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Comment by JoeAltmaier | Hacker News Reader

Are they including deadheading in their 'utilization' metric? Half the time (or more) the car is going to be self-driving to its next pickup point.

Also, any savings in parking needed is going to be eroded by road congestion - in a perfect scenariao cars are all going to be moving around most of the time. That seems like a much worse problem at present - roads are often at peak usage, while reducing parking is only a cost-saving feature.

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With mass carsharing, the number of cars should fall so this should be less of a concern.

I agree though - if your cities are already gridlocked, this won't make them somehow less gridlocked. You still need mass transit, cycling and walking no matter who's driving the cars.

There's concept called "platooning". If distances between cars could be reduced, road passenger density could be improved dramatically.

The number of cars is irrelevant to congestion. The number on the road is important. And with car-sharing there are exactly the same number of useful car trips (people getting to work or home) plus the deadheading. Its strictly (much) worse.

>if your cities are already gridlocked, this won't make them somehow less gridlocked

I disagree. Human drivers use roads very inefficiently: they overreact, have high latency and drive overly selfish.

Computers can drive cars closer together, can maintain higher speed with the same safety margins and without causing traffic jams by overbreaking, and they have no problems obeying traffic laws (and thus won't cause a literal gridlock).

The only reason why self-driving cars won't notably reduce road utilisation is because of induced demand

And because with car-sharing it doubles the number of car-miles. Because, deadheading (driving to the next pickup point).

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)

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.

> Are they including deadheading in their 'utilization' metric? Half the time (or more) the car is going to be self-driving to its next pickup point.

Author here. You are completely right, I didn't include self-driving time in the second table. To correct my omission, let's add a column for 3X utilization where self-driving cars spend half of the time driving alone.

Shared self-driving car (3x)

-----------------------------

Used: 3 x 5% x 2 = 30%

Parked: 70%

Number of cars in the city: N/3

Parking places needed: (N/3) x 70% = N x 23.3%

Parking reduction: (N x 95%) / (N x 23.3%) = 4.07x

-----------------------------

So, in that theoretical case, we would need 4x less parking. However, this is just a rough calculation, for better precision you would use some traffic simulation application.

Thanks for feedback!

Thank you! So nice to have hard numbers.

Of course with proper scheduling, this worst-case could be chipped away to get better numbers. The best case is nearly zero deadheading (with likelihood of nearly zero). Some kind of curve could be constructed? I'd love to see that too!

True, but for that we need a traffic engineer with experience in traffic flow simulation software :) https://www.researchgate.net/post/Whats_the_best_software_fo...

> roads are often at peak usage, while reducing parking is only a cost-saving feature.

In many European cities, parking area could be converted into additional lanes by removing a few marker lines. Reduced demand for parking area can increase throughput too, in these cases.

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