Evaluation of safety effectiveness of composite shoulders, wide unpaved shoulders, and wide paved shoulders in Kansas.
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2013-03-01
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NTL Classification:NTL-HIGHWAY/ROAD TRANSPORTATION-Design;NTL-HIGHWAY/ROAD TRANSPORTATION-Construction and Maintenance;
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Abstract:Incremental increases in paved shoulder widths have been studied and are shown in the Highway Safety Manual. While
each incremental increase in shoulder width is beneficial, there is evidence that suggests the relationship between safety
improvements and incremental increases in shoulder width may not be linear. It is possible that the net safety gains for wider
shoulder increments are not as high as incremental benefits of the initial increments of shoulder width. Thus, a highway agency
may have opportunities for greater system-wide safety benefits from paving longer roadway segments with a narrower shoulder
rather than paving shorter roadway segments with a wider shoulder. This approach is tempered by consideration of long term
degradation in shoulder width and slope over the life of a facility due to normal pavement maintenance activities. Practitioners
must balance long-term sustainability, cost, expected operations and safety benefits of proposed improvements. For new and
reconstruction projects, the cost of additional shoulder width is minimal compared to retrofitting an existing facility. Determining
the benefits of various shoulder improvement approaches fits within the Kansas Department of Transportation’s (KDOT)
“Practical Improvements” approach to maximize benefits relative to the construction and maintenance costs required.
Among the 8,300 miles of rural two-lane highways in Kansas, approximately 25 percent of them are equipped with
composite shoulders consisting of three feet of pavement with the remainder aggregate or turf. Their safety effectiveness was
studied using the Empirical Bayes (EB) approach and the cross-sectional approach. Three developed Safety Performance
Functions (SPFs) were used to create Kansas-specific Crash Modification Factors (CMFs) for composite shoulders compared with
segments with no or unpaved shoulders. It was found that upgrading narrow unpaved shoulders to composite shoulders can reduce
shoulder related crashes by up to 61 percent and fatal and injury crashes by 31 percent. It was also found that wide paved shoulders
can provide more safety benefit than composite shoulders, and wide unpaved shoulders can provide slightly less safety benefit than
composite shoulders. Based on these results, 20-year projections were developed projecting the safety effectiveness that can be
achieved through implementing these safety improvements.
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