American history is much more about rapid and cheap transport than about extremes of population density.  Even New York, our densest major city by far, became dense relatively late in American history.  To this day, the United States is not extremely dense, not say by European or East Asian standards.

But in American history, themes of horses, faster ships, safer ships, turnpikes, canals, our incredible river network, railroads, cars, and planes have been absolutely central to our development.  America has put in a very strong performance in all those areas.  When it comes to density, we have a smaller number of victories.  The moon landing was mobility, but not density.

Many of our Founding Fathers were in fact a bit suspicious of density.  So why not play to your own cultural and also geographic strengths?  After all, the United States is arguably the most successful country.

American SMSAs are so often more impressive than are American cities per se.

These days I see an urbanist movement that is more obsessed with density than with mobility.  I favor relaxing or eliminating many restrictions on urban density, and American cities would be better as a result.  Upward economic mobility would rise, and Oakland would blossom.  But still I am more interested in mobility, which I see as having a greater upside.

One issue is simply that urban density seems to lower fertility.  It is not obvious the same can be said for mobility.

And do you really want to spread and replicate the politics of our most dense areas?

Is not mobility rather than density better for raising a class of young men who will fight to defend their country?

Do not mobile, scattered immigrants assimilate better than densely packed ones?

The density crowd is very interested in high-speed rail, which I (strongly) favor for the Northeast corridor, but otherwise am not excited about, at least not for America.  Otherwise, the density crowd works to raise the status of a lot of low-speed means of transport, for instance bicycles.  Bicycles are also precarious, and their riders break the traffic laws at a very high frequency.  I do not wish to ban bicycles, but I do wish we could program them not to run red lights.  (I wonder how much the demand for them would then fall.)

I prefer to look to a better future where higher-speed transport is both affordable and green.  Ultimately, low-speed transport is a poor country thing.  It is also a poor country thing to have a lot of different speeds on your roads at the same time (I will never forget my first India visit in 2004).  High variance of speed also can prove dangerous, as evidenced by the research of Charles Lave.

I do not want to see the United States moving in poor country directions.

If you are obsessed with mobility, you will attach great importance to Uber, Waymo, self-driving vehicles more generally, and better aviation.  To me these are major advances, and they all can get much, much better yet.

I do not know if current plans for Neom, in Saudi Arabia, can prove workable or affordable.  Nonetheless, the idea of rapid transport along “The Line” at least represents an attractive mode of thought.  A better direction for future exploration than bicycles.

These points were obvious to many people in the 1960s.  The Jetsons had their (safe) flying cars.  The ultimate innovation in Star Trek was the transporter.

Jane Jacobs was obsessed with the West Village, an amazing part of America.  Yet, as far as I can tell (I haven’t read all her work), she didn’t write much about how to get more people visiting, and learning from, the West Village.  Hers was the perspective of the insider who already lives there.  That is one valid perspective, but not the only one.

Robert Moses was obsessed with building the Cross-Bronx Expressway.  That was a mixed blessing (see Robert Caro), but it did reflect his interest in mobility rather than density per se.

Today the world is full of anti-tourist movements, opposed to at least some kinds of mobility.  I prefer to push back on most of those, using Pigouvian fees to protect Venice and other locales when needed.

Ireland strikes me as the one country today that truly should be obsessed with density, not mobility.  Before 1840, the country had many more people than it does today.  And it could once again, easily.  In the meantime, there are far too few structures and the cost of living is very high.  Dublin and Belfast also need more cultural infrastructure (requiring higher populations) to be bigger draws for talented foreign workers.

The correct answers here really are going to depend on the countries and regions under consideration.

Switzerland, a highly successful country, also pays great heed to mobility.  The Swiss tunnels through the Alps are some of Europe’s greatest achievements, though today we take them for granted.  And the Swiss are trying to do road upgrades without slowing traffic.  You don’t have to put more people in Bern if it is easier to get to Bern, and away from Bern.

Mobility often gives you more algorithmic freedom than does density.

So, at least amongst the urbanists, perhaps density is these days a wee bit overrated?  After all, the net flow of American citizens still is to the suburbs.