"To qualify as an urban area, the territory identified according to criteria must encompass at least 2,500 people, at least 1,500 of which reside outside institutional group quarters."
https://www.census.gov/programs-surveys/geography/about/faq/...
The US is big. That the above is urban makes sense in the context that I'm only about a 15 minute drive from a small city and am only about an hour drive from a large city. There are lots of places in the US that are 100+ miles from the nearest Walmart.
You're likely quoting the census, who defines any town with more than 2,500 people as urban.
I promise you, if you went to a town of 2,500 people, you would not have the word "urban" in your head.
EDIT: After a little reflection, this really blows my mind. Over 60 million Americans live in towns of less than 2,500 people. This country is massive.
I'm not suggesting that there are no solution here -- I'm genuinely asking what the solutions are.
The car-free zone makes sense if it's smaller than the area that has public transit. Note that even in the complete absence of public transit, you can still comfortably implement about a quarter-mile radius of car-free area, since a quarter mile walk is only 5 minutes.
In the US what we're mostly talking about is closing individual downtown streets to through traffic (like, say, Market between 5th to 13th, ignoring the City Hall death circle). On NYC 14th street, any car is allowed, as long as it turns off within 2 blocks. This greatly limits the usefulness of the street to through traffic, (and therefore makes it a much better pedestrian and bus environment) without changing the parking situation at all.
Look at many continental European cities.
A mid-sized German city will be full of low to mid density buildings and walk up apartments, few skyscrapers. Mixed use of retail and residential, and very good transit by American standards, connecting to intercity rail transit. Highways frequently terminating or routing outside the city. Plenty of greenspace.
Leipzig, Germany https://www.google.com/maps/place/Leipzig,+Germany/@51.33310...
Even the US used to have this, though they are long drowned in sprawl. Any city that was of decent size pre-WWII usually has street car suburbs - a dense mix of single family homes, shops and apartments usually built around a transit node that connected to the core and region. Look at Shaker/Cleveland Heights and Lakewood in Cleveland. There is a healthy mix of uses and density centered around transit. You can see the pattern they developed around, even if it's vestigial at this point.
Low density means there is at least a full building worth of space between houses and ideally several buildings worth.
It means you can stand on every side of your house and look and have difficulty seeing your neighbor.
"a dense mix of single family homes, shops and apartments"
"a healthy mix of uses and density centered around transit."
Not a big fan of that kind of density, nor are many others. I'd at least like to have my own backyard, and not have my neighbors peering in from a second story.