Problem Statement No. 2014 LA-35/TN-66
A general problem occurs at many bridge locations along highways where the required length-of-need for bridge approach rails cannot be met within the existing right-of-way (ROW) limits. These conflicts occur when existing driveways, roads, or other objects are within the ROW. It is not unusual to have less than a 15-ft length between the end of the bridge and the conflict. Solutions to this problem have included using short radius guardrail, a shortened guardrail section, or a crash attenuator. Typically, these solutions are not practical for the site location or are not cost effective.
The scope of this research study is to investigate extreme site constraints at bridge ends encountered by State Departments of Transportation (DOTs). A categorization methodology will be provided for determining proper impact conditions and evaluation criteria for future design concepts. The researchers will supply impact conditions and evaluation criteria for future hardware designs for roadside safety application to be applied at extreme sites at bridge ends.
Typically, a rigid longitudinal barrier is used to contain errant traffic at a highway bridge location. These rigid longitudinal barriers present an obstacle at their terminations for oncoming traffic. There are several methods designers use to alleviate these obstacles. Often a guardrail terminal system is used as an approach rail to the bridge location; however, a general problem occurs at many bridge locations along highways where the required length-of-need (LON) for the bridge approach rail cannot be met. The length of need is defined as the length needed for a traffic barrier typically used to protect and shield fixed features or hazards. A typical equation used to determine the length-of-need is the following (1):
where: LH represents lateral extent of hazard, LR represents the runout length, L1 represents the length of tangent section of rail advance of hazard, L2 represents the distance from edge of pavement to tangent section of guardrail, b/a represents the flare rate of guard rail. Alternate solutions to these obstacles include using short radius guardrail, a shortened guardrail section, or a crash attenuator. Historically, short radius guardrails have been used at most locations as crash attenuators might not always represent a feasible or economical solution.
Crash cushions or impact attenuators are devices used to shield and protect fixed features. They are typically employed in areas where use of a long barrier installation is not feasible. When impacted by the errant vehicle, crash cushions absorb the impacting energy by deformation and decelerate the vehicle, leading it eventually to a stop, or redirecting the vehicle.
There are two main types of classifications for crash cushions: temporary and permanent. Temporary crash cushions are generally employed in work zone areas. Crash cushions can also be classified as redirective or non-redirective, gating or non-gating, and self-recoverable or non-self-recoverable. Redirective crash cushions absorb the kinetic energy of the impacting vehicle and deflect the vehicle back in the opposite direction. On the contrary, non-redirective crash cushions do not have this ability. Instead, non-redirective crash cushions allow the vehicles to penetrate the system while at the same time reducing the vehicle’s speed. Gating crash cushions allow the vehicle to penetrate the crash cushion for part of the length. In contrast, non-gating crash cushions do not allow penetration and have the capability to redirect an errant vehicle. Self-recoverable crash cushions are able to restore themselves with little or no maintenance after an impact. Crash cushions are selected based on these classifications as well as their reusability.
Several studies and tests have been conducted by Southwest Research Institute (SwRI), Midwest Roadside Safety Facility (MwRSF), and Texas A&M Transportation Institute (TTI) on various short radius guardrail systems (2). These were evaluated under multiple performance criteria including Americna Association of State Highway and Transportation Officials (AASHTO) 1989 Guide Specification for Bridge Railings, National Cooperative Highway Research Program (NCHRP) Report 230, and NCHRP Report 350 (3,4,5). Currently, these systems are limited to Test Level 2 (TL-2) under NCHRP Report 350 performance criteria.
Silvestri et al. conducted a research study to identify the best practices used to alleviate problems where length-of-need requirements for bridge approach rails cannot be met (6). The guide document was developed through a literature review and survey of State DOTs. The survey addressed data concerning: practices or standards for bridge barriers when LON cannot be met, practice variation according to design speed, different types of crash cushions used, and installation of a short radius guardrail in front of a slope. From the information collected, it appears that use of short radius guardrail practice at bridge locations where LON cannot be met is generally the option preferred by the DOTs. Although few States indicated that their DOTs make somewhat frequent use of crash cushions at bridge locations where LON cannot be met, their employment is very limited by other States due to their higher installation and maintenance costs. Also, use of crash cushions might become not practical and undesirable on road sections with multiple drives and side roads, considering their size. Some State DOTs prefer to relocate the obstacle/drive access to a point beyond the proposed length of need. When that is not feasible, DOTs have different preferences on how to shield the obstacle, which includes use of short radius guardrail, crash cushions, but also Wood Post Controlled Release Terminal (Alaska DOT), T-Intersection or adjustment of the LON equation (Louisiana DOT), and nested thrie beam transition from concrete bridge rail end block, then attachment of short radius rail as necessary (South Dakota).
Abu-Odeh is currently working on a research study effort funded by the Texas Department of Transportation which aims at developing a MASH TL-3 compliant short radius guardrail (7). This system is being designed to address sites at which the intersecting roadway/driveway is greater than xx feet from the bridge end and has a ROW distance of xx ft or greater. If successful, this design will provide a much needed solution for a very common safety problem. However, some sites may have space constraints that are too severe for the proposed design or possibly any MASH compliant design.
The scope of this research study is to investigate extreme site constraints at bridge ends encountered by State DOTs. A categorization methodology will be provided for determining proper impact conditions and evaluation criteria for future design concepts. The researchers will supply impact conditions and evaluation criteria for future hardware designs for roadside safety application to be applied at extreme sites at bridge ends. This project does not aim to develop a hardware solution for these site conditions. This study aims to develop criteria for the development of hardware solutions.
The information compiled from this research will supply impact conditions and evaluation criteria for future hardware designs for roadside safety application to be applied at extreme sites at bridge ends. The research will provide background information that could be used for justification for site specific design exceptions.
TTI will generate a report providing results of a literature review and a survey of typical site constraints at bridge ends encountered by State DOTs. In addition, the report will include a categorization methodology for determining proper impact conditions and evaluation criteria for future design concepts.
A chart to categorize extreme site conditions at bridge ends will be suggested for use by the Departments of Transportation. Results of this research could be used in the future to develop new barrier for use with these extreme sites conditions.
The researchers will perform a literature review and will develop a survey to address State DOT personnel with the scope of investigating typical site constraints at bridge ends encountered by State DOTs.
The researchers will evaluate the material collected from Task 1 and will determine site constraints encountered at bridge ends. Next, the researchers will sub-divide the site constraints into categories depending on geometric site constraints. The applicable MASH impact condition matrix will be determined for each category. MASH evaluation criteria will be investigated and adapted, if necessary, to category constraints. When adaptation is required, other evaluation criteria, such as the Federal Motor Vehicle Safety Standards (FMVSS), will be considered. A chart will be developed to characterize impact conditions and evaluation criteria deriving from these extreme sites at bridge ends.
The researchers will generate a report providing project results. The report will include a chart categorization for determining proper impact conditions and evaluation criteria for future design concepts. In addition, the researchers will supply impact conditions and evaluation criteria for future hardware designs for roadside safety application to be applied at extreme sites at bridge ends.
|TTI Research Supervisor:
Dusty R. Arrington
Associate Transportation Researcher
Texas Transportation Institute
Texas A&M University System
College Station, Texas 77843-3135
|Pooled Fund Technical Representative:
Chris Guidry, P.E.
Assistant Bridge Design Administrator
Louisiana Department of Transportation Bridge Design
P.O. Box 94245
1201 Capitol Access Road
Baton Rouge, LA 70802