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Suggested Citation:"Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Safety Performance of Part-Time Shoulder Use on Freeways, Volume 1: Informational Guide and Safety Evaluation Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/26394.
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3 P A R T - T I M E S H O U L D E R U S E I N F O R M A T I O N A L G U I D E Introduction Part-time shoulder use is “a transportation system management and operation (TSM&O) strategy that allows use of the left or right shoulders as travel lanes during some, but not all, hours of the day” (Jenior et al. 2016). This document provides an overview of part-time shoulder use, presents the results of past operational studies, and presents the results of safety research conducted through NCHRP Project 17-89 Safety Performance of Part-time Shoulder Use on Freeways. The Federal Highway Administration (FHWA) recognizes three types of part-time shoulder use operation:  Bus-on-shoulder (BOS) – open only to authorized buses and usually at the driver's discretion.  Static part-time shoulder use (S-PTSU) – open to vehicles only during predetermined hours.  Dynamic part-time shoulder use (D-PTSU) – open to vehicles in response to real-time traffic conditions (Jenior 2019). This document primarily discusses S-PTSU and D-PTSU operation. For simplicity, this operation is referred to herein as “PTSU operation”. Any reference in this document to “PTSU” is referring to shoulder use by all vehicle types during a few hours of the day; it is not referring to BOS operation. NCHRP Project 17-89 initially explored the safety performance of sites with BOS operation, and the analysis found that the presence of BOS operation did not have a statistically significant effect on total crash frequency or fatal and injury crash frequency. PTSU operation in the United States (US) has been exclusively on freeways, and findings presented in this document are from studies of freeways. a. Dynamic operation, I-70 Colorado. b. Static operation, I-66 Virginia Figure 1. Dynamic and Static PTSU Operation. The first US PTSU facility opened in the state of Washington in the mid-1970s. Facilities opened in Massachusetts and Hawaii in the 1980s, and Virginia in the early 1990s (Jenior 2016). There are currently PTSU facilities in 12 states. Most PTSU facilities in the United States are open to all vehicles, are not tolled, and have the same speed limit as the adjacent freeway through lanes. Reasons that transportation agencies have implemented PTSU include the following:  Operational improvements and capacity increases approaching those expected with the addition of a general-purpose lane.  Reduced cost and right-of-way requirements compared to a conventional widening project.

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Left or right shoulders can be strategically opened as travel lanes, and "part-time shoulder use" is defined as using a shoulder "some, but not all, hours of the day.”

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 309: Safety Performance of Part-Time Shoulder Use on Freeways, Volume 1: Informational Guide and Safety Evaluation Guidelines provides an overview of part-time shoulder use, presents the results of past operational studies, and presents the results of safety research conducted through NCHRP's Safety Performance of Part-time Shoulder Use on Freeways project.

Supplemental to the document is a Freeway Analysis Tool, which includes BOS data, S D PTSU Data, and a Prediction Tool, as well as NCHRP Web-Only Document 309: Safety Performance of Part-Time Shoulder Use on Freeways, Volume 2: Conduct of Research Report.

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