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A representative from the IL Tollway called requesting guidance on the use of a trailing end terminal to protect the upstream end of the rigid concrete parapet.

Recently, you inquired about the implementation of the MGS with trailing end terminal for use in protecting the upstream end of a rigid concrete parapet. For your special situation, the concrete parapet was positioned approximately 4 ft behind the back side of the steel line posts. It should also be noted that you were referencing the ILDOT Standard 631011-06, Traffic Barrier Terminal, Type 2.

It was stated that there are special situations where the parapet is farther away from the traveled way than desired for the guardrail offset. In addition, there may be other circumstances that do not allow for the guardrail to be flared back toward the upstream end of the parapet and anchored to it. Therefore, preliminary guidance was requested for safely positioning the downstream region of the MGS and trailing end terminal to longitudinally overlap the upstream end of the parapet so that the MGS would shield blunt end impacts on the parapet end. Based on engineering judgment and in the absence of crash test results, we believe that a reasonable positioning would be to align post no. 5 with the upstream end of the parapet. This configuration would place approximately 22 ft of guardrail past the upstream end of the parapet.

It should be noted that future research should be directed toward determining the length of need where downstream guardrail systems with trailing end terminals are effective in safely containing and redirecting high-energy impacts with pickup trucks as well as small cars under the MASH safety guidelines.

Post #12 (the middle post in the asymmetrical transition piece) was omitted to avoid conflict with a drainage structure. See attachments. Is this allowed or what recommendation would you have for this detail?