I have never had problems hearing HD subchannels in NYC where the regular FM signal is available. You only lose the HD signal when you go through a tunnel that also blocks the main signal.Since I no longer live in the market, can someone describe the quality of HD2-3-4 signal reception in "the City"? I would think the tall buildings play havoc with the lower bit-rate signals.
I find you have to be about 25 miles or more from Midtown to begin losing the HD signals. You begin to notice it when you are in a valley or go down a steep hill. The HD stations are audible, for instance, while traveling along I-287, the interstate highway that rings NYC through most of New Jersey, plus Rockland and Westchester Counties in New York State.