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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2022. Selection and Placement Guidelines for Test Level 2 Through Test Level 5 Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/26679.
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1   The roadside safety community has been interested for several decades in developing selection and placement guidance for the multiple test levels of median barriers. The variety of median widths and terrains combined with evolving testing specifications and lack of conclusive data on median crossover crashes have been obstacles to success. The ongoing implementation of the Manual for Assessing Safety Hardware (MASH), combined with new data collection efforts and the availability of new analysis tools, has overcome some of the primary obstacles to developing median barrier guidance. (AASHTO 2016) The objective of this research was to develop, in a format suitable for consideration and possible adoption by AASHTO, proposed guidelines for the selection and placement of MASH Test Levels 2 through 5 (TL2-TL5) median barriers. These guidelines are based on traffic volume and mix, roadway and median geometry, median barrier placement, in-service performance, cost–benefit and risk analyses, and barrier type (i.e., shape, material, rigidity, etc.). These guidelines are suitable for use by government transportation agencies at the state and local levels. It is anticipated that the results will be integrated into an updated edition of the AASHTO Roadside Design Guide (RDG). The approach to the guideline development, as stated in the project statement of work, is risk- based; the frequency and severity of crashes with and without median barriers are estimated and the risk of observing an incapacitating or fatal injury crash is calculated. The third version of the Roadside Safety Analysis Program (RSAPv3) was developed to perform a cost–benefit analysis. NCHRP Project 22-12(03), “Development of Guidelines for Bridge Rails” expanded RSAPv3 to document the risk analyses that are the basis of the cost–benefit analyses. (Ray 2021; Ray 2012b) The statement of work suggested the use of RSAPv3. As this research progressed, the scope was extended to include MASH roadside barriers in addition to median barriers. Additionally, NCHRP Project 15-65, “Development of Safety Perfor- mance Based Guidelines for the Roadside Design Guide,” was advertised and awarded. (Ray 2018) NCHRP Project 15-65 has developed an updated approach to roadside design guidance develop- ment based on the encroachment probability model programmed with RSAPv3 and conceptual- ized through a governing equation. The existing guidance in the AASHTO RDG (AASHTO 2011) is being updated to use the systematic approach under development in NCHRP Project 15-65. For these reasons, and so this median and roadside barrier guidance will fit seamlessly into the ongoing update to the AASHTO RDG, the new guidance presented herein for both median barriers and roadside barriers was developed using the NCHRP Project 15-65 governing equation. This report summarizes the guideline development effort. Substantial technical research was undertaken, much of which is detailed separately in Appendices A through E to allow the reader to focus on the guideline development. The guidelines for assessing median barrier need and selecting the median barrier material are shown in Figure 23. Table 12 can be used to select the appropriate barrier test level. Cost–benefit guidelines are presented in Section 5.3. Guidelines for shielding fixed objects on slopes flatter than 2:1 are presented in Figure 25. C H A P T E R 1 Introduction

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The variety of median widths and terrains combined with evolving testing specifications and lack of conclusive data on median crossover crashes have been obstacles to developing median barrier guidance.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 996: Selection and Placement Guidelines for Test Level 2 Through Test Level 5 Median Barriers develops, in a format suitable for consideration and possible adoption by AASHTO, proposed guidelines for the selection and placement of Manual for Assessing Safety Hardware (MASH) Test Levels 2 through 5 (TL2-TL5) median barriers.

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