State DOTs routinely encounter variations in approach guardrail transition (AGT) installations that are different than what was tested. This can include differences in rail height, post installation, and soil grading to name a few. There is no guidance for state DOTs to assess whether these variations are still crashworthy. Further, there is no guidance for how these variations may perform on lower speed roadways. Kiani (1) previously explored variations in AGTs for MASH TL-3 compliance. The research effort focused on situations where a post could not be installed (i.e., a missing post in the transition region), or improper grading (i.e., lower embedment depths) were encountered. The computer simulation analysis focused on a MASH TL-3 compliant thrie beam system with a curb. The simulation analyses generally found that the two field variations had a significant effect on the MASH crashworthiness in terms of vehicle instability and accelerations. Additional research was recommended to investigate these variations further and to investigate other AGT variations. There is a need to continue investigation of other modifications to AGTs that can occur during design/construction. The information compiled from this research will provide the FHWA and State Departments of Transportation with an increased understanding of the performance of AGTs with variations other than what was tested.

 

The project objective is to evaluate installation variations that could influence AGT performance. For example, DOTs are seeking guidance on what rail height could cause problems for a TL-3 AGT system. This project will follow-up the work performed by Kiani (1) which explored variations in a MASH TL-3 AGT. Specifically, this project aims to develop a proposed risk matrix to determine at what speeds different modifications could be a concern. For example, at what speed would a lower rail height of a TL-3 system not likely be a concern for a 25-mph road. This risk matrix would also be applied to the previous work by Kiani (1) on missing posts or reduced post embedment.

Benefits

Many existing approach guardrail transition installations have been installed with modifications. Computer modeling would allow states to better evaluate existing hardware and determine when certain modifications are a concern and what roadways these modifications could be acceptable. In addition, this research report can assist states with questions that may arise during guardrail transition design and construction when modifications to the AGT are proposed.

Products

The TTI research team will provide a final report that will include the results of the computer simulation analyses and guidance for the variations in AGTs.

Work Plan

The work plan for this research includes the following tasks.

Task 1: Literature Review and State DOT Survey
The TTI research team will review the previous study by Kiani (1) and any other relevant literature focused on MASH evaluation of variations in AGT systems. The research team will distribute a survey to state members of the Roadside Safety Pooled Fund to collect and prioritize information on variations to AGTs that are encountered by the states. The research team will perform an initial brainstorming to identify some of these variations prior to sending out the survey. Also, the research team will utilize some of the survey information collected by Kiani (1). Once the information is collected, the research team will select the variations to be analyzed in Task 2. It is anticipated that four different variations will be considered for further evaluation in Task 2. However, the final number will depend on the type of variation and the level of effort required to model and evaluate it.

Task 2: Finite Element Analysis of AGT Variations
The research team will build computer models of the AGT variations that are selected as part of Task 1. The previous modeling effort by Kiani (1) will be used as a base point for some of the models. For each variation, the research team will evaluate the system according to MASH TL-3. This will consist of impacting the system with the 1100C FE model and 2270P FE model. The impact speed will be 62 mi/h and the impact angle will be 25 degrees. The research team will assess each computer simulation according to the MASH TL-3 evaluation criteria. The research team will consider different impact locations, if necessary, depending on the variation. If the system does not the MASH TL-3 evaluation criteria, then the research team will perform an evaluation for MASH TL-2. This will consist of performing the same computer simulations with an impact speed of 44 mi/h. The research team may consider other impact conditions depending on the initial performance and type of variation. The research team will discuss these alternative impact conditions with the tech rep prior to any further evaluations. The research team will develop Guidelines for AGT variations based on the computer simulation results. The guidelines will include the AGT variations investigated in this study and the AGT variations investigated by Kiani (1). It is anticipated that part of the guidelines will be a risk matrix for the investigated AGT variations.

Task 3: Final Report
The research team will generate a final report of the findings from the research. The results will include the literature review and state survey results, the finite element analyses, and computer simulation results. The research team will include guidelines and recommendations in the final report for the AGT variations that were evaluated. It is anticipated that the results will be output in a form of risk matrix for each AGT variation investigated.

Time Schedule

Started: March 2025
Time frame: 18 months

---

March 20, 2025