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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

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 https://en.wikipedia.org/wiki/Personal_rapid_transit

jcranmer 3y ago
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.
- kalverra 3y ago
  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.
caditinpiscinam 3y ago
I don't understand your point about time; how does 20 small vehicles not take up more room than a bus?
- quantumwannabe 3y ago
  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.
  - yongjik 3y ago
    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.
    - quantumwannabe 3y ago
      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.
      - deleted 3y ago
        [deleted]
      - jcranmer 3y ago
        > 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).

jcranmer 3y ago

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.

kalverra 3y ago
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.

caditinpiscinam 3y ago

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

quantumwannabe 3y ago
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.
- yongjik 3y ago
  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.
  - quantumwannabe 3y ago
    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.
    - deleted 3y ago
      [deleted]
    - jcranmer 3y ago
      > 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).