Comment by holowoodman | Hacker News Reader

Comment by holowoodman | Hacker News Reader

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Public transit also doesn't scale. Germany, during the Covid years, introduced a cheap country-wide flat fee ticket (~50 bucks per month, all you can ride) for public transit. Which lead to road traffic going down measurably. Which lead to trains and busses being packed, people traveling more and farther on those. Leading to higher cost, degraded service and packed bus stations and rail lines. Building more of those isn't possible geometrically as well, cities are already packed. We are currently, at tremendous cost and effort, moving railway lines and stations underground, like Stuttgart 21, for that reason.

The point is, moving people inherently is bad. Shifting from cars to public transit reduces the badness a little, but you still need infrastructure that scales O(p * s * f), where p is the number of people, s is the average distance travelled per journey, and f is the frequency of journeys. Scaling doesn't change the slightest bit with public transit, you just have a different constant factor which is irrelevant for scalability.

So the solution isn't public transit. It is the avoidance of any unnecessary travel, meaning that we actually need something like a tax on non-home-office jobs and stores that you personally have to visit (as opposed to shopping online). We need better delivery infrastructure so people don't need to travel. We need close-to-home shopping options. Because actually moving goods instead of people scales logarithmically, the network of countrywide, regional, local distribution centers, bigger shops, smaller shops behaves like a tree.

> you just have a different constant factor which is irrelevant for scalability.

So then why bother with cities at all? High rises are just a different constant factor relative to 40 acre farms after all.

I feel like this is analogous to a case where someone says that an algorithm only differs by a constant factor but it turns out that because of that difference it hits the cache for 99% of use cases and as a result you see better than 100x speedups for all real world workloads.

Cities with cars obviously work up to some size. The sheer number of personal vehicle trips that a single rail line can replace is huge. And just to give you an idea of how large the scale factor here is, consider that you can pack at least 3 rail lines into the width of a typical two lane road, and that large cities commonly have 4 lane arterials.

If the political will existed to reallocate the space it could be done and the viable density would scale accordingly. On the extreme end we have Tokyo as a practical example, and even they are far from saturating all the available space for building rail lines.

> moving goods instead of people scales logarithmically

This is obviously false. There's some average parcel size, and an associated maximum capacity for a delivery van. Thus any given delivery run has a limit on the number of shipments it can service. Obviously that scale factor is significantly larger than the one for passenger rail versus cars, but it's still "just" a constant factor and thus irrelevant for scalability by your own logic.

In fact by your own logic I'm fairly certain that you will find that life as a whole is unscalable. Better nuke the planet I guess.

> So then why bother with cities at all? High rises are just a different constant factor relative to 40 acre farms after all.

Because of the logarithmic scaling of infrastructure (like supermarkets, doctors, hospitals), and because of the square root scaling of people-density per area vs. total travel distance to said infrastructure.

>> moving goods instead of people scales logarithmically > This is obviously false. There's some average parcel size, and an associated maximum capacity for a delivery van.

You do move bananas by ship from continent to continent. Then you split it up, move it by smaller ships up-river. Then you split it up, move it by truck to each cities distribution center. Then you split it up, move it by smaller truck to each store. Then people buy it.

Obviously logarithmic and the whole reason for ships and freight trains to exist, otherwise we would all get our tropical fruit in person by airplane.

And since we are on a CS-heavy site, talking about scalability, constant factors are irrelevant. That is what scalability means. It is the extrapolation to big numbers, where those constants no longer matter anymore. Of course there might be a local equilibrium for sufficiently small numbers. But that is always temporary as humanity keeps growing.

I take it you've never examined thev theoretical landscape of matrix multiplication algorithms?

The splitting you describe could be carried out just as easily with human transportation as with fruits and vegetables. I still object that it isn't logarithmic in either the space or trip requirement though. It's linear with one of those constant factors that you say are irrelevant. Just as you can never move a human using less than a human sized volume, you can never move a banana using less than a banana sized volume. Hence, linear.

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