Safety Performance of Part-Time Shoulder Use on Freeways, Volume 2: Conduct of Research Report (2021)

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3 C H A P T E R 1 - I N T R O D U C T I O N Introduction The American Association of State Highway and Transportation Officials (AASHTO) Highway Safety Manual Supplement (HSM Supplement) (AASHTO 2014) provides predictive methods for evaluating the safety performance of freeways and ramps. The specific models in the HSM Supplement can be used to analyze a variety of freeway designs but do have some limitations. One limitation is that the influence of the “safety shoulders as travel lanes” (i.e., part-time shoulder use) is not accounted for by the predictive method. The crash prediction models (CPMs) presented in this document are intended for analysis of freeways with part-time shoulder use (PTSU) or freeways on which PTSU is being considered. Models in the current HSM Supplement should continue to be used for projects not involving PTSU. The Federal Highway Administration (FHWA) recognizes three types of part-time shoulder use operation (Jenior et al. 2019). These types are identified in the following list.  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 all vehicles during predetermined hours.  Dynamic part-time shoulder use (D-PTSU)—open to all vehicles during time periods specified in real-time based on observed traffic conditions. This document primarily discusses S-PTSU and D-PTSU operation. For simplicity, these two types of part-time shoulder use operation are collectively referred to herein as “PTSU operation.” When it is necessary to differentiate between static part-time shoulder use and dynamic part-time shoulder use, the terms “S-PTSU operation” and “D-PTSU operation” are used. Any reference in this report 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. This report describes the development of CPMs for freeways with PTSU operation. CPMs for BOS operation were initially explored but ultimately not developed; the presence of BOS was not found to have a statistically significant effect on crash frequency or severity. BOS operation permits only a small number of vehicles to drive on the shoulder, drivers of buses receive training prior to being authorized to drive on the shoulder, and BOS operation is restricted to low-speed conditions. These factors may contribute to the lack of a statistically significant effect of BOS presence on crash frequency. The PTSU models are used to predict the average crash frequency associated with one direction of travel on a freeway. Each CPM includes a safety performance function (SPF), one or more adjustment factors (AFs), a calibration factor, a severity distribution, and a crash type distribution. The SPF, AFs, and calibration factor are multiplied together to form the predictive model equation. One predictive model is described herein for each of the following site types:  Freeway segment  Ramp entrance speed-change lane  Ramp exit speed-change lane The CPMs include an AF for PTSU operation. This enables the predictive model to be used for the analysis of freeway sites with or without PTSU operation.

4 Report Organization This report is organized into the following chapters:  Chapter 1—Introduction.  Chapter 2—Literature Review.  Chapter 3—Modeling Approaches. This chapter provides definitions for the site types modeled in this project and the framework of the crash prediction models.  Chapter 4—Project Database. This chapter describes data collection activities and provides tabular summaries of the database assembled for this project.  Chapter 5—HSM Predictive Model. This chapter describes models that predict the frequency of fatal-and-injury (FI) and property-damage-only (PDO) crashes. These models are proposed for inclusion in a future edition of the Highway Safety Manual (HSM).  Chapter 6—Crash Severity and Crash Type Distribution. This chapter describes the distributions of various crash severity levels and crash types. These distributions were prepared for use with the CPMs described in Chapter 5.  Chapter 7—Advanced Crash Predictive Models. This chapter describes predictive models that were estimated using two advanced regression analysis techniques—random parameter modeling and latent class analysis.  Chapter 8—Supplemental Safety Findings. This chapter presents the results of additional statistical analyses that were conducted to assess the safety effects of different PTSU design and operational features. Also presented are the results from a sub-annual analysis to separately model periods of the day when the shoulder is open to traffic and closed to traffic.  Chapter 9—Conclusions. This chapter presents major conclusions from the project and recommendations for future research. Related Documents In addition to this report, National Cooperative Highway Research Program (NCHRP) Project 17-89 produced several other resources. These resources are identified in the following list.  PTSU Informational Guide—a concise document summarizing major findings from this project and background information on PTSU and the operational benefits of PTSU.  PTSU Safety Evaluation Guidelines—a document that describes how a practitioner can effectively use the CPMs developed in this project. The document contains a proposed chapter for a future edition of the HSM.  A spreadsheet tool implementing the CPMs described in this report.