A downtown without street trees in places warm in the summer will forever have their few customers fighting for parking spots right in front of the door. Even in good downtowns, there are inevitably gaps. Fill them. And many virtuous cycles spin off from street trees which benefit environmental health, economic health, and public health.
This is just a short walk away in my new hometown of Tuscaloosa. See how delicious all that shade looks, between the building shade and tree shade? The continuity of shady walks downtown are one of the strongest components of Walk Appeal in warm weather. Many say their downtowns have a parking problem; what they actually have is a Walk Appeal problem. Street trees are a Walk Appeal superfood, and therefore help the "parking problem" disappear.
There's a problem with street trees downtown that most people don't realize: most of them hang too low, blocking the signs. Band signs over storefronts can be as high as 16' above the sidewalk, so the species of tree chosen should be happy to have its lowest branches that high. This requires proper trimming, of course. Fail at that, and Main Street merchants have been known to poison the street trees so their signs can be seen better.
There's one exception to the downtown lowest limb rule: if a street will forever be fronted by restaurants instead of retail, like in this picture of Espanola Way in South Beach, it's fine to have the limbs hang low, shading the diners better. And transpiration from their leaves puts cooling moisture in the air around them, creating a similar effect to misting fans.
Residential streets need only have street trees with branches that clear the heads of tall people. This is a street in Tuscaloosa with good-to-excellent homes, but the continuous canopy of street trees knits it all together into a great street. I enjoy it often!