We've spoken before about Saudi Arabia's "The Line" project. At first, I wasn't sure if it was real, but it is, and it's now under construction. We then spoke about whether a 170-kilometer line is an optimal urban form for a city, and the answer, according to this study, is that it's not. The problem with a line is that it actually maximizes the average distance between inhabitants. This makes sense because you could have two people living and working 170 kilometers apart.

On the other hand, if you maintain the same built-up area and take the opposite kind of geometry -- a circle -- you actually minimize the average distance between inhabitants. It's for this reason that older cities (the ones that weren't masterplanned) have tended to grow radially and not linearly (unless there were geographic features forcing it to grow in a certain way). So there is a strong argument to be made that The Line is a suboptimal plan for a new city.

But here's what's interesting: many cities already follow a somewhat similar approach. They don't do it as absolutely as The Line, but they do it in the way that they zone for higher densities and a mix of uses only on their main corridors. Example:

This creates a similar kind of effect when it comes to walkability, ability to support higher-order transit, and overall agglomeration economies. All of the urban activity gets concentrated along one corridor, maximizing the distance between people. In extreme examples, you also get inhabitants that are forced into different mobility options. The corridor is supposed to be transit-oriented, but all of the surrounding areas are really only conducive to driving. This creates a mismatch that is less an ideal for everyone.

So this post is our regular reminder that, when it comes to planning cities and bringing people together, circles tend to be better than lines. This doesn't necessarily mean that you need to adopt some sort of radial street network, à la French model. (Although I'm now thinking about the effects of this vs. an orthogonal grid.) It just means that urban density works a lot better when it's clustered, especially around transit. And generally, circles make for better clusters.