I've never seen this before, but after a little research it seems many places are doing this for LEED credits, aka Leadership in Energy and Environmental Design.
I won't pretend to understand the motivation behind it, at least until I can get more information. Still, I would like to know if these signs are popping up in your neighborhood.
I will say that I found a pretty interesting quote from one person , who simply left it as "A symbolic measure, and largely unenforceable."