Elon Musk doesn't understand geometry | Hacker News Reader

Elon Musk doesn't understand geometry

humantransit.org

24 pointsfree0ne32 comments

Please don't editorialize the title. The original title is actually "Basics: Technology Never Changes Geometry."

The OP's main point is that in dense urban areas, replacing high-capacity vehicles with small-capacity ones presents a geometric challenge, because the surface area covered by streets is fixed. Were we to replace every bus carrying an average of, say, 40 people, with 20 small self-driving vehicles, each carrying an average of two people, the street surface area occupied by the small vehicles will be significantly greater and traffic will slow down to a crawl. Imagine a gazillion small vehicles stopping everywhere to pick up and drop off passengers. As the OP correctly points out, finding ways to use space efficiently is the core problem of urban transportation.

The additional vehicles could simply drive around the "hot" zones...

The author forgot about time. 20 small self-driving vehicles that come every 30 seconds uses no more road space than a 40-person bus that comes every 10 minutes.

Musk's self-driving taxi idea also isn't new. It's essentially the same thing as Personal Rapid Transit [1] which was originally envisioned back in the 1950s as a way to solve real world public transit system issues:

>Most mass transit systems move people in groups over scheduled routes. This has inherent inefficiencies. For passengers, time is wasted by waiting for the next vehicle to arrive, indirect routes to their destination, stopping for passengers with other destinations, and often confusing or inconsistent schedules. Slowing and accelerating large weights can undermine public transport's benefit to the environment while slowing other traffic.

>Personal rapid transit systems attempt to eliminate these wastes by moving small groups nonstop in automated vehicles on fixed tracks. Passengers can ideally board a pod immediately upon arriving at a station, and can – with a sufficiently extensive network of tracks – take relatively direct routes to their destination without stops

[1] https://en.wikipedia.org/wiki/Personal_rapid_transit

And if you look at the history of PRT systems, they haven't been terribly successful. It's not a new technology--it's 50 years old--and yet there are still so few examples of its employment.

I went to WVU where we have still actively use the PRT system built in the 70s. It's the school's main system of transport, and when it worked, it was pretty great. When I was there, it was infamously unreliable, but supposedly that's improved a lot lately. I do often miss it, and wonder why more places don't implement something similar.

I don't understand your point about time; how does 20 small vehicles not take up more room than a bus?

The bus and the 20 AVs have the same throughput (40 passengers in 10 minutes). While the AVs take up more physical space than the bus, they require the same amount of road space to transport the same number of passengers in the same amount of time.

You have to have extremely short bus headways before the road space efficiency tilts back towards its favor.

Seems like you're basically saying "It doesn't matter because the same road can support either a bus or 20 cars."

(1) This is a bad metric because it means everything is equivalent until traffic jam occurs. (Why stop at 20 cars? How about 2,000 cars?)

(2) More importantly, everything cannot be equivalent because traffic jams do occur in any respectably-sized city. Having 20 cars instead of one bus absolutely does contribute to having more frequent and severe traffic jams.

You're making the same mistake as the author. What matters is how many people a lane can move in a given amount of time, not how large each burst of people is.

We stop at 20 cars in 10 minutes because that's the equivalent of the bus. If the bus ran twice as often (i.e. 5 minute headways), we'd have 40 cars in 10 minutes, but that's only one car every 15 seconds which is far from a congested state, especially with no traffic signals (like a rail/guideway based PRT or a hypothetical future self-driving system where cars coordinate with each other).

PRT is essentially just a bus system with a large number of small buses and a large number of routes as opposed to a traditional bus system with a small number of large buses and a small number of routes.

> but that's only one car every 15 seconds which is far from a congested state, especially with no traffic signals (like a rail/guideway based PRT or a hypothetical future self-driving system where cars coordinate with each other).

Not really. In the case of a guideway-based system, the minimum headway is determined by the time it takes to a) slow down to a safe diverging speed for the switch, b) physically pass over the switch at that speed and clear it, c) physically reset the switch to a different direction, and d) the following car has to be have sufficient distance to be able to stop in case the switch faults during step c. And you generally want to have some padding in there so that you can recover the schedule in case things go wrong. (Or you can dispense with switches entirely, in which case it's ruled by the amount of time vehicles have to dwell at a station for passengers to embark and disembark. Which, yeah, 15s is going to be too tight for that.)

The idea of using smart technology to improve capacity of roads by doing with safety margins is horrifying to me because a) the technology is really nowhere near that, b) the design is fundamentally hostile to pedestrians and cyclists and quite frankly anyone other than robodrivers, c) you still have the problem in urban environments that there's no room for grade-separated junctions, so the possible capacity gains are sharply limited, and d) safety margins exist for a reason! Just ask a certain submersible CEO (for whom you'll need a medium, for he recently stopped being biology and started being physics).

> We stop at 20 cars in 10 minutes because that's the equivalent of the bus.

I think you're underestimating the capacity of a bus system significantly. I live in Dublin, which is a city of about 1.5 million people, not super-dense, hardly the world's greatest metropolis. The double-decker buses used almost exclusively in the Dublin transport system hold 90 people (from time to time Dublin bus experiments with bendy buses, but they tend to get stuck on roundabouts). Let's say the average car holds 1.5 people (I think this is generous, it's probably closer to one, but let's go with it). So a bus is equiv to 60 cars.

I can think of a few streets nearby which have stops for ~20 bus routes, with other routes running through them but not stopping. Some will be every-20-minutes routes, but some will be closer to every-five-minutes at peak. At peak time, there will be multiple buses per minute on many streets. You will see buses queuing up behind each other to load and unload. There is _no way_ that the equiv capacity in cars could even _fit_ on the road, even if traffic wasn't a concern.

These, by the way, are normal bus routes, not BRT; that would be even worse.

Here's the practical equivalent of 240 cars embodied in buses (plus 272 in the tram): https://s3.eu-west-1.amazonaws.com/prod-mh-ireland/607c5cf1-... - It's hard to see how you'd fit those cars in the equiv space. At peak times, all of those buses will be full.

I can see how that's true in the case of a single lane street: the 20 individual vehicles or the 1 bus both need one lane either way. But is it still true for higher volume cases? For example, I imagine that if you replaced a major subway line with a highway, the highway would have to be a lot larger to accommodate the same number of passengers in individual vehicles.

If policy makers had the authors perfect understanding of geometry :

* We would be living in tiny cubicles and sharing amenities with N people (N is a function of whatever factors the authorities deem important.)

* Strapped to vertical planks for flights.

* Have perfectly circular(/spherical?) cities with poorer people commuting from farther rings.

* Work in tiny cubicles (This has already happened, thanks beancounters!).

* Children carted away to boarding schools to not disrupt routines of productive people and ensure optimal education (sounds familiar?).

* Industrial workers deployed to worksites for N months (N is a function beyond your control..)

My city's traffic planners designed the traffic light phases with safety (and possibly throughput) as their primary concern.

Apologies for the caps, but I have very strong feelings about this:

THERE'S NOT A SINGLE INTERSECTION THAT I CAN WALK THROUGH IN ONE PHASE

People being people just go on red. Drivers run through intersections way past late amber - especially on left turns. Someone assumed spherical humans in a vacuum and is probably happy with the results.

And by the time you explain it, you've already lost the narrative. A fact that we've learned in the massive social media sphere is that "Louder = right". And, I hate this.

Someone screams their viewpoint? They must be right.

Someone responds back with trite phrases that catch people? Obviously correct.

Someone retorts with an overly simplistic comment? Of course, naturally correct.

Someone exerting power in a situation against someone with little power? Might makes right.

> Musk assumes that transit is an engineering problem, about vehicle design and technology. In fact, providing cost-effective and liberating transportation in cities requires solving a geometry problem. This confusion is very common in transport technology circles.

It's actually a psychology problem and I wish more urban/transit planners understood that.

You'd be hard-pressed to make buses go bumper to bumper like cars in traffic, meaning there's space to spare even when most people use public transport to move around. Geometry is actually a constraint only on the terminals - parking lots consume a lot of land.

Meanwhile people choose to drive not because it's efficient (it's not, far from it), but because for them it has advantages which can't be summarized as a single number like average travel time. If you ask around you'll find that most people don't think too hard about the exact amount of minutes their commute takes. They just want to be on time and once that's taken care of, other factors become more important.

To solve this you have to change how public transport is perceived.

I'm not American so bear with me here: One way to do it is to give a familiar experience. If statistics are to be believed, half of US K-12 students get to school by bus. Also, people regularly fly. Private transport via air is comparatively rare. This is all mass transit.

To me the core features of these two experiences are:

-Everyone sits.

-Everyone is accounted for.

-Changes are minimized, if present at all.

-There's a schedule and it's treated seriously.

It appears that Elon Musk, despite being generally unhinged, knows more about human nature than transit planners, which is weird, because that's a low bar to clear.

there are a few problems with the article: first, busses in US cities are either too full on clogged streets or too empty on empty streets to have an average load factor high enough to make them less polluting than cars. Cars. even ordinary sedans, used as shared robotaxis could readily beat busses environmentally. But once you get purpose-built mini-bus robotaxis, that will dominate in efficiency.

I'm all for better social amenities, but, in the US, with maybe a couple exceptions, robot-rides-on-demand is the likely winner, and it will be a near ideal free market: No barriers to entry, relatively low capex to get started, intermediaries enabling any size fleet to get customers, etc.

No, Elon does understand geometry — this author doesn’t understand the free market.

On the rare occasion that the “public transit” busses sharing our roadways are full, they are indeed very efficient.

Unfortunately - these situations are rare. The rest of the time, other solutions are vastly more desirable to the public. Simultaneously, these huge, lumbering 90%-empty beasts pile up traffic behind them, forcing massive exhaust plumes from the unnecessarily idling traffic.

Stop subsidizing sub-standard solutions, and let the transit-using public decide.

Alternatively, you could incentive people taking transit by making cars more expensive (by pricing in externalities).

>(by pricing in externalities).

In practice, this means someone pockets the change and nothing gets done about the so called externalities.

What do you mean? Can you give examples?

Suppose you want to force people to drive less — you increase gas prices with a tax.

Citing this extra tax, some lawmaker spends money on something completely irrelevant that was lobbied to them. Public transport is a fairly distant concern for most of them.

Oil companies increase crude prices to get on the gravy train, so does every other entity in the industry.

Car dealerships jack up prices because they want to maintain their profit levels.

Meanwhile middle class people who rely on cars get effed.

Nah, you do congestion pricing in cities where transit is feasible.

You don't ride buses much do you? Of all the times I've ridden regular bus routes, there's only been one route where the bus being 90% empty would not be extremely atypical--and even then, that was only when I was picking up the bus during off-peak hours. More frequent have been the times where there being anything other than standing room available was the atypical situation.

If you're in an urban environment at least as densely packed as, say, San Jose, then the bus system is likely to have actually quite substantial ridership. By the time you get to the kinds of dense urban environments of the standard major US cities (NYC, Chicago, DC, Philly, Boston), I quite assure that the buses are routinely packed. The story might be different if we're talking about, say, Bismarck.

Furthermore, I should point out that if a bus is stopped "pil[ing] up traffic behind them", that's because it's letting passengers embark or disembark, which is a strong signal that the bus probably isn't 90% empty.

> Of all the times I've ridden regular bus routes, there's only been one route where the bus being 90% empty would not be extremely atypical

You're falling victim to the inspection paradox: https://towardsdatascience.com/the-inspection-paradox-is-eve...

Interesting. What evidence do you have to back up your claim that the user has fallen victim to this paradox?

Using your experience as a passenger to gauge how full buses usually are is about as straightforward of an example of that paradox as you could ask for. As an extreme example, imagine there are 10 bus routes, 9 of which are always empty and 1 of which is always full. If you surveyed every passenger, they'd tell you that 100% of the buses they've ever been on are full, but in reality, only 10% of the buses are.

You know you can just look this stuff up, right? The major urban transport systems publish data on this; it's how they decide which routes to cut and which to beef up.

I know. My point is that that's what you need to do, rather than trying to use (anec)data from passengers' perspectives, as the post I originally replied to did.

In Chicago, where I live, route cutbacks are major drama. If nobody is riding the busses, as the ancestor comment suggests, the routes get cut. You know that's happening because people are constantly pissed about it. It was that comment making the extraordinary claim, and ordinary evidence is sufficient to refute it.

On-demand AVs will probably kill mass transit in the US. The poor will get the equivalent of EBT cards to ride, so there will be a social aspect to it. But framing it as some kind of Randian triumph ignores that there are places in the world where mass transit is very efficient because the cities are different. Private cars will be for the rich and should be taxed appropriately. Meeting your fellow citizen while riding an AV mini-bus will open your eyes to social diversity.

There are tons of real-world situations in which individuals, by making choices that benefit themselves, create a situation that is worse for everyone overall. It's basic microeconomics https://en.wikipedia.org/wiki/Tragedy_of_the_commons

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