Years ago, and I don't know if its true anymore, there was an unwritten rule that someone would go as far as a work commute to go to church. So I guess it's important to know how far someone will commute and now?
Two considerations are important.
Some studies (referenced here)suggest that the phenomenon of the ultra-commuter is overstated, and that there is a near-universal 30 minutes maximum average commute in US cities.
Also the number of younger adults driving continues to fall, with the pickup of public transportation use. The cautionary tale is not that we will become more dependent upon transit for Sunday service attendance (though I would be if I didn't walk to church) but that young adults increasingly substitute driving/in-person experiences for online experiences.
The result is the same. Unitarian Universalists too often have regional congregations -- and many of these are small -- that I wonder if many "covered areas" are in fact unevangelized. This also suggests that some in-town neighborhoods in major cities are completely unserved.