I think barrier=crash_attenuator would be the best. Then one could maybe define crash_attenuator:type=* or maybe crash_attenuator=* to specify the specific kind as fitch_barrier or water_barrel or whatever.
Hm, actually, maybe highway=crash_attenuator would be more accurate 
Borrowed this from oogle (strictly for topic illustration) at a Rome-Ancona junction.
The one I had actually in mind is similar guardrail harmonica construct with a big green sign instead with white left-righ arrows from ground to above the guardrail where the way splits instead of the painted yellow in the picture. Did not find it in the various road sign wiki’s that I know.
edit: Found this in Bing street side, after observing it in Bing aerals today.
it has semblance to the green sign I have in mind… maybe the German autobahn. This one is a yellow cover over whatever is underneath… water filled barrel maybe(???). I like the guardrail construct better, looking to be able to absorb a lot more kinetic energy.