Evaluating the Performance of Longitudinal Barriers on Curved, Superelevated Off-Ramps (2025)
Chapter: 1 Introduction
CHAPTER 1
Introduction
1.1 Background
The safety performance (i.e., crashworthiness) of longitudinal barriers has traditionally been evaluated under idealized impact conditions (i.e., a straight linear section of the barrier is installed on level terrain and the impacting vehicle is freewheeling with minimum roll and pitch effects). This protocol has evolved to provide a “practical worst-case” impact condition that is reproducible and comparable. In reality, barriers get installed on tangent and curved sections of mainline highways and can be impacted under various conditions. Commonly, similar barriers are installed for safety continuity from mainline highway sections into tighter, curved, superelevated roadway sections for on- and off-ramps. These barriers may have varying radii and superelevation to safely accommodate transitioning traffic. In some situations (e.g., urban areas), tighter radii are more often needed for on- and off-ramps where land space is limited. Vehicles traversing these curved situations are often traveling at speeds higher than posted, raising concerns about leaving the roadway. Thus, similar barriers are continued through these sections. Because the possibility of leaving the travel lane can be increased by higher speeds for the curve, whereby the vehicle traverses the lateral superelevation and shoulder slope before impacting the barrier, the nature of the impact can influence the adequacy of the barrier.
To analyze the safety effectiveness of barriers used on curved, superelevated roadways, NCHRP has undertaken multiple research efforts:
- NCHRP Project 22-29, “Performance of Longitudinal Barriers on Curved, Superelevated Roadway Sections”
- NCHRP Project 22-29A, “Evaluating the Performance of Longitudinal Barriers on Curved, Superelevated Roadway Sections”
- NCHRP Project 22-29B, “Evaluating the Performance of Longitudinal Barriers on Curved, Superelevated Off-Ramps”
These efforts developed a better understanding of the safety performance (i.e., crashworthiness) of barriers used on curved, superelevated roadway sections and recommended options and guidelines for improving barrier selection, design, and deployment in pursuit of enhanced highway safety. The need for these research efforts was predicated by the limits of safety performance evaluation of longitudinal barriers, which, under current and past crashworthiness evaluation criteria, was focused on idealized impact conditions. It was recognized that the barrier can be impacted in a variety of ways, but little effort had been made to understand the nature of impacts under such conditions and to adapt barrier deployments for addressing the variations in impacts on curved, superelevated roadway sections.
Recent research has found that departments of transportation (DOTs) have individually developed practices for the installation of barriers for curved, superelevated roadway sections to offset
limited guidelines in both the AASHTO Policy on Geometric Design (the Green Book) (1) and Roadside Design Guide (2). NCHRP Project 22-29 research was undertaken to advance the understanding of barrier performance using state-of-the-art analysis approaches and translate the findings into enhanced guidelines for the design, selection, and installation of concrete and steel W-beam longitudinal barriers installed on curved sections of roadways. Curved roadway sections are generally constructed with superelevation to compensate for the centrifugal forces exerted on vehicles and to make it easier for the driver to control the vehicle through the curved section. The analyses provided a means to analyze the combined effects of curvature, superelevation, and shoulder slope on vehicle dynamics for varying vehicle trajectories, orientations, and speeds.
NCHRP Project 22-29B research found that dynamic effects can significantly affect the interface between the vehicle and the barrier during a crash as the vehicle leaves the road. On curved sections, the vehicle is more likely to leave the road at a higher angle and consequently impact the barrier with higher impact severity. The higher impact severity can lead to increased forces on the occupants (and hence occupant risk metrics), more intrusion into the occupant compartment, and ruptured barriers or unusual interactions between contacting components. In addition, a higher impact angle can increase vehicle instability and may lead to vehicle rollover, override, or penetration behind the barrier. The higher impact angle increases the tendency for vehicles to climb rigid barriers and for tire and post snagging to occur for semi-rigid strong-post barriers. Furthermore, the road superelevation will cause the vehicle to approach the barrier at a different orientation (roll and pitch) or height relative to the barrier than would be the case on a flat surface. This is particularly critical when a shoulder has a negative slope relative to the roadway superelevation.
NCHRP Project 22-29 and NCHRP Project 22-29A research could not fully address barrier performance for all curved, superelevated section conditions, so it addressed a range of the most common conditions. The resulting insights provided the basis for proposed new approaches and criteria for barrier design and placement on curved, superelevated off-ramps. This additional research effort, NCHRP Project 22-29B, was initiated to continue the analyses for “tighter” curves typical on highway ramps, where opportunities to achieve the desired design and placement options are typically more limited. This research was initiated with such roadways being called curved, superelevated, off-ramps (CSORs). Examples of longitudinal barrier installations on long- and short-radius CSORs with concrete and steel longitudinal barriers are shown in Figure 1.
1.2 Objective
The objective of this research was to gain additional insights and establish guidelines relative to the crash performance of longitudinal barriers placed on varying curved, superelevated ramp sections. The goal was to apply the analysis approach and models from the completed research in a focus on ramp situations to develop appropriate recommendations for the selection, design, and installation of barriers for such ramp situations. These efforts employed the successful approach used in the previous research to consider the broad set of variables (e.g., road curvature; superelevation; barrier design and type; vehicle size; impact angle and speed; shoulder slope and width; barrier location, height, and orientation) relative to the effectiveness of longitudinal barriers on curved, superelevated highway ramps. The research began with reviews of current practices and designs for barriers on highway ramps. The efficacy of the typical designs for barriers on ramps was critically evaluated using vehicle dynamics analyses, finite element crash simulations, and full-scale crash testing. These efforts are believed to have resulted in new insights on the relative effectiveness of various barrier design, selection, and deployment practices for applications of longitudinal barriers on curved, superelevated off-ramps.
1.3 Research Approach
This research effort followed the successful approach used in NCHRP Research Projects 22-29 and 22-29A. In these efforts, vehicle dynamics analyses and crash simulations were applied to determine the effects of typical vehicles leaving the roadway and impacting a roadside barrier. These analytical tools have been demonstrated to be effective for evaluating vehicle-to-barrier interfaces and crash impact outcomes for a broad range of conditions (e.g., speed, impact angle, surface profile, vehicle type, barrier designs, and road features). This depth of analysis had been shown to provide micro-level indications of barrier effectiveness that would allow agencies to generate recommendations for design guidelines.
Vehicle dynamics analyses and crash simulations were applied to analyze a set of short-radii curved, superelevated ramp sections and a range of vehicle types and impact features. This approach provided a wealth of data for prospective impacts at varying angles over a broad array of typical CSOR design conditions. The research used vehicle dynamics analyses to provide broad insights into variations in barrier effectiveness, supported by deeper analyses using finite element simulation to provide an understanding of impact physics. This was followed by crash testing to validate the findings.
1.4 Organization of Final Report
This final report is intended to provide a synopsis of relevant information and knowledge acquired or developed in the research efforts and to translate the findings into new insights and guidelines for effectively addressing specific needs for barriers used on all varieties of curved, superelevated highway ramps. The report incorporates the methodologies and findings derived from previous efforts under NCHRP Projects 22-29 and 22-29A. The report includes the following:
This document is intended to (1) provide a concise overview of the relevant findings from related research efforts; (2) document the background studies and analyses completed focusing on issues and considerations for “tight” CSORs; (3) present the conclusions drawn from the various aspects of this research effort; and (4) offer recommendations for improving the design, selection, and deployment of longitudinal barriers on curved, superelevated ramps. The findings provide useful insights into the design, selection, and deployment of common barriers used on curved, superelevated ramps. The findings are also presented in a manner that readily allows agencies to assess their current design guides or inspect specific barrier installations in the field and determine potential effectiveness. The report documents the methods and depth of the research undertaken.
