NCHRP_Report_1089 presents guidelines for the use and placement of the 31-in. Midwest Guardrail System (MGS) adjacent to curbs under the Manual for Assessing Safety Hardware (MASH) Testing Level (TL)-3 impact conditions. The guidelines consider the effects of curb geometry, fill condition behind the curb, curb termini, block-out depth, barrier offset from the curb, and barrier height relative to the roadway on performance of the guardrail. The new surrogate measures were evaluated against established crash-based measures and spanned a range of site conditions, crash types, and crash severity. The guidelines describe various types of surrogate measures and offer a framework for organizing the surrogates; outline options for collecting and measuring these surrogates; identify treatments that could benefit from surrogate-based evaluations; and provide case study examples.
Curbs are used to control drainage, separate pedestrian facilities, limit right-of-way, provide access control, and limit erosion. However, the need for curbs often competes with guardrail installation. Most research on curb and guardrail has used testing criteria from NCHRP Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features. The American Association of State Highway and Transportation Officials (AASHTO) and Federal Highway Administration’s MASH agreement requires the use of hardware evaluated with MASH testing conditions. Some crash tests have involved curb placed near guardrail. Many of these tests have failed due to guardrail rupture, barrier override, or vehicle instability. Traditionally crash testing has focused on the performance of pickup trucks striking guardrail with a nearby curb. Some recent crash testing has indicated that research is needed on small cars hitting guardrail near a curb due to the difference of vertical loading upon impact compared with that of a pickup truck. Crash testing guardrail installed near a curb with pickup trucks has shown the limitations in using only computer modeling to set guardrail placement recommendations. More recent crash testing results, involving use of a different guardrail, and changes in crash test criteria call into question how well guardrail performs near a curb. Research was thus needed on the performance of the guardrail system most commonly used, the 31-in. MGS.
Under NCHRP Project 22-39, “Guardrail Performance at Various Offsets from Curb MASH TL-3 Applications,” the University of Nebraska-Lincoln was asked to develop installation guidelines for the placement of 31-in. tall W-beam guardrail in combination with roadside curbs to satisfy MASH TL-3. A 6-in. tall AASHTO Type B curb was selected as the critical curb, and the steel post version of the MGS with 8-in. deep timber blockouts was selected as the critical barrier configuration. Two vehicle trajectory tests were conducted with MASH vehicles (small car, pickup truck) traversing over the curb to evaluate trajectories behind the curb. Two full-scale crash tests using the same types of MASH vehicles were also conducted on the MGS offset behind a roadside curb according to MASH TL-3 criteria. Both crash tests resulted in vehicle containment and redirection and passed MASH TL-3 safety performance criteria. Simulations were also conducted to evaluate performance with reduced impact speed or angle. These simulations showed no negative effects to the performance of the MGS behind curb. Finally, simulations with 4-in tall curbs showed significant reductions in the vertical trajectories of the vehicles, the lower height curbs thus posing no additional risks to the performance of the MGS placed in combination with curbs.
