What is the correct Guardrail height?

Highway guard rail is one of the most well researched transportation devices. The height to which it’s installed, measured vertically from the pavement edge, is an important consideration. I’ve had to look this number up many times, and did so again to prompt me to write this article. So what is the correct height?

Following research by NCHRP, AASHTO is now recommending guardrail be installed at 31 inches above the pavement. This measurement is from the pavement edge to the top of the guardrail.

This has recently been changed. The 2006 version of the AASHTO Roadside Design Guide contained a value of 27.75 inches. Crash testing research showed that it did not meet NCHRP Report 350 Test Level 3 criteria. Thus, FHWA and AASHTO have revised the recommended guardrail height to 31 inches in the 2011 edition of the manual.

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