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. For example, a steep slope needs curb to control drainage and erosion. This steep slope may need shielding. If a pedestrian facility is near curb, the guardrail will have to be further offset from the face of curb. Typical solutions for curb placed near a guardrail, like using a sloped curb or reducing curb height, can be difficult to use and may cause problems for sight-impaired pedestrians who may unexpectedly enter the roadway. Shorter curbs may not have the hydraulic capacity needed. In addition, fill condition behind the curb, curb transition, and termination adjacent to guardrail may cause vehicle instability at impact. Using curbs to solve one problem may create an entirely new problem.
Most research on curb and guardrail used the NCHRP Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features testing criteria. The American Association of State Highway and Transportation Officials (AASHTO) and Federal Highway Administration’s (FHWA) joint Manual for Assessing Safety Hardware (MASH) agreement requires the use of MASH evaluated hardware.
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 additional research is needed on small cars hitting a guardrail near a curb due to the difference of vertical loading upon impact versus that of a pickup truck. Crash testing guardrail installed near a curb with pickup trucks has shown the limitations in using just computer modeling to set guardrail placement recommendations. More recent crash testing results, that involve the use of a different guardrail, and change in crash test criteria call into question how well guardrail performs near a curb. Further research is needed and should be conducting using the most commonly used generic guardrail system, the 31-in. Midwest Guardrail System (MGS).
The objective of this research is to develop guidelines for the use and placement of the 31-in. MGS adjacent to curbs under MASH TL-3 impact conditions. The guidelines are to consider the effect 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 the performance of the guardrail. It is anticipated that a combination of previous research, computer simulation modeling, and full-scale crash testing will be required to develop and verify the guidance to demonstrate MASH compliance.
STATUS: Research is in progress.