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: Recommended Procedures for the Safety Performance Evaluation of Highway Features. Each test level is defined by impact conditions (speed and angle of approach) and the type of test vehicle the barrier is designed to redirect (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.
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 were not provided.
Under NCHRP Project 22-31, “Recommended Guidelines for the Selection and Placement of Test Levels 2 through 5 Median Barriers,” Roadsafe, LLC developed proposed guidelines for the selection and placement of Test Levels 2 through 5 median barriers suitable for use by all government transportation agencies at the state and local level.
This project is complete and the final report has been published as NCHRP Research Report 996, "Selection and Placement Guidelines for Test Level 2 through Test Level 5 Median Barriers." The appendices are available on the TRB Publications Summary Web Page.