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Suggested Citation:"Chapter 2 - Definitions." 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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2 Definitions The words median and median barrier are defined slightly differently in the AASHTO and State DOT literature. (AASHTO 2006; AASHTO 2011; Caltrans 2012) The National Transpor- tation Safety Board (NTSB) has asked FHWA and AASHTO to provide a consistent definition for cross-median crashes, citing a lack of consistency throughout the states. This section presents the definition of terms used consistently in this report and ultimately in the guidelines that are the result of this research. Reference is made throughout this report to these definitions. • Median: The portion of a divided highway separating the traveled ways for traffic in opposite directions. • Median barrier: A longitudinal barrier system intended to reduce the risk of an errant vehicle crossing the highway median. Median barriers are designed to be impacted from either direction of travel. • Median-related event (MRE): Any event where an errant vehicle enters the median. MREs represent all vehicles that encroach left into the median, regardless of the outcome (i.e., crash or no crash). • Cross-median event (CME): An event where an errant vehicle fully crosses the median and may or may not collide with another vehicle from the opposite direction. CMEs are a subset of MREs. • Cross-median crash (CMC): A cross-median crash is one in which an errant vehicle crosses the median of a highway and strikes or is struck by a vehicle from the opposite direction. CMCs are a subset of CMEs, which are, in turn, a subset of MREs. C H A P T E R 2

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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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