So I guess I was wrong. I thought "The Line" in Saudia Arabia was never going to be built -- at least not in its current incarnation. But apparently it is now under construction, and it is still planned to be 170 km long and house some 9 million people when it's complete. I suppose something could happen between now and when all 170 km are complete, but I'm happy to accept that, for the time being, I was wrong.

Now it's time to ask ourselves what a 170 km long city would even be like. To answer that, here is a fascinating article from npc Urban Sustainability describing how mobility and urban interactions are likely to work in this kind of a linear city. And it turns out that a line is actually an optimal urban form if you're trying to both maximize commute times and maximize the average distance between inhabitants:

One of the most critical aspects related to The Line is distance. If its 9 million inhabitants are homogeneously distributed in the city, each km will have roughly 53,000 people. If we randomly pick two people from the city, they will be, on average, 57 km apart. Although The Line occupies only 2% of the surface of Johannesburg, if we pick two random people in Johannesburg, they are only 33 km apart. Keeping the surface fixed, a line is the contiguous urban form that maximises the distance between its inhabitants. In The Line, people are as far away from others as possible. Considering that a walkable distance is 1.0 km, in The Line, only 1.2% of the population is at walking distance from others. Active mobility is not viable in The Line since distances are too long. The plan for The Line has no cars but also gets rid of most active mobility. Although in The Line, basic needs could be satisfied within 5 min, most journeys to school, work, leisure or visiting other people will depend on public transport.

So what would be better? Well if you're trying to minimize the average distance between people and increase urban interactions, then the optimal form is generally the one that cities have been using ever since they were first created:

We can think of a city called The Circle, where we take the same tall buildings as in The Line but put them next to each other, forming a circular shape. A circle that occupies the same surface as The Line (34 km2) has a radius of only 3.3 km. In The Circle, the expected distance between two random people is only 2.9 km. In The Circle, a person is at a walking distance of 24% of the population (and within 2 km, they could reach 66% of the destinations), so most of their mobility could be active. In The Circle, a high-speed rail system is unnecessary since people could walk or cycle to most places, and buses could supply the rest of the journeys. The Circle occupies roughly the same surface as Pisa, Italy, but has 50 times its population. A round urban form is the most desirable since it reduces commuting distances and the energy required for transport.

This is one of the challenges with building large scale cities from scratch. It's easy to become enamoured with a particular plan or symbol; whereas in reality, cities don't care about these kinds of visual representations. They don't care that, in plan, the city may look like an eagle (see "Helicopter Urbanism", a term that was supposedly coined by Jan Gehl). What matters is how interactions between humans play out at street level.