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The Illinois Department of
Transportation is developing a new Highway Standard to implement the
non-blocked MGS.  We are also updating design guidance in the Bureau of
Design and Environment Manual according to the Implementation Guidance section
of “SAFETY PERFORMANCE EVALUATION OF THE NONBLOCKED MIDWEST GUARDRAIL SYSTEM
(MGS)”, TRP-03-262-12.  In that guidance we do not find any limitations or
guidance on transitions to end treatments (anchors or crashworthy ends.) 
We do note a requirement for placing at least 25 feet of MGS with blockouts
between the asymmetrical transition from thrie beam to w-beam at an approach
guardrail transition and the non-blocked MGS.  From this, our
interpretation is that the non-blocked system does not introduce constraints for
connection to an MGS downstream anchor or to a proprietary crashworthy end
terminal.  However, review of manufacturer literature for various
proprietary crashworthy end terminals shows some variance and uncertainty.

From this, it appears that we
should consult with the various manufacturers regarding their guidance for
connection and/or transition to the non-blocked MGS.  Do you think this is
the appropriate course, and what are your comments and suggestions?

We cannot make recommendations regarding the omission of blockouts within proprietary terminal systems. Typically these systems have used blockouts in there as-tested configuration, so it is difficult to anticipate their performance without them in place and the performance may vary between different types of terminals. Thus, we would recommend keeping the as-tested blockout configuration.

As far as transitioning blockout depth from existing terminals to non-blocked guardrail, we would recommend the following approach. Begin transitioning blockout depth no sooner than 12.5’ from the end of the terminal system. At that point, you can transition the blockout depth from 8”or 12” to 0”.

With respect to trailing end terminals, we believe we need to be conservative as well. Previous MASH testing we conducted on the trailing end terminal impact 31.25’ from the end with the pickup truck and 9.375’ from the end with the small car. We believe that the presence of blockouts may affect the results of both of these tests in terms of vehicle stability, vehicle capture, vehicle snag, and/or occupant risk measures. As such, we would recommend not converting to the non-blocked system until 50’ from the final anchor post in the trailing end terminal.

Thanks