In a near future, Humans are reluctant as ever to release the grasp on outdated infrastructure. They end up building a highway that consists of a giant conveyor belt, with 'entry gates' that speed up vehicles enough to enter. The idea behind this is that the belt travels at 70mph while the cars on it also travel 70mph, decreasing travel times while keeping it relatively safe to travel at such high speeds due to the baseline of the belt.

Ignoring cost and energy requirements for such a structure, what challenges would one face using this method? How would a vehicle reliably and safely dismount the belt other than an exit ramp that slows you in stages?

EDIT: I was thinking perhaps something similar to the idea about the train that never stops; a secondary platform that you merge on to which then slows down and deposits you back on to the normal streets -- but how would this work with high traffic density?

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    $\begingroup$ I'm not sure why there's a vote to close this as idea generation. However, I would suggest you change reliably to safely in your second question. Running off the side of the belt at 140 MPH is a reliable way to exit the belt, after all. $\endgroup$ – Frostfyre Jun 9 '15 at 14:10
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    $\begingroup$ This has been fully covered in an existing sci-fi story, might have been Asimov, I cannot remember the title I'm afraid. They used multiple parallel belts at ever increasing speeds as you moved over- so the incremental speed increase was manageable $\endgroup$ – Marv Mills Jun 9 '15 at 14:23
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    $\begingroup$ Heinlein, 1940. "The Roads Must Roll." google.com/… $\endgroup$ – WhatRoughBeast Jun 9 '15 at 14:42
  • $\begingroup$ "In a near future" - I believe you mean "Throughout all of human history" $\endgroup$ – corsiKa Jun 9 '15 at 19:51
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    $\begingroup$ @MarvMills Asimov used something like that for foot passengers boarding trains in The Caves of Steel? $\endgroup$ – Leushenko Jun 9 '15 at 20:02

There is still a danger. When a vehicle veers off the road into a pole they will hit at 70 mph + 70 mph = 140 mph.

Also merging in will be a problem. Relative to the static structures the cars on the belt will be going the 140 while those merging in will come on the belt at 70 (accelerated under their own power). A collision between those two will be a 70 mph collision. They will need to be separated until the speed difference is low.

Then consider traffic jams. The belt will force the cars ahead which is a problem when there just isn't space where they want to go. That's a pileup waiting to happen.

Actually getting on and off will probably be by first accelerating to 70 mph on a static road then coasting onto a new lane with the belt at 70mph and then accelerating again to 70 mph before merging into the main stream.

  • $\begingroup$ While you're at it, don't forget that if these are bi-directional highways 140+140 = 280 mph for a cross over head on collision. You better have some incredible airbags in your car. $\endgroup$ – Justin Ohms Jun 9 '15 at 17:26
  • $\begingroup$ I don't think traffic jams would be a serious issue - all cars are going 70 mph just due to the conveyor belt, so if everyone has to slow down for a jam, they're all moving slowly relative to each other and to the track underneath them. $\endgroup$ – Kevin - Reinstate Monica Jun 9 '15 at 19:39
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    $\begingroup$ @Kevin but jams form at off ramps when the local streets can't handle the volume. So if all people coming in need to turn off on a full off ramp that can't swallow them... well you can guess the rest. $\endgroup$ – ratchet freak Jun 9 '15 at 19:47
  • $\begingroup$ That's a terrifyingly good point o_o $\endgroup$ – Kevin - Reinstate Monica Jun 9 '15 at 19:48

Well to begin, you are still traveling at 140mph. the car is only traveling at 70mph in relation to the belt. so the trees going by on the side of the road are still going by very fast. What happens when there is an accident on the belt? do cars get sent off and pinwheel 20 times through the ditch and surrounding countryside? or do they all somehow get 'kept' inside the belt and bounce around like pinballs into each other?

You are also right, speeding up and slowing down to match speeds outside of the 'highway' would be problematic, especially since you are not going to get all 4 wheels to merge onto a different speed belt at the same time.

I think you'd be better off having a magnetic system, where the 'highway' is static but the 'force' it provides moves, like magnetic pulses.

Asimov had a similar system in the Robot series, but it was for pedestrians, and there were many belts of different speeds depending on how far you were traveling, and the got progressively faster toward the center, but in small enough increments you could do so at a walk. So you might be able to do it with vehicles if they are of the 2 wheeled variety, and you have several belts of increasing speed.

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    $\begingroup$ the 2 wheelers will just flip around when leaving as soon as the front wheel leaves the belt. An unlimited slip differential will allow the wheels of a normal car to rotate at different speeds. $\endgroup$ – ratchet freak Jun 9 '15 at 15:02

Since changing speeds is so problematic, the belts should be subdivided into much finer gradients, so each time you step or drive onto a belt, your speed only changes by 10mph, for example. For a beltway designed to carry cars, this would lead to am improbably wide "highway" (and the machinery for the belt would make the right of way even wider)

Perhaps it would be more realistic to have a beltway sized for people. Each belt could be less than a metre wide per speed gradation, and so long as certain rules are enforced to keep people from being unbalanced and falling across the various beltways (for example, you could not carry a suitcase in one hand, you could become unbalanced or strike a person in a belt beside you. All luggage would have to be in roll along suitcases, or in "shopping carts"), then travel would be fairly safe and secure. Each direction would be enclosed to prevent people from being affected by wind or rain, and at the same time an enclosed conveyor belt like this might also diminish headwinds and buffeting inside the tube with some clever design (the air needs to be moving at almost the same speed as the people, so there will be a very interesting ventilation system inside).


Self-Guided Cars

Well, if this is in the near future, you could always say the cars are self-guided. Then there isn't as much risk of them going off the road or crashing into each other.

You could also say the cars are linked to some central traffic computer that controls the belt speed and the individual cars. That way, if there's going to be a traffic jam or other problem, the central traffic computer can slow all the belts and reroute cars to avoid accidents and such.

If humans want to drive, you can just have them drive on normal, non-belted roads. If they want to use the belts, they have to engage their car's auto-pilot.


I want to raise some maintenance issues with this design. First of all how would the belts be replaced as the wear out, if the belt is just one long continuous stretch, a couple of miles would be out of commission. Secondly. how would the belt be lubed, often times large belts like this need to be lubed, see space shuttle crawler. Also, how often would the belts need to be replaced? After all they are being worn from both sides (the wheels of the car and the conveyor wheels). Another thing what happens to the drivers if the belt snaps, how bad would the injuries be. So the maintenance issues alone would be a huge challenge for the average driver.

  • $\begingroup$ That's a point in the aforementioned The Road Must Roll story by Heinlein. Basically, his solution is to have worker's rights ignored and strike dealt with the death penalty, with the horrible conditions workers have to deal with being a Dickensian unavoidable evil. While many readers today will cringe at it, having the society justifying itself that way (without necessarily the author agreeing) could make for an interesting story. $\endgroup$ – Eth Apr 20 '18 at 14:16

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