Part-time shoulder use on freeways is an increasingly popular and practical performance-based design strategy for both reducing congestion and improving bus travel time and reliability. There are three types of part-time shoulder use: bus-on-shoulder (BOS), static shoulder use, and dynamic shoulder use. BOS is limited to authorized buses driven by trained drivers. Static shoulder use and dynamic shoulder use allow all or most vehicles to use the shoulder when it is open. Static shoulder use opens the shoulder to traffic on a fixed schedule while dynamic shoulder use opens the shoulder to traffic in response to overall traffic conditions.
Sixteen states have one or more part-time shoulder use facilities in operation on freeways, many of which have opened within the past five to ten years. In 2016 FHWA published a guide on part-time shoulder use. That publication identified the lack of tools for quantifying the safety of part-time shoulder use as a major research gap. Only two safety studies of part-time shoulder use in the United States were cited, and each was focused on a single facility. Moreover, the current Highway Safety Manual (HSM) freeway models do not explicitly consider the potential safety performance of part-time shoulder use.
Research is needed to help engineers and planners better understand both the safety performance of existing part-time shoulder use operations and the specific operational and design elements that influence (or appear to influence) their safety performance.
The objectives of this research were to (1) develop quantitative tools for practitioners to use to evaluate safety performance of freeways with part-time shoulder use as a function of temporal, operational, and other conditions when the shoulder is open or closed to traffic, and (2) use the tools to determine the safety performance of part-time shoulder use, in order to better inform state DOT decision making on their application.