4 ms·

Well, you cannot pack trains of 2K people at 90 seconds, because you reach a limit with people getting in and off in the stations, the whole system slows down

by yholio 6y ago

Well, you cannot pack trains of 2K people at 90 seconds, because you reach a limit with people getting in and off in the stations, the whole system slows down with trains waiting their turn to enter the station, so real throughput decreases drastically. Additionally, infrastructure costs are much higher for similar peak rates since subway tunnels remove 3x more dirt volume, and require proportional structural materials (7.3 m bore holes vs the current Borring prototypes of 4.2m) Also, there is a more fundamental problem with reaching high thoughtputs with mass transit: you need to force people out of optimal routes from their distributed destinations and origins, into enforced stations. This makes sense for point to point links like LA to SF, but it's very inefficient when trying to cover a 2D city: the more routes and stations you add, the less you are likely to reach peak capacity of any one link. If you are going for city-wide subway, you will pay much, much more to build large, expensive tunnels that will stay mostly idle at the perifery of the system.

lmm 6y ago
> you reach a limit with people getting in and off in the stations, the whole system slows down with trains waiting their turn to enter the station, so real throughput decreases drastically. Yes, and this would be an even bigger problem for your proposal, that's the point. > Also, there is a more fundamental problem with reaching high thoughtputs with mass transit: you need to force people out of optimal routes from their distributed destinations and origins, into enforced stations. This makes sense for point to point links like LA to SF, but it's very inefficient when trying to cover a 2D city: the more routes and stations you add, the less you are likely to reach peak capacity of any one link. If you are going for city-wide subway, you will pay much, much more to build large, expensive tunnels that will stay mostly idle at the perifery of the system. You're hugely exaggerating the problem. You get some inefficiency yes - if we imagine a grid system then many journeys require a change of line and the average journey is 1.27x longer than an ideal path between two points, assuming people don't change their actions at all (e.g. living on the same line where they work). But that's outweighed by the efficiency gains of mass transit - in a city with a decent subway system that longer journey with a change will still be significantly faster than driving. As for tunnels sitting idle at the periphery of the system, in the real world that's really not the issue you make it out to be, for a number of reasons; housing concentrates along commuting corridors, the places where the population thins out are, by the same token, the places where land is cheap and the train line can run on the surface, and if necessary then trains can turn short or split/join. If you look at the usage levels that subways actually achieve, they're incredibly cheap for what they do.
- yholio 6y ago
  >> trains waiting their turn to enter the station > Yes, and this would be an even bigger problem for your proposal, that's the point. No. Trains must make every station. A low occupancy pod will only diverge from the high speed lane when it has reached a destination or pickup point. > in a city with a decent subway system that longer journey with a change will still be significantly faster than driving. I don't think anyone disputes that. But this happens because there is no comparable point to point transit system and the existing road network is inefficient. The point of building tunnels is remove those conflict areas and provide direct point to point routes with very high throughput. If, for the same price you can have similar throughput using individual vehicles, then it makes absolutely no sense cram people into comunal transport, force them into unnecessary stops, changes, and waiting times, making them compete for real-estate prices in transit corridors etc.
  - lmm 6y ago
    > No. Trains must make every station. A low occupancy pod will only diverge from the high speed lane when it has reached a destination or pickup point. So you'll have to build twin tunnels through stations (and since they'll require an ADA-compliant evacuation walkway, they'll have to be almost full-size subway tunnels even if you're using smaller personal pods), which will be a fiddle to use TBMs for. You'll have to have a system for diverting or not, and that's going to require maintenance and break down sometimes. And what does the actual mechanics of pods stopping for people to get out look like at busy stations? You'd need giant shunting yards to route pods to free "platforms" (like a garage forecourt only much bigger), and you'd have to build them underground in the places that are most expensive to do tunnelling and remove spoil from, and they'd have a complex layout that would have to be built in a labour-intensive way - all the lighting and ventilation and fire-suppression systems would have to be fitted into that layout (underground stations are already the most expensive parts of building subway projects). Or you'd have pods backing up onto the main line. Or you'd have to batch them together in a, uh, train. > I don't think anyone disputes that. But this happens because there is no comparable point to point transit system and the existing road network is inefficient. Tunnels are still only going to go from some places to some other places. Roads are already planned carefully. But there's only so much capacity you can get out of individual vehicles. > The point of building tunnels is remove those conflict areas and provide direct point to point routes with very high throughput. It's not going to be high throughput though. Every transportation system has a high throughput when you're just looking at maximum capacity along a plain line going at speed - it's all the fiddly interfaces where the throughput goes down. It'll be the same for a tunnel system (if it's ever built).

by yholio 6y ago

lmm 6y ago

> you reach a limit with people getting in and off in the stations, the whole system slows down with trains waiting their turn to enter the station, so real throughput decreases drastically. Yes, and this would be an even bigger problem for your proposal, that's the point. > Also, there is a more fundamental problem with reaching high thoughtputs with mass transit: you need to force people out of optimal routes from their distributed destinations and origins, into enforced stations. This makes sense for point to point links like LA to SF, but it's very inefficient when trying to cover a 2D city: the more routes and stations you add, the less you are likely to reach peak capacity of any one link. If you are going for city-wide subway, you will pay much, much more to build large, expensive tunnels that will stay mostly idle at the perifery of the system. You're hugely exaggerating the problem. You get some inefficiency yes - if we imagine a grid system then many journeys require a change of line and the average journey is 1.27x longer than an ideal path between two points, assuming people don't change their actions at all (e.g. living on the same line where they work). But that's outweighed by the efficiency gains of mass transit - in a city with a decent subway system that longer journey with a change will still be significantly faster than driving. As for tunnels sitting idle at the periphery of the system, in the real world that's really not the issue you make it out to be, for a number of reasons; housing concentrates along commuting corridors, the places where the population thins out are, by the same token, the places where land is cheap and the train line can run on the surface, and if necessary then trains can turn short or split/join. If you look at the usage levels that subways actually achieve, they're incredibly cheap for what they do.

yholio 6y ago
>> trains waiting their turn to enter the station > Yes, and this would be an even bigger problem for your proposal, that's the point. No. Trains must make every station. A low occupancy pod will only diverge from the high speed lane when it has reached a destination or pickup point. > in a city with a decent subway system that longer journey with a change will still be significantly faster than driving. I don't think anyone disputes that. But this happens because there is no comparable point to point transit system and the existing road network is inefficient. The point of building tunnels is remove those conflict areas and provide direct point to point routes with very high throughput. If, for the same price you can have similar throughput using individual vehicles, then it makes absolutely no sense cram people into comunal transport, force them into unnecessary stops, changes, and waiting times, making them compete for real-estate prices in transit corridors etc.
- lmm 6y ago
  > No. Trains must make every station. A low occupancy pod will only diverge from the high speed lane when it has reached a destination or pickup point. So you'll have to build twin tunnels through stations (and since they'll require an ADA-compliant evacuation walkway, they'll have to be almost full-size subway tunnels even if you're using smaller personal pods), which will be a fiddle to use TBMs for. You'll have to have a system for diverting or not, and that's going to require maintenance and break down sometimes. And what does the actual mechanics of pods stopping for people to get out look like at busy stations? You'd need giant shunting yards to route pods to free "platforms" (like a garage forecourt only much bigger), and you'd have to build them underground in the places that are most expensive to do tunnelling and remove spoil from, and they'd have a complex layout that would have to be built in a labour-intensive way - all the lighting and ventilation and fire-suppression systems would have to be fitted into that layout (underground stations are already the most expensive parts of building subway projects). Or you'd have pods backing up onto the main line. Or you'd have to batch them together in a, uh, train. > I don't think anyone disputes that. But this happens because there is no comparable point to point transit system and the existing road network is inefficient. Tunnels are still only going to go from some places to some other places. Roads are already planned carefully. But there's only so much capacity you can get out of individual vehicles. > The point of building tunnels is remove those conflict areas and provide direct point to point routes with very high throughput. It's not going to be high throughput though. Every transportation system has a high throughput when you're just looking at maximum capacity along a plain line going at speed - it's all the fiddly interfaces where the throughput goes down. It'll be the same for a tunnel system (if it's ever built).