State Departments of Transportation (DOTs) commonly deploy concrete median barriers as both permanent and temporary installations to separate opposing traffic flows. The crashworthiness of these barriers is typically validated through full-scale crash testing in accordance with current roadside safety standards. In certain cases—such as navigating around fixed roadside features like bridge piers—DOTs may need to flare the barrier’s length of need (LON) to maintain protection while adapting to site constraints.

However, flare geometry can significantly influence crash dynamics. As the flare rate steepens, the angle at which vehicles may impact the barrier increases. For rigid and semi-rigid systems, this can result in more severe impacts and greater risk of vehicle redirection into adjacent lanes or opposing traffic—an especially critical concern on two-way roadways or in constrained crosssection environments.

To mitigate these risks, the AASHTO Roadside Design Guide (RDG) provides recommended maximum flare rates based on design speed and barrier type (Table 5-9) (1). These values, however, were developed based primarily on passenger vehicle performance and do not reflect the behavior of heavier vehicles such as Single Unit Trucks (SUTs), nor do they fully address the challenges posed by lower-speed, space-limited environments where many concrete barriers are installed.

Texas A&M Transportation Institute (TTI) conducted research evaluating the performance of a flared, cast-in-place concrete barrier system under the American Association of State Highway and Transportation Officials (AASHTO) Manual for Assessing Safety Hardware (MASH) Test Level 4 (TL-4) conditions using the SUT vehicle (2, 3). The system consisted of a 75-ft long, 40- inch tall reinforced concrete single-slope barrier. The intermediate segment flared outward by 18 inches, with upstream and downstream transitions implemented at flare rates of 20:1 and 30:1, respectively. The study found that SUT stability is strongly affected by barrier height and flare rate—factors not currently captured in the RDG flare rate guidelines.

Moreover, many concrete barrier installations occur on lower-speed roadways with constrained environments, where fitting a 20:1 flare may not be feasible. Therefore, there is a need to establish practical flare rate guidelines for such scenarios for cast-in-place concrete barriers on lower-speed roadways.

Objective

The objective of this research is to determine acceptable flare rates for cast in place concrete barriers at different low impact speeds that satisfy MASH evaluation criteria using finite element (FE) computer simulations. This project aims to provide recommended flare rates for cast in place concrete barriers as guidelines in a similar format as Table 5-9 in the RDG. The guidelines will specify the recommended flare rate for rigid concrete barriers based on passenger cars performance, along with the corresponding minimum MASH allowable barrier height for SUT impacts.

Benefits

The project will develop recommendations based on research for the flare rate of cast in place barriers for lower speed roads. These recommendations will make it easier to install rigid concrete barriers at lower speed roads.

Products

The TTI research team will provide a final report that will include the design recommendations for flare rates of the cast in place concrete barriers at 30 to 50 mph impact speeds.

Work Plan

The work plan for accomplishing the objective of this project consists of four tasks described below.

 

Task 1 – Literature Review

The research team will review previous and ongoing research projects to determine flared cast in place concrete barrier systems that have met MASH evaluation criteria. The research team will use precious crash test reports to develop FE models and conduct system validations in Task 2. The results of the literature review will also be used to inform and guide the research team for the parametric studies that will be conducted in Task 3.

 

Task 2 – FE Modeling and Validation In this task, the research team will use engineering drawings of previously crash tested cast in place barriers to develop FE models. The research team will validate the models against the crash test reports using computer simulations. Validation will consider vehicle dynamics during and after impact, and occupant risk metrics. The research team will develop rigid single slope concrete barrier models at different heights as needed. 

 

Task 3 – Parametric Computer Simulations of Flared Rigid Concrete Barriers

In this task, the research team will conduct a series of finite element (FE) simulations to evaluate the performance of flared rigid concrete barriers under various impact conditions. The simulations will consider impact speeds of 30, 40, 45, and 50 mph, using the MASH 1100C (small car), 2270P (pickup truck), and 10000S (SUT) FE vehicle models. The study will focus on the single slope concrete barrier (SSCB) with heights ranging from 32 inches to 56 inches. The objective is to determine the allowable flare rates that meet MASH crashworthiness criteria for a given impact speed and barrier height. The proposed methodology consists of the following steps:

1. Initial Flare Rate Assessment: The research team will first determine the maximum allowable flare rate using the passenger vehicle models (1100C and 2270P) at each selected impact speed, assuming a barrier height of 56 inches

2. Minimum Barrier Height Evaluation for SUT: Using the flare rates established in Step 1, the research team will then identify the minimum barrier height required to maintain acceptable performance under SUT impact conditions at the same speeds.
Given that the single slope barrier profile generally results in more severe occupant risk metrics compared to the F-shape barrier, it is anticipated that the F-shape barrier will also meet MASH evaluation criteria under the same impact conditions for passenger cars. However, the research team will also perform a set of simulations with the F-shape profile using only the final recommended flare rates to verify their compliance.

 

Task 4 – Guideline Development and Final Report The recommended flare rates for cast-in-place concrete barriers will be presented in the form of design guidelines, formatted similarly to Table 5-9 in the AASHTO Roadside Design Guide. For each selected impact speed (30, 40, 45, and 50 mph), the guidelines will specify the recommended flare rate for rigid concrete barriers based on passenger cars performance, along with the corresponding minimum allowable barrier height for SUT impacts. All components of the research will be thoroughly documented in a final report. This report will detail the literature review, FE modeling, model validation, simulation studies, and the methodology used to develop the flare rate guidelines for cast-in-place concrete barriers at low impact speed conditions. A draft version of the final report will be submitted to the project’s technical representatives for review. Feedback received during this review will be incorporated into the revised final report. 

 

Time Schedule

Started: August 2025 Time frame: 18 Months

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September 11, 2025