[jcranmer]

> 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).