I came across this beautiful Willow Oak (Quercos Phellos) on Hudson Street, and was struck by the beauty of the bare roots (pictured below). Proof that given a large enough tree pit, enough water, not too much vandalism from pedestrians, dogs, or parked cars, and a bit of luck, street trees can mature to a ripe old age. Older people have longer ears, and older trees have larger roots. Large roots can take over a tree pit to create a beautiful, sculptural element on the sidewalk that has interest in and of itself.
The large roots of the Willow Oak also support a large canopy (pictured below in winter). The tree canopy provides shade, cleans the air, cools the climate, and provides beauty and interest year round. Larger trees may not have room in the tree pit for additional plantings, but large specimens are so majestic they don't need any adornment to make a beautiful statement on the street.