BACKGROUND

Longitudinal barriers are roadside safety devices commonly used to prevent errant vehicles from impacting hazards located alongside the traveled way. These barriers have been successfully designed in many different variations ranging from rigid concrete parapets to semi-rigid beam guardrail to relatively flexible wire rope barriers. Historically, the development and testing of longitudinal barriers has been done under the assumption that the barriers are installed in relatively straight sections parallel to the roadway. While the barriers designed under this assumption have performed well, their behavior when installed on curved roadways is largely unknown. However, despite this lack of understanding of their safety performance on curved roadways, longitudinal barriers are commonly installed on superelevated curves to protect errant motorists.

NCHRP Project 22-29, "Performance of Longitudinal Barriers on Curved, Superelevated Roadway Sections," and NCHRP Project 22-29A, "Evaluating the Performance of Longitudinal Barriers on Curved, Superelevated Roadway Sections," demonstrated that the safety performance of a traffic barrier decreases as its curvature increases. The barriers with the sharpest curvature are found on off-ramps. Although spectacular accidents on off-ramps are occasionally reported in the media, there has been no study of barriers on off-ramps since the early 1980s. In that pioneering study, a series of full-scale tests was conducted of Jersey-shape barriers on a sharply curved off-ramp that also had superelevation and downgrade. While the test vehicles were all smoothly redirected in 40 mph impacts, the results indicated that the barriers were near their performance limits. Today, vehicles often travel at speeds substantially greater than 40 mph on off-ramps, and clear guidance is needed on selecting and designing traffic barriers for curved off-ramps.

Based on these considerations, research is needed to quantify the performance of longitudinal barriers on curved, superelevated off-ramps and provide guidelines for their safe application.

OBJECTIVE

The objective of this research was to provide proposed guidance to the AASHTO Technical Committee on Roadside Safety on selecting and designing traffic barriers for curved, superelevated off-ramps for possible incorporation in the AASHTO Roadside Design Guide.

STATUS

Completed. Publication decision pending.