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AASHTO is mostly silent on warrants. It gives some very
basic guidance in Chapter 13.  The description for TL-2 is specific about
“posted speeds are reduced”.  So VDOT currently restricts the posted speed
where we use TL-2 bridge barriers. We chose 40 mph, that may be overly
conservative so we are considering increasing the posted speed limit. 

The speed for TL-2 testing is 44.x mph.

If I compare 44 to the crash testing that is valid for “high
speed arterials” (AASHTO language) they are at 62mph.  High speed is
clearly above 62 these days (the max limit in Virginia 70, it was 80 in Montana
when I visited this year)

Is there somewhere where guidance is provided related to the
interpretation of what the high end of “reduced speed” is.  

Our speed limits on unposted roads is 55mph.  I am
looking for any published guidance about how to consider using TL-2 on a
routine basis and maybe allowing TL-2 for use on the unposted roads.

In addition to the guidance we would consider our current
warrant process which is based on the unpublished report from 1980’s or 1990’s,
not the unpublished report from 2014.

(http://www.virginiadot.org/business/resources/bridge/Manuals/Part2/Chapter25.pdf
page 17 and following.)

Is there guidance about extending the posted speed above the
crash testing speeds?

I am generally aware of three documents that have addressed warrants for bridge railings.

The first document is AASHTO’s Guide Specifications for Bridge Railing (1989).

The second document was the draft report for NCHRP Project 22-8. In that document, a figure was provided that guided conditions (speed and traffic volume) under which barrier placement was suggested for TL-2, TL-3, and TL-4. You can also obtain a copy of this figure in MwRSF report no. TRP-03-98-03.

Finally, there was a more recent NCHRP 22-12(3) study that led to guidelines for barrier placement on bridges. Roadsafe LLC completed this studied several years ago. I have been waiting to see if AASHTO would adopt these guidelines. However, I have not seen that occur as of yet. The study report is attached. In general, the authors denoted that test level selection is a function of percent trucks, encroachments, and level of risk acceptance.

Now I know that these three reports may not answer your question directly. In terms of speed, the 22-8 plot may actually give some help in seeing that Test Level guidance is not just correlated to impact speed in crash tests. Traffic volumes affect level of risk as well. We could have a high-speed rural road that carries very little traffic volume. In this case, one may argue that the risk is low enough that a TL-2 barrier, or even lower is appropriate, even if highway speeds are 60 mph or higher.

Further, we know that impact speeds with hardware are lower than encroachment speeds (i.e., when an errant vehicle first leaves the traveled way). Even with highway speeds, including excessive driving, we design our hardware for a slightly lower impact speed based on real-world crash data, which was used to select TL-3 impact speeds in MASH.

In your case, unposted rural roads, often with lower traffic volumes, are known to be 55 mph and would easily be considered TL-2 or less.