I wonder what the root cause of these findings is. It seems to be a combination of attention diversion and something else-- after all, trees and buildings (presuming no extra stuff like birds, shouting neighbors, etc.) seem about equally distracting. One thing that occurs to me would be to do studies with subjects who are blind, deaf, both, and neither, as well as possibly autistic vs. not, etc. Also, comparisons between different cities, especially if you include small cities (still fairly urban feeling, but not a million people either) would be interesting to see.