Median barriers can be divided into six test levels, as defined by the 2009 AASHTO Manual for Assessing Safety Hardware (MASH) and NCHRP Report 350, which represent the types of vehicles the barrier is designed to redirect. A test level is defined by impact conditions (speed and angle of approach) and the type of test vehicle (ranging in size from a small car to a fully loaded tractor-trailer truck). The longitudinal barrier is the only classification for which all six test levels are defined at this time. Longitudinal median barriers are also grouped into three general categories: flexible, semi-rigid, and rigid. Although a rigid barrier and a flexible barrier can be designed to satisfy a given test level, they will have different applications. The rigid barrier will produce higher vehicle decelerations and prevent any lateral deflection while the flexible barrier will produce lower accelerations. Less rigid barriers result in less energy dissipated by the vehicle; hence, accelerations imparted to the occupants inside the vehicle during an impact are lower as compared to vehicle impacts with rigid barriers. On the other hand, flexible barriers have been shown to have larger lateral deflections, thus limiting their use in narrow medians. The crash test procedures for median barriers have seen much progress over the years, as evident by the publication of NCHRP Report 230, NCHRP Report 350, and most recently, the MASH. Currently, the AASHTO Roadside Design Guide (RDG) is the primary national guideline available to states in preparing their own policies for roadside design. The RDG offers guidance for the selection of median barriers, citing a higher percentage of heavy trucks in the traffic flow, adverse geometries, and higher accident rates as conditions that may warrant barriers with a performance level higher than Test Level 3. However, thresholds for these values are not provided. Additional guidance on the selection and placement of median barriers would complement recently developed guidance for bridge rails and would help better define these thresholds while helping to balance public funds with improved safety.
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 Test Levels 2 through 5 median barriers. The guidelines were based on traffic volumes and characteristics, roadway and median geometry, median barrier placement, in-service performance, cost benefit, risk analysis, and barrier type (i.e., shape, material, rigidity, etc.). The guidelines included charts with associated site-specific adjustment factors for the selection of the appropriate median barrier test level and for use in the selection of median barrier type and placement within the median. The guidelines should be suitable for use by all government transportation agencies at the state and local level. It is anticipated that the results will be integrated into an updated edition of the AASHTO Roadside Design Guide (RDG).
Research is complete. Final Report is under review.