurrence, and the values noted for all crashes for undivided highways seem reasonable Average shoulder width (ft)
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Another issue that should be addressed in future research is the lack of AMFs for shoulder width greater than 8 ft since the literature indicates that the safety effects for such shoulder widths are unknown. Median Width Past Research The most important objective for the presence of medians is traffic separation.
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The team determined that the values noted for the only model with median width influence are reasonable and in accordance with current trends and literature. The only available AMF based on the models developed in this research is for multi-vehicle crashes; there is a 1% reduction for every additional foot of median width added.
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From page 6... ...
"Estimating Safety Effects of Cross-Section Design for Various Highway Types Using Negative Binomial Regression," Transportation Research Record 1500. Transportation Research Board, National Research Council, Washington, DC (1995)
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