Which side of the street would you rather walk on?
The first two photos were taken across the street from each other, on Arch St. between 21st and 22nd. These trees are all the same species - Callery pear (Pyrus calleryana) - and they all appear to be around the same age. But as you can see, the trees in the photo on the right (south side of the street) look much nicer.
Why? Because the trees on the north side of the street were pruned using a technique called “topping,” which amounts to chopping off branches wherever the person doing the chopping damn well pleases. Topping not only results in fugly trees, but prevents trees from healing and growing properly, which can lead to both disease and dangerously unsupported branches and limbs.
PHS has a wonderful five-minute video on how to properly prune trees. I’ll find some examples of good pruning to share soon.