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The question sounds somewhat paradoxical so I'll explain. In case of ships: recent trends of overcapacity and cut throat competition (and a few years ago: expensive fuel) lead to slow steaming:

So I started to wonder about possibility of similar trend in autonomous cargo transport cars - saving fuel in case of overcapacity would lead to reducing the transport speed for purely economic reasons.

Assumptions:

  • no great breakthrough in engine technology

  • self driving car would have a comparable price to a model with driver

  • no regulatory limits influencing the choice

  • cars can have smaller engine for start, if that seems like a good business

  • the vehicle has choice of either driving slowly or just arrive early just wait until later

What's the minimum speed for self driving cargo cars, below which savings from reduced air friction would be offset by other kinds of friction or by inefficient engine operation?

Bonus question: any idea what speed would be a minimum speed (optimized for lowest energy consumption) for a self driving electrified rail?

(I actually think that near future with futuristic, self driving... and often very slow cars is a nice twist, which is technically speaking quite rational)

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    $\begingroup$ Hypermiling strategies will vary considerably based on the type of vehicle and the gearbox configuration. For instance I get great mileage at 45 and 75, but not so much in-between. $\endgroup$ – apaul Apr 7 '17 at 19:59
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The big inefficiency isn't engineering related, it's human related.

Traffic can be a big problem:

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Any fuel savings and efficiency math is best done by working at a consistent speed. Think about it - every time you brake you've converted gasoline into heat. However, any cargo system that operates on a strict passage system that must be shared with commuters will have traffic. There isn't traffic in the ocean because it's wide open.

While the open road will be able to find a good trade-off at around 40 mph (below which drag in air is a linear relationship with velocity instead of a quadratic relationship - this is the answer for your electric rail problem), in the real world the traffic will necessitate constant stops and starts that will kill more efficiency then reducing speeds below current speed limits. (Going to higher than current speed limits will, however, be a major cost killer).

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    $\begingroup$ @Shadow1024 Important to note is that traffic congestion patterns are dependent on how many self-driving cars and human-operated cars occupy the road. If all cars are self-driving, then congestion will be reduced. However, if the majority of cars is still human-operated, then self-driving cars cruising at half the maximum speed on the freeway are going to cause congestion instead. Even if the slower speed is fuel efficient, if you're going to clog the roads with your self-driving cars, it would defeat the entire purpose. In short, you may want to specify how you envision the future. $\endgroup$ – Sazanami Apr 7 '17 at 20:11
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    $\begingroup$ @Sazanami I would have gone in that direction, but he had a near-future tag. Especially in a world where every car was self driving, and roads were self building (or, everyone owned self driving flying cars), this definitely stops being an issue. Especially with flying commuter cars, self-driving cargo cars slowly deploying goods at slow speeds seems a logical extension, but that's nowhere near-future. $\endgroup$ – Mark Apr 7 '17 at 20:59
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    $\begingroup$ @Sazanami: Congestion would still be an issue with self-driving cars, since it's often a reaction to unpredictable events, such as (to take a recent real-world example) someone setting a large fire under the highway. $\endgroup$ – jamesqf Apr 7 '17 at 23:31
  • $\begingroup$ @jamesqf I agree that it won't be eliminated completely, but the majority of traffic jams are the result of unpredictable events caused by drivers themselves. Most notably, accidents, but sudden lane changes are also a major cause of slowed down traffic (at least where I live - drivers simply don't keep enough distance here to cushion the shockwave caused by sudden brakes). $\endgroup$ – Sazanami Apr 8 '17 at 8:55
  • $\begingroup$ @Sazanami: I don't think the drivers are the proximate cause there. Rather, they're in the middle, reacting to those unpredictable events. Self-driving cars would experience the same events and react to them, though their reactions might not be the same as that or the average human. Who, for instance, might brake or swerve if he sees a semi crossing the road ahead, instead of trying to drive under it :-) $\endgroup$ – jamesqf Apr 8 '17 at 16:53
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The tradeoff for slow steaming is the cost of the fuel vs. the opportunity cost of not shipping things faster. In the case of many transoceanic shipping cases, the cost of fuel wins out.

With self driving cars, this is more complicated. People in the kinds of nations which are interested in self driving cars are in a rush, all the time. We're trying to cram everything we can into each day. Such a customer will be completely uninterested in saving a few bucks at the cost of a few minutes.

However, as technologies mature, this might change. If self driving cars open up the possibility of working (on the clock) in your car, the disadvantages of slowness start to fade. Your day may start with you getting in your car, and punching in. Then your car takes a very slow and gas-efficient trip to work over the course of maybe 2 or 3 hours. You then get to work, interact face to face (telepresence is awesome, but will never replace face to face), you spend a few hours, then hop back in your car for a 2-3 hours trip home. Punch out when you get home and kiss the wife (or husband) hello.

Workers who can only do their job when present (such as most factory workers) obviously don't have this opportunity. But as robotics starts to help automate more jobs, we may find that the nature of our jobs change to better leverage this slow-steaming commute. A forklift driver might be trained to take customer service calls during their commute. Businesses might find synergistic jobs which fit well with this half-commuting/half-present work balance.

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  • $\begingroup$ Except that for a 2-3 hour trip, your car will need a bathroom, coffee machine, &c. $\endgroup$ – jamesqf Apr 8 '17 at 4:29

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