This decreases density further, making greater travel distances.

This is approaching the problem backwards. You don't want to spread people further out with token green spaces in between miles of concrete. If for no other reason than those areas are way less effective at being "green space" that green space well separated from human civilization. The kinds of trees you can plant along boulevards or in inner-city parks or along highways are quite limited to a subset that can subsist on poor nutrient quality from the soil and much reduced sunlight availability from the pollution they are meant to "keep at bay".

Its the same reason "urban farming" is ridiculous. You end up spending way more in technology to make urban settings livable for plant life than you get back in value from having the plants in the first place.

If you want environmental boons like you describe, you want to dramatically expand nature preserves and increase the density of human settlement so we consume less of the livable footprint of the Earth with our societies and pollute the world less all the same.

This isn't a problem to hide behind a row of malnourished trees.

This would be incredible. Have you heard about the massive wildlife overpasses in Canada?