The appropriately named Jarrett Walker is the author of Human Transit, a seminal text on transportation and cities that draws on his decades of experience in urban planning; he has the distinction of being called “an idiot” by Elon Musk on Twitter, when he pointed out that Musk’s Boring Company tunnel proposals could not possibly work due to their low capacity.

Walker’s overarching thesis is that city transit is undermined by “elite projection,” where rich people pretend that the way they like getting around – in private vehicles that go from door to door – can possibly work at urban scale, despite the fact that simple geometry shows that this is a physical impossibility.

As in, “It doesn’t matter how tightly you pack self-driving Ubers together on our roads. If all the people who make your coffee and empty your wastebin are in private vehicles rather than on buses and trains, the roads will be at 5 or 10 times their physical capacity.”

This emphasis on private vehicles leads people to seize on technological fads to defend the indefensible – hence the vogue for describing the smartphone as the key technology for transportation, or self-driving cars, or data-driven custom shuttle routes that re-route themselves based on demand signals from riders’ phones.

These all share the geometric flaw: even the smallest cars, packed as tightly as possible, multiplied by all the people who rely on buses and trains, will overflow all the roads we have now and all the roads we could ever build.

There is another flaw: when you make it cheaper to ride private vehicles (rather than public transit), you siphon transit riders out of the buses and trains, and put them on the roads, increasing congestion: so adding “efficient rideshares” actually makes transit worse, not better

Walker tried to explain this to Elon Musk on Twitter, discussing how his proposed Boring Machine tunnels’ narrow bores meant that on the one hand, they couldn’t carry enough people to make an appreciable difference in traffic, and on the other, that his proposal for allowing private cars to run through the tunnels is nuts: “The amount of the city that you would have to level to create enough of those elevators to get everybody’s car into the tunnel at 5:30 in the evening, it’s preposterous; it cannot help being. Anything that is that inefficient has to be only for elites.”

Musk called him an idiot.