Interestingly, if sidewalks measure "urbaness", most surburbs have extensive sidewalks and walking trails within a given development.

NYC in the example is very urban, but Staten Island is about 3x as dense as the (very suburban) suburb where I live -- and it gets very rural out here very quick (we're mostly converted farmland at this point). Staten Island seems fairly suburban to me, lots of single-story family homes with yards. It's almost the definition of a suburb except it happens to be administratively within NYC boundaries.

Manhattan is about 30x as dense as where I live and there's nowhere within 15-20 miles of where I live that even looks as urban as Manhattan. Mile after mile you get multi-story highrises or tiny squashed multi-story town homes that are mostly stairs. Even if you ignore the biggest skyscrapers, there's very few other places quite like it.

Part of it is that we're mixing two things that are a bit arbitrary.

How "urban" a city is and what the limits of a "city" are. Both of these are fungible by the community, as they should be. Urban living where I grew up would probably still include a backyard, in SF it probably includes a garage, in NYC it includes neither. The city of SF is pretty strictly defined, but the city of San Jose certainly has more fungible boundaries as well (santa clara vs milpitas vs san jose).

I am astonished at how much sidewalks cost per mile. We have engineered a system so complex that cities literally cannot afford to do much of anything.

I live in a rather nice area that has sidewalks only where new homes have been developed. There are huge stretches of roadway (some at 40mph) where riding on a bike equates to having a death wish. (Yeah I know bikes are supposed to be on the road, but at speeds above 20mph or so that is also complete bull!)

Sidewalks are cheaper than roads by a ton.

We have a mileage here where the city does all the sidewalk work. It's pretty awesome, no more tripping on cracks. I think it's like $15 a year for my $200,000 home.

It seems like the national consensus is that the threshold for "urban" is low enough to allow for a few more classifications of density which would probably be related to things like yards being squeezed out, single-family being replaced by high-rise, and presence of effective mass transit in spite of the American spite for such things. The term "downtown" would often fill such a role, though with even more variability than "urban". It's worth keeping in mind that our national capital doesn't even have any skyscrapers, and the term "urban sprawl" is widespread and widely understood to be something that actually happens with some frequency.

US mass transit may be meager but I don't think I agree that Americans are spiteful of mass transit. E.g...

http://m.metro-magazine.com/news/292617/poll-americans-favor...

Sure, half the population is willing to give it lip service, but we're really good at coming up with excuses, barriers, and delays, and really bad at coming up with funding. Getting a new passenger rail line built is about as easy as getting a new nuclear reactor built.

Seattle has a huge city limits, but much of it is undeveloped parkland or somewhat industrial. I don't know of any populated neighborhoods that lack sidewalks.

Circa 2008, lots of areas around Greenwood didn't have curbs or sidewalks... everything north of 85th got annexed by Seattle in the 50's and none of it had sidewalks or curbs or storm drains. I bet most of the other sidewalk-less areas are similar annexations.