Comment by davnicwil | Hacker News Reader

Comment by davnicwil | Hacker News Reader

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I'm first in line to agree with you, I think cars in cities are awful because of these two problems, and honestly I support all sensible efforts to reduce the number of (current generation) cars on city streets.

I'm also a daily user of public transport in a major world city, however, and whilst buses and trains work they also have the fundamental problems highlighted above that are in my view insurmountable, and which (self driving) cars solve. So I'm in a dilemma over whether I think they'd be an improvement, but I ultimately believe they would be.

But yes, absolutely pollution and space saving would have to be addressed. I'm optimistic they could be:

Pollution: hopefully solvable with electric. Yes, of course, there are efficiency gains that trains/buses would perhaps always have (having to move many small things rather than one big thing carrying the same number of people) but I can see electric reducing the absolute level of pollution from cars to a point where it basically doesn't matter. Maybe overly optimistic, but I hope that'll happen.

Space: Driverless cars, particularly electric driverless cars, can be much smaller and can drive much closer together. Different cities have different challenges, but I don't see why we couldn't get to a situation where roads might effectively have capacity for 2x vehicles across and 2-3x vehicles along if the vehicles can be coordinated to drive closer together and are physically smaller (remember that on-demand unlocks certain design constraints - you don't have to provision for peak - if the average number of passengers is 1.5, then most cars on the road could be ultra small 2 seaters). Again lots of conjecture and optimism here, and I am probably assuming that 'manual' cars would eventually disappear or be banned at least from cities too, but nonetheless I think there is hope here.

Public transport will always be able to pack more people into the same space.

There is a minimum amount of space occupied by the vehicle's own matter. The ratio of that material to the occupant is a scaling law: the larger the vehicle, the higher the volume-to-surface, the denser the passenger-to-vehicle ratio can be.

A standard R160 subway car carries up to about 250 people in an area that would be about the same as 5 Corollas positioned nose to tail, a starting ratio of about 10:1. If cars are made twice as dense, it's still 5:1.

This is ordinary batching. The only thing that can carry more people more densely than a bus or train is the footpath.

Yes you're absolutely correct - I guess my main question is could the space savings, like the pollution savings, be 'good enough' for it not to matter in practice, assuming that driverless cars become the 80% usecase for road traffic and today's 'massive 5 seater car for an average of 2 people' model goes away.

It's also important to point out that of course, buses and trains shouldn't go away - they could indeed also have a bunch of problems solved by being self driving too, most notably the 24/7 running thing - they'd just be another option in the mix.

> Public transport will always be able to pack more people into the same space.

While that's true, that's also just an argument for something we already understand. Are there possible arguments to be made about the number of busses and routes because it's not taking people point to point. What about the lul-time of day, where on-demand model is more efficient than driving empty busses around. They reduce schedules obviously. But now you're punishing passengers that start a shift at noon. There are more things to solve than just "the least amount of pollution and space."

Totally unrelated but similar argument. The FAA introduced a new system and process for creating standard flight paths a few years ago. Flights around the nation started doing new things. TONS of people are now inundated by jet noise. They're now discussing just going back in certain areas to the old routes. They ONLY optimized for efficiency of route and didn't consider anything else.

> There are more things to solve than just "the least amount of pollution and space."

The argument I was responding to was that self-driving cars would be sufficiently more efficient users of space that it would obviate the need for public transport. This was demonstrably false.

> But now you're punishing passengers that start a shift at noon.

Sure, but (a) that's a financial question, not a capacity question and (b) rush hour is when both finance and capacity collide.

Public transport is a more efficient option per passenger-kilometre and per square metre of road or cubic metre of tunnel than self-driving cars. It is always going to be. Self-driving cars will have other virtues and will reshape transportation, but ascribing magical powers to them does nobody any good, especially if it leads to defunding of subways and buses.

>A standard R160 subway car carries up to about 250 people in an area that would be about the same as 5 Corollas positioned nose to tail, a starting ratio of about 10:1. If cars are made twice as dense, it's still 5:1.

You're not accounting for the space between trains. They will only take up 5-10% of the available rail space because you need gaps between the trains.

You need gaps between cars, too. Two seconds of following distance in good weather conditions, four seconds of following distance in bad weather conditions.

At highway speeds, 4 seconds of following distance is 120 meters.

> You're not accounting for the space between trains.

NYC's least efficient system is block train control, requiring 300M separation between trains.

In 300M you can fit about 66 Corollas, again packed bumper to bumper. That's 66 * 5 people = 330 people, or approximately 1.5 subway cars.

Subway trains are 6 or 8 subway cars long, depending on track and time of day.

With CTBC some platforms can run at 60 trains per hour (30 local, 30 express). That's 8 subway cars every 2 minutes, or approximately 1,000 people per minute.

To move 1,000 people at all with the Corollas you will need 250 * 4.55M of them, or approximately 1.1 kilometres of cars, bumper to bumper.

>With CTBC some platforms can run at 60 trains per hour (30 local, 30 express). That's 8 subway cars every 2 minutes, or approximately 1,000 people per minute.

Unless your trains are going at 10 kph you will have more of a gap than 300 meters if you see a train every 2 minutes.

Or the trains can move much faster and much closer because of improved control.

You should recognise this argument: it's the same logic given for why self-driving will solve everything.

But you're always limited by loading time. If you take X seconds at each stop then you will end up with X * speed distance between each car. Even at 30s loading and 50 kph you're looking at 400 meters between each train.

Packing more self-driving cars onto roads is an appealing notion, but I don't think it holds up to scrutiny. Even if a computer detects hazards instantly and continuously negotiates with and anticipates the movement of other vehicles, the physics of moving cars remain the same. Safe stopping distances won't be dramatically different. Road surfaces, stray animals, mechanical failures, and any number of other hazards don't care who or what is driving a car.

Say you have 3 lanes of traffic. Each lane is occupied by 100% driverless cars.

Let's say, there is an object completely blocking the middle lane, and partially blocking the two outer lanes. What will the driverless cars decide to do?

My guess, is they wait there until the road is completely unblocked, there's no way for the cars to understand the proper course of action. It will be great fun to bring absolute gridlock with an empty cardboard box in downtown NYC once this all kicks off. Naturally, by then, card board will be illegal there.

Why would driverless cars be smaller? They still need to be able to carry the same number of passengers; the only difference there is that one of those passengers is no longer also a driver.

With autonomous cars, robotaxis become feasible. If you're buying a car for yourself, you're going to want a certain amount of space for passengers or cargo in case you need to do something like drive your family to the airport. You'll also want long range in case you decide to go on a road trip. Even if 90% of the time you'll be commuting alone 20 minutes to work, you'll want this extra range and capacity for the times you need it.

If you're deploying a fleet of robotaxis, most of your fleet can be vehicles that only carry one or two passengers at a time on short trips. If a family wants a ride to the airport, then they'll specify that they need a larger vehicle for more passengers and luggage, etc. So you can buy a fleet of mostly smaller cars with a shorter range, and compliment them with a few minivans for long range trips. Overall your fleet will be quite efficient.

One thing that is often not accounted for is car seats. Do you have to bring your own?

Yep, and given that some big percentage of car seats are not properly installed, do you then have to take the requisite minutes to install and uninstall each seat properly? There are a lot of challenging edge cases.

> the vehicles can be coordinated to drive closer together

How do you make this work? There's no standard in self-driving systems, no standard in sensors, and no standard for vehicle to vehicle communication.

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