GREENSBORO, N.C. — In one sense, the Proximity Hotel in Greensboro shouts its environmental bona fides from the rooftop. One of the first sights that greet guests as they turn in to an otherwise nondescript office park off Green Valley Road are the 100 solar panels perched atop the handsome hotel, which from afar looks like an old textile warehouse lovingly brought back to life. Visitors with low-emitting,fuel-efficient vehicles can pull into a preferred parking spot closer to the front entrance, where a U.S.