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Comment by bumholio | Hacker News Reader

It's quite easy to envision some ramps where the vehicle steers out of the main underground lane and ascends on a 20% incline for 10m, stops into a station not dissimilar to any other public transit station, with multiple platforms so multiple passengers can embark at the same time in different pods.

When passengers are secure, another ramp takes them back down - or even the same one, with proper synchronization on less busy stops. It would then drive the passengers to any station in the network with almost the same flexibility as a self driving car.

I would say the cost differentials to subway are massive, one order of magnitude for similar passenger throughputs. And it's not some unproven, highly speculative technology like self driving cars, it's something that could be built today with low tech (but I concede, high capital investment).

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Like most people you vastly underestimate just how many passengers a subway moves. You need to move ~10 people per second through to match a subway's capacity. (2 directions * almost 200 per car * 10 cars per ~5 minutes.)

Self driving pods could work but you would basically need to dig tunnels as big as current subway tunnels to deal with the traffic + breakdowns etc.

At the limit, with proper design a stream of pods can act almost like an endless, continuous train - no subway network can beat that. But even if we accept a 2s spacing between pods, human level reaction times, that's still around 1 passenger per second per lane (with minimal 2 person/pod packing), or 15% of your peak subway estimate per direction. You should also factor the much higher efficiency of a system with no train changes, where the pod occupancy is optimized to avoid those empty returns to the busy area, where you don't have to dig a whole tunnel worth of capacity (6 tracks) at the terminus stations since one is enough for the traffic there, that can dynamically switch track direction depending on demand during the day etc.

Bear in mind, a subway tunnel for a single train has a diameter of about 7m, or about ten times the area (and dirt volume to be moved, and concrete to be poured) of a 2x2m lane.

You're ignoring the near-fixed costs to building tunnels: right-of-way, ventilation, maintenance. Even then, your 2 second gap is a pipe dream, not technically but logistically. You need scheduling flexibility to maintain such small gaps. Whether it's long wait times at stations, long detours to avoid congested routes, or keeping the system significantly below capacity, you can't assume that both a) pods can get on and off at will and b) there's a continuous flow of pods.

>You should also factor the much higher efficiency of a system with no train changes, where the pod occupancy is optimized to avoid those empty returns to the busy area

How do you avoid empty returns to the busy area? You still need to supply the pods, and if your solution is underfilling pods on the way back, that's exactly the same as sending a light train.

>where you don't have to dig a whole tunnel worth of capacity (6 tracks) at the terminus stations since one is enough for the traffic there

This is only an advantage at small scales. At higher scales, one train tunnel is just enough at the terminus stations, and the busier routes have more.

> your 2 second gap is a pipe dream, not technically but logistically. You need scheduling flexibility to maintain such small gaps.

A 2 second gap means that a pod needs to sync and squeeze between two vehicles 4 seconds apart, or 45m at 40Km/h. That's three seconds of 0.3g on a 40m acceleration track, after which it can merge with no assistance from the other pods (in a practical system, they would clearly cooperate). Assuming you achieve 90% of full 2s capacity, a pod will wait an average of 20 seconds for it's slot - queues should not form outside of very busy stations, that would be connected to very thick pipes anyway.

> How do you avoid empty returns to the busy area?

You might need to return empty, you might not. There might be a triangular route available that can maintain fullness throughout. You could build a buffer in anticipation of the rush hour. The train absolutely needs to follow the same route back at the same time interval and there is no way to reallocate part of the return capacity to the forward, overloaded route by a simple lane direction reversal. (to clarify, I am talking about maximizing peak capacity at rush hour on a given overloaded segment, there would be significant empty runs during the rest of day like in any other system).

> At higher scales, one train tunnel is just enough at the terminus stations

That's because you only have underground trains to places where a full tunnel makes economic sense, so the city is substantially under-served.

2 second gaps needs ~20 passengers per pod and a lot of pods which get pricy. That's looking a lot more like a subway train than the kind of routable cars people like to talk about. Further, subway trains move at 60MPH making 2 second gaps a safty issue.

>2 second gaps needs ~20 passengers per pod

Sorry, I don't follow. Why can't a 2 person pod reach 40 Km/h and maintain 22m spacing between pods?

As for the number/price of the pods, the relevant metric is price per passenger. A Subway train costs about $10.000 per standing seat, with the bulk of costs going to infrastructure, not rolling stock. So with cheaper infrastructure and a $20.000 2 person pod, we are cost competitive with standing seats in a packed subway, at much higher passenger comfort.

> Why can't a 2 person pod reach 40 Km/h and maintain 22m spacing between pods?

2 seconds at 40 Km/h is safer but that has other problems.

Remember, Pods don't have zero length they take up space which decreases the pods per hour. A 2 person 3 meter pod with 22 meter spacing at 40 Km/h = 2 * 40,000m / (3m + 22m) = 3,200 people per hour if they are all 100% full.

That's a long way from a subway's capacity unless you want to build 20 lanes per subway you are replacing. Going faster means more pods, but you also need to increase following distance. Slower is safer, but means trips take longer.

Running small pods underneath roads doesn't seem like it would really increase throughput much at all, while being significantly harder to build and maintain than the area directly above it.

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