View Q&A

I have a quick question for you.  At what point does the superelevation on the outside of a curve become a problem for MGS placement?  We have the following situation:

9’ lane

4’ turf shoulder

Where the 5.5% super is present the turf slope would match then we’d have a 13.5% grade break at 1.5’ in front of the barrier. – is there a minimum offset the rail would need to be located behind that break?

Or, have them provide a 2.5% slope in front of the barrier to limit the grade break to 7%.

What are your thoughts?

There is a potential concern with the superelevation becoming an issue due to the negative slope effectively lowering the height of the guardrail relative to the vehicle traversing the shoulder.

The best available guidance or related information was a study done by GMU regarding barriers on super-elevated curves – see attached. They did some trajectory analysis and found that there were potential capture problems for guardrail at certain guardrail offsets and shoulder slopes. However, I don’t know that this analysis was ever fully proven out.

Optimally, we would want to limit the offset of the barrier down the negative slope if possible to minimize the variation in the vehicle trajectory relative to the barrier. Making the grade break more forgiving should help as well. However, the best information we currently have is in the reports above.

Let me know if you have further questions.

Thanks!