But it’s NC State and not Top4, so don’t expect anything to come out of it.
It still costs money. And where should the exits be? If they are far apart this doesn't work. And the prospect of being underground for a long trip is not that thrilling.
You're basically proposing a boiling the ocean kind of solution.
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).
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.
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 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.
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.
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.
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.
Everything that is expensive and unreliable is because they assume an embedded wire is not going to be allowed/done by the people managing the roads.
All while avoiding the seemingly insurmountable difficulties of fully autonomous driving on congested and chaotic city roads.
If you could build the pods on the cheap, cities would love it as an affordable alternative to subway and tunnels - it's much cheaper to dig a 4x2m concrete box under the main road then massive tunnels and stations.
In time, as the network expands, full self driving can be added and the vehicles can exit the network and drive the last miles on regular roads to the traveler's front door in the suburbs.