The weak economy has forced developers to stop construction on several unfinished buildings in New York. Last week, for example, Boston Properties suspended construction on an office tower just south of Columbus Circle after a future tenant canceled plans to lease space. Meanwhile, experts say the stalled projects could hurt real estate values. "What it shows in physical detail is weakness in the current environment," said Jonathan Miller, a real estate consultant. "It adds to the idea that there's not a sense of urgency."