Two double-decker buses collided in Times Square on Tuesday, with one hurtling into a crowded plaza, injuring at least 15 people and sending scores more scurrying to safety, the authorities said. The crash, which took place just before 3:30 p.m. at the corner of 47th Street and Seventh Avenue, sent at least one traffic light tumbling to the ground and debris flying onto a sidewalk crowded with tourists and shoppers. “We are very, very fortunate that there were not more injured,” Deputy Fire Chief Roger Sakowich said.