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I have another question about the MGS. See attached for the IL Tollway standards for gutter used adjacent to MGS. On our mainline high-speed sections, G-3 gutter is used where necessary to handle the pavement drainage and/or to prevent sideslope erosion. We currently offset the guardrail post 6" behind the back of gutter, which means that the distance from the flowline to the face of rail is 11.75". What are your thoughts on this configuration versus the 6" high curb with a 6" offset that was tested?

For your information, the post used to be at the back of gutter and we used a 6" blockout for the guardrail. When we switched to the MGS, we decided to keep the offset to the rail the same as it was and push the post back.

Dean Sicking, John Reid and I have reviewed the attached CAD details that pertain to the MGS with alternative curbs used within the Illinois Tollway. As you recall, the MGS was successfully crash tested with a 6-in. tall, AASHTO Type B curb. In this scenario, the MGS was installed with the rail face placed 6 in. behind the midpoint of the curb face, or 7 in. behind the curb toe. The rail height was 31 in. above the level roadway surface.

In the IL Tollway detail, the MGS rail face is positioned 11.25 in. behind the toe of the G-3 gutter. In addition, the top of the rail is positioned 32.5 in. above the roadway relative to the bottom of the curb or swale. The curb height is 5.25 in. tall, as measured between the curb toe and the back of the curb.

Although there are slight differences between the successfully crash-tested system and the IL Tollway detail, we believe that the noted system with MGS in combination with the G-3 curb would provide a crashworthy system. However, we do not have physical or scientific evidence to support this opinion and would like to conduct a brief analysis to investigate the alternative scenario. As such, we used LS-DYNA to evaluate and compare the two scenarios since we have experimental data to validate the 6" Type B curb cases.

Dr. John Reid has made a very brief comparison between the two noted curb geometries " the 6-in. tall AASHTO Type B curb and the Illinois Tollway's wedge-shaped curb. This initial investigation included both an examination of vehicle trajectories and motions with and without the guardrail in place behind the curb. From this study, the use of the wedge-shaped curb in combination with the MGS (located per your prior CAD details) does not appear to degrade barrier performance over that observed for the MGS with the 6" Type B curb. As such, MwRSF is not concerned with placing the MGS behind the wedge-shaped curb using the previously noted details.