It's an ornamental pear; I think Pyrus calleryana. There are lots of pears out now. Small single white flowers, often red or other colourful anthers on the stamens. Douglas Justice says their terrible smell is an identifying feature. I've never noticed that myself.
Thanks. I didn't detect any smell but these trees seem to be an ideal host for lichen and moss. They could be found on trees lining both sides of the street for four blocks.
