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Comment by halomru | Hacker News Reader

It only doubles the car miles if there's nobody nearby to pick up. That would mean that most people are headed in a similar direction (e.g. all from suburbs towards city). But if all head in the same direction, the lane for return traffic has virtually no humans, and car-miles on it don't matter.

So either the traffic patterns allow pickup to be optimized to do much better than doubling the car-miles, or traffic patterns are so that it doesn't matter.

(of course that's overly simplistic, but I don't see how it's a less valid simplification than yours)

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Of course doubling is a simplification. Even one more car in the congested direction makes auto-driving car-sharing strictly worse for congestion. The question is how much worse. In Silicon Valley for instance, where everybody lives everywhere and works somewhere else, its a mesh of commute traffic and doubling may be a good estimate.

The important case of commuting toward the city is interesting. Car-sharing breaks completely there - everybody starts work at about the same time (commute traffic confirms this). So we'll need as many shared cars as we already have. No savings at all.

Any savings comes down to off-commute traffic. Its a second-order effect. Maybe even too insignificant to bother with the complexity of sharing.

It adds almost no cars in the congested direction. That's because there are already many cars going there.

If people are going from everywhere to everywhere, there will be almost no extra cars.

Again, the deadheading will double the trips. Compare one trip per person to work and parking their car, versus that trip plus the auto-driving car that got them there driving to the next pickup point. That's the extra bit. It could be any length trip. Depending on timing (uncorrelated departure times) then it averages half the width of the city. In any case its strictly worse in the auto-driving situation.

It's quite useless to state that "it doubles the trips, regardless of trip size", isn't it?

Yeah, when all directions are congested it will add a lot of very short trips.

"virtually no humans"

Something interesting to think about is logistics optimization. I have nothing against the big brown trucks but it should somehow be possible to haul my amazon packages from distribution center in the city by the railroad tracks to the burbs. Or distro center to distro center package movement.

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