Robocars vs. PRT


Readers of this blog will know I used to talk a bit about Personal Rapid Transit (PRT) but have switched to a belief that it is now likely that robocars might fulfill the PRT vision before actual PRT can. To understand that, it is necessary to explore just why PRT has never really come about, in spite of being promoted, and possible for almost 40 years. The Morgantown Personal Rapid Transit has run since 1975, though it uses large vehicles and only has 5 stations, so it doesn't realize the PRT vision of personal cars that go point to point in a network of stations. The ULTra system, with personal cars (which run on tires in a simple track) is being built at Heathrow airport.

I wrote an article on the reasons I have rejected classical, track-based PRT and then opened discussion on it in the Google transport-innovators group. The thread was quite vigourous. I had expected PRT fans to not welcome the concept, and to believe that robocars are still very distant science fiction, for indeed that is a valid objection.

I had not expected such a love of the general concept of shared transit that I would see people arguing that even if robocars were arriving soon, it would still be better to fill our streets with custom elevated guideways for a PRT system. Indeed, some advanced that we should not be building roads at all, that people would give up entirely on vehicle ownership in a PRT or robocar world and that providing garage to garage (or door to door) service was not necessary in the U.S. market, or could easily be done by just running PRT tracks to every house.

I understand the frustration in the PRT world. The ideas make a lot of sense, but no city will buy them. I contend that's because municipal transit planners are highly averse to innovation. They are happy to buy 100 year old technology for their cities. They think farecards and web sites that can tell you when a bus will get to your stop are space-age innovations. Nobody wants to be the planner who bet on an untested technology that failed. That's a career-ending risk. They would rather bet on old technology, and in spite of how well it is understood, see it go 100%, 200% or more over budget.

I predict that, once the technology becomes more real, robocars will win because they will be built bottom-up on a simple, already existing platform (roads) without any requirements to build infrastructure or run it. They will be bought by individuals, in particular by early adopters. Early adopters have money to burn on the latest hot new toys. They will happily waste it and buy the cooler model 8 months later. Cities don't buy this way, they can't. Cities buy technology that's already obsolete before they even put it out for bid, and it's very obsolete a decade later when it goes into operation.

Worse, transit requires monopolies. Either the city runs the transit as a monopoly, or it grants a franchise to a private company to build and run it. (That's far more rare, since most transit runs with heavy subsidies in the USA.) Monopolies mean corruption (as they get large, they end up having more influence on the city officials than the customers do) and they mean monopoly-style customer service.

While robocars are still over a decade away, I fear that even though PRT could be built today, it will take it a decade to get over the marketing humps it has not managed to overcome in 40 years. By that time, robocars should be much closer to reality, and we'll reach a point where even a transportation planner will realize the robocars will arrive soon enough to affect transit planning in the present.

Rather than being viewed as the enemy, robocars should be viewed as a way to realize the PRT vision without those deal-blocking new infrastructure requirements. But the PRT community is not yet ready to agree.

Read Robocars and PRT


Municipal planners may be entirely justified in their reluctance to buy PRTs. Even if PRTs in general take off, the chances that the very first widely deployed PRT system won't be completely obsolete in a decade or two are very small.

Some of the people on the transport innovators group seem to be a little, uh, off-kilter. Like, not understanding some basic physics.

Really, a road is just a very specific kind of rail, which happens to be very versatile but not provide very much guidance.

In 10-20 years, almost surely. But then one has to consider what they are buying, namely light rail, bus rapid transit and in some cases heavy rail (which is what BART is.)

The PRT vendors say that their PRT lines will be vastly cheaper per mile and per station than the traditional lines, which makes some sense since the lines are intended to support only small, lightweight cars, and that's much cheaper than "light" rail, and the stations are thus smaller too. Their main extra expenses are the need for offline stations (ie. extra line of track some distance before and after all stations), many more cars if you want the same peak load, and the new control systems.

Of course, since even traditional transit line builders can't seem to estimate the cost of their lines, nobody is ever going to trust the predictions of a new technology until it proves itself a few times, which creates a chicken and egg.

But indeed, the one thing you can build that won't be obsolete is roads. Right now roads promote gasoline burning cars, which will make them lose some favour. As we get efficient cars, whether it be by robocar or by battery breakthroughs or some new fuel, that will change.

That raises the question of why nobody's designing a PRT system which runs on traditional light rail lines - it would just be small cars with hopefully more extensive lines, but basically get the advantages of a PRT with the same sorts of compatibility benefits as roads.

What stories have you heard about why BART uses wide rail? A bit of web searching turns up the general opinion that it was and continues to be a disaster, and the story that it was supposedly better for keeping cars from getting blown over on the never built golden gate bridge extension. You say something about there being funny stories related to that in the newsgroup discussion.

I doubt PRT would ever run on light rail tracks. Getting a robocar to follow a preset path painted on the ground (or a series of magnetic spikes dug in the ground) is essentially a solved problem. The hard problem is assuring the vehicle won't hit anything.

In that case, rails gain you a modest improvement in rolling resistance, which is not worth it.

What can be worth it with rails would be access to simpler electrification. You can do electrified tire vehicles (like trolley buses) but it is a little easier on rails. But rails have all these downsides -- you can only go where they go, if something is stuck on the rails you can't get around it.

So most people in PRT are very big on elevated guideways, which they believe would be cheaper than roads or rails-in-roads because they are only for light vehicles. They also assert they would be narrow (or have a screen gap between the rails) so as not to block light or the view. I think they underestimate the objections the public will have to overhead guideway vehicles, not just due to visual obstruction, but also sound (especially on rails) and people looking in 2nd storey windows. Elevated guideways of course allow you to build a safe system today, without robocar technology, and have meant this for decades. However, they have been a much harder sell than PRT advocates imagine.

As for broad gauge, the rumour I heard was that BART was negotiating with Union Pacific to have them pay for a share of the transbay tube. UP, it is alleged, decided that they need not pay up front, and could just rent passage through the tunnel when needed. BART, it is suggested, out of spite switched to broad gauge to make sure that could never be true.

Broad gauge does give more stability in the wind, it is argued, and for the general ride, but just about everywhere in the world decided over a century ago that it just wasn't worth it considering the advantages of standard, which included not just interoperability, but a worldwide market for parts and rolling stock. One of the reasons BART is so expensive is that it must use custom equipment.

BART's other problem is that it is 3rd rail, which means it can't have grade crossings and must be fenced off when at grade. I think with modern technology we might be able to design a safe 3rd rail system for at-grade, which would use both GFCI and smart circuits that only activate a section of rail when receiving a command from a train. (Seems it should have been possible to make a rail that only activated when the weight of a train was on it, but I don't know if anybody ever did that.)

I think this is the first time I've ever heard someone say that non-grade-separated rail design is *preferable*.

If nothing else, the Houston Metro ought to show why grade separations are a good idea.

At-grade is vastly cheaper. If you can do it safety with new technology, it means you can do it for far less, and that means getting a lot more for the same money. The choice in most cases will be to have the at-grade system or to have nothing. If you have $1B for separated and that will buy you 5-10 miles at today's prices, or hundreds of miles at grade, which are you going to pick? And in many cases, the at-grade is already there.

I will also add that steel on steel is typically quite noisy. While pneumatic tires are slightly less efficient in rolling resistance, they make a smoother and quieter ride.

I've tried to do work on the NJ Turnpike (one of the smoother and better maintained roads in NJ) and on the Northeast Corridor and other rail lines, and I found rail to be decidedly smoother. Granted, this may be at least partly the result of the pretty snowy January, but, still, it seemed to me that rail made the better ride (and this includes light rail, in the form of the Hudson Bergen Light Rail Line).

Yes, to be more correct, what I mean is that tires make a smoother ride on the same surface. The rails themselves are usually much smoother than road pavement, and thus the steel wheels on rails ride is smoother. A tire ride on pavement as smooth as rails is would be best.

I have optimism, as yet unproven, that computer controlled suspension might allow very smooth rides on any kind of surface. A real time laser scan of the upcoming service would cause control of magnetic suspension for example.

Can't agree with you about robocars, Brad. Think of the liability. Roads are just too uncontrolled an environment. I still say that roads are important and cars are important, but so is an electric infrastructure, and that means rails. is a form of transit which doesn't require a monopoly (anybody conforming to the standard can build a guideway). Yes, it needs new infrastructure, but in places like India, they have NO CHOICE but to build new infrastructure because their roads are completely clogged. Drive in Mumbai or Bangalore if you want to see what I mean. You'll also see why robocars could never survive in that environment. The first time a robocar killed somebody, there would a a HUGE hue and cry to have them banned from India. As it is, if you cause a fatal accident, there's a very good chance that you'll get strung up from the nearest lightpole.

Because cities are not going to tolerate multiple companies laying track down the same streets, and it's unlikely they would have differing companies on adjacent streets very often. Like phone and cable companies, there will be only one choice of track on your street. This is also how standard rail works, with deals among all the interconnecting rail companies. You would want them to all interconnect which means lots of government regulation of standards etc. Not conducive to innovation.

I do address the liability question. I do agree that people are more afraid of being killed by computers than by other people. So they will delay adoption. But some place will adopt them, one without a liability system like in the USA, and they will get so safe that people start comparing having computers kill 100 people vs humans killing 10,000 and even the most fearful will give in. We let computers fly our planes, drive our people movers and run our elevators.

Dual mode PRT is a better plan than single mode, but RUF still has to get over the idea of installing all these rails, which means it's a city thing.


I'm one of the TI group PRT people with whom you debated.

My basic point is this: streets are already full, and robocars will not change that for 30 years at least. Furthermore, I am skeptical that, even in the distant future, robocars will be able to achieve the capacities that will be required while running on city streets. So even if robocars arrive, we will still have clogged streets unless there is some sort of off-street solution that can relieve the capscity crunch.

Elevated PRT would provide additional capacity within 5 years, and do so in a way that is safer than robocars will ever be. No matter how advanced the technology, a robocar that mixes with pedestrians and bicyclists cannot ever be as safe as a dedicated solution, without robocars sacrificing capacity by running very conservatively and slowly.

So even assuming a robocar future, PRT today still makes sense today. In the future, assuming robocars take hold, a dual-mode robocar/PRT vehicle will be able to take advantage of both on-street and off-street capacity, relieving the streets of a significant portion of the 10-lanes of robocars that will be required to support capacities in the busiest corridors. The elevated guideway need not be a long term waste, even as it solves a vital short term need.

As for the claim that PRT proponents have "a love of the general concept of shared transit", all I can say is that in my case it has nothing to do with a love for shared transit and everything to do with a hatred of the effects of private transit. I love private transit, but I hate the fact that our cities are clogged by tens of thousands of private vehicles parked every day. I hate waiting in traffic whenever I have to go anywhere. I hate that my neighborhood is bisected by a 6-lane state route that is bumper-to-bumper with non-local traffic. I hate hearing about some child killed trying to cross that 6 lane route on his bicycle.

PRT can resolve those issues today, while preserving private transit 100% since PRT does not touch the street. All PRT does is give people an option to leave their vehicles at home for many of their trips, or perhaps use a local park-and-ride avoid clogging up the entire route to their destination. And the guideways would still be useful in a robocar future, so why the objections? Your issues with monopolies ring hollow given that we've lived with local infrastructural monopolies for 100 years as a necessary evil to provide a service that could not be provided in any other way - phones, electric power, cable TV. Why not transit?

It is an interesting question as to whether one can solve traffic congestion with capacity, either with roads, or PRT. Urban designers now know that if you see congestion, and so put in a new freeway, it doesn't take terribly long before that freeway is clogged. Initially traffic that was on streets diverts to it, and eventually new housing is developed along it which fills up the capacity. At least that's true in a growing economic region.

This is less true for transit than highways because transit is not as convenient, but transit as good as cars, as PRT hopes to be, should have the same result. Put in a good PRT line and people should start using it until it overloads, and starts having waits. I know PRT companies say that their lines will be so cheap that this can be solved by just adding more cheap lines everywhere, but this has yet to be seen.

Robocars would eventually face the same problem, though the actual capacity of our roads if we don't fill them with human driven SUVs carrying one person is quite a bit higher than today's, so the gap is long.

I have speculated -- though nobody can really predict this well -- that PRT or robocars might actually alter this problem by causing downtowns to spread out, as distance from "hubs" stops being an issue for many businesses. In the PRT or robocar world, why build your cinema in expensive downtown real estate if everybody is coming to you via PRT or robocar, and one location is as good as another. You would only need walkable clustering if you expected people to go to multiple destinations in your cluster and want to be able to walk between them. This is what a shopping mall wants to be. Most office buildings, however, would have no particular need, other than prestige, to be close to the "core."

(BTW, while "transit" literally means going from A to B, in common use it means public transit, ie. publicly owned transit. So PRT is transit but private cars are not. Taxis are hard to classify.)

PRT is a fee-based system, so high demand is not as much of a problem as it would be with roads, because you can raise user fees to reduce the demand. If the PRT guideway is running at capacity with $1 fares, try increasing to $1.25 and if you're still running at capacity then you would be generating 25% more revenue - making the case for expanded guideway more economically and politically feasible. Except for toll roads, autos don't have this revenue-generating demand throttle.

As for the idea that demand will expand to meet supply, that is likely true, but is that a problem? If PRT is so successful that previously low-density areas become more dense with new housing and commerce, this is sprawl in reverse. In the extreme case, demand and density grow so large that very high capacity systems become economically feasible - e.g. metro rail lines, underground PRT/GRT, etc. I view this as a good thing.

As for your last point about PRT increasing sprawl, I think this can be managed with good city planning. If the initial PRT is mainly confined to dense core areas (perhaps with a few lines branching out into remote park-and-ride lots), then those core areas would tend to grow fastest. Later, the system can expand out naturally as core density grows. If you start out with a sprawling low-density network, you may indeed increase sprawl, but it doesn't have to be that way if you're smart about focusing the highest density of guideway and stations in the city core.

Well, a government or monopoly run PRT would not be able to raise prices to the level of outlandish profits, without public acceptance of that funding something like a tax. Of course outlandish profits have hardly been a problem in transit systems. PRT might not increase density. Several of the PRT companies are promoting rail systems they claim do 120mph or even more on longer haul routes. This would not increase density.

Density of course, is not some sort of pure good we seek. In fact many would argue there is an optimum density, which is denser than suburb but not as dense as skyscraper. Some suggest an average height of 3 storeys. Higher densities seem to increase crime rates (though there may be other causes), litter/vandalism rates (because the more people share a commons, the less each individual cares about trashing it) and even sociability. In a 40 storey high-rise appartment you are one of a giant crowd, your neighbourhood is too large to be cohesive.

Density largely increases walkability, which we like for social and environmental reasons.

However, I predict that both PRT and robocars would change the character of urban density and concentration, though I think it is too early to tell just how. The reason for this is that the PRT and robocar journey can be a "working" journey, where you can read, use the internet, watch videos etc. In that situation you can consider that everything within 5 miles is "adjacent" to you, with a travel time under 10 minutes, little of it wasted.

That has both upsides and downsides. I do think that both types of vehicles can deliver energy efficiency and pollution levels low enough that we don't worry about the environmental side of it. It is the health and social aspects that are more interesting.

Like everything else, markets will control our land use patterns. With robocars, any location with enough people within 5 miles can be a "local" location to those people for shopping or entertainment. If the land is significantly cheaper than in the dense core, where to locate will be a no-brainer. Location will no longer be about proximity to transportation, but instead about things like views, crime rates, cost and some amount of walkability to cafes and restaurants.

Especially when anything you need to shop for can come to you by deliverbot unless you have to go and physically browse.

I mostly agree with your last comment, but I disagree with your point about density. I don't think there's a limit on density, and if there is, I don't think we've reached it. Judging by the exhorbitant real estate values in some of our most dense cities (skyscrapers and all) I think people are more than willing to pay top dollar to pack themselves into tight spaces, just to be in the middle of it all.

Similarly, there is no such thing as minimum density - some choose to live on a 5000-acre ranch in Montana. And between those extremes, there is a continuum of preferences.

But the problem is that suburban sprawl requires automobiles, and automobiles limit density of the core by increasing the need for freeways, wider roads and parking. So people buy their suburban dream house with two-car garage and commute into the city to work; soon the roads are overburdened and the formerly smoothly dense city center is soon slashed by freeways and pockmarked by parking lots. Neighborhoods decay, more residents move to the suburbs, and the cycle continues.

As an analogy, some types of malignant tumors grow so fast that they actually starve out their cores - the outer layers expand so fast that there isn't enough blood supply to the core, and the center dies, and the result is a hollow ball of a tumor with live malignancy encircling dead malignant tissue in the middle. This is what happens with cities that expand too fast and rely on automobiles too heavily.

So the problem is not sprawl per se, it's sprawl coupled with over-reliance on the automobile. But efforts to artificially limit automobile use in the city (e.g. tolls, reduced parking) only accelerate the decline because people need the freedom of cars - stop building parking lots and you only accelerate movement to the burbs. The hope of PRT is that it can provide that freedom without the negative effects of autos, and (more importantly) that a larger percentage of people will accept PRT as a replacement for their private cars. Eventually, the hope is that PRT is so popular that parking needs actually dwindle, and all those half-empty parking lots become too valuble to waste on parking.

Are these valid assumptions? We won't know until it happens (if it happens). We already know that buses and trains can't do it unless they reach the level of coverage of Manhattan, but PRT is much closer to autos than buses or trains - is PRT close enough to autos to get people to leave their cars at home? I think so, but again we can't know until it happens.

So back to density: the automobile inherently limits density and destroys density, but people will not accept limiting auto use without a viable alternative, so density is effectively capped in our auto-centric cities. PRT aims to be the alternative that can provide near auto-like service with no density-destroying side effects (parking, freeways), meaning that neighborhoods can be as dense as they want. There will no longer be an infrastructural limit on density, and density can expand to meet the needs of the city rather than shrink to meet the needs of the suburbanites.

I don't say there is a minimum density, or that we are over a maximum density. I speak of an optimal density, but that is different depending on what you are trying to optimize for.

There is a minimum density to support shared public transportation, I think. And for each person, there is a maximum density beyond which they don't want to live there.

People like to be downtown to get quick and easy access to a variety of restaurants and hangout places, interesting shopping, and other people -- and transportation. If they can start getting those things other ways, I predict a change in their view of optimum density. That's because, as I said, density comes with some negatives, more negatives than just the difficulty of serving it with private cars.

I think if you examine flight to the suburbs, "I could not longer get around in my car in the dense area" is not that high on the lists. I think the #1 reasons are related to children. Yards to play in, streets to play in, lower crime and the self-reinforcing factors of better schools and similar child-raising neighbours. In fact many who flee to the suburbs resent having to use their car to do everything where before they could walk.

I now see what you're getting at - that optimum density in a PRT world may well be less than optimum density in today's transit-centric city. I think that's a good possibility, since there will no longer be a need to pack people in to make high density transit feasible.

My point was that PRT would not artificially reduce density with destructive infrastructure, as cars do. It may well be as you say, that PRT cities may be overall less dense than the typical transit-dense city (New York) is today. But it will be a density limited naturally by local preference, not imposed by the inherent density-reducing infrastructure required by the automobile.

And even though I generally support the idea of a robocar future, I remain concerned that an exclusive robocar-on-the-streets approach will continue the current pattern of neighborhoods being clogged by non-local vehicle movement and endless parking requirements. Public vehicles on elevated guideways solve the density issue regardless of whether it's human driven or robocars on the streets below them. Robocars and PRT are complementary, even synergistic.

Let me first address parking. Today, the city subsidizes parking greatly in several ways.

  • First, they provide free or cheap on-street parking for anybody on most streets.
  • They often provide free or cheap city owned lots.
  • They provide free parking to residents of congested areas by offering special permit stickers that allow long street parking.
  • The provide building codes that demand that houses have garages, or that new stores or residential towers have lots of parking. (Sometimes however, they do the reverse, and recently some cities have been insisting the residential towers have only 1 spot for 2 units.) Where parking is required, those without cars are still forced to pay for it.

Of course, we drivers love free parking.

In the robocar world, I think parking problems go away. I should write a special piece on this:

  • Just about anywhere is usable as a parking spot because by today's definitions, robocars never park, they just stand. They are never driverless. They can happly stand in front of fire hydrants or blocking driveways. On command, they move out of the way.
  • Likewise, robocars can double park and triple park, and again, move out of the way on demand.
  • Robocars will always park at valet density, or even denser, in parking lots.
  • Robocars don't need to park close to their riders destinations, and in fact can park quite far away if the next trip has sufficient advance notice or is in fact even roughly scheduled the way most commutes are.
  • When lots of robocars are stopped, by definition there is not much traffic. In these low traffic times, robocars can double park on the streets, narrowing them -- which is fine due to the lower traffic. At rush hour, all robocars will be in use, and on-street parking will be forbidden to those not in use.
  • Smaller robocars, notably 1 person robocars, should be able to park tightly -- three in a typical non-compact parking spot. Valet parked, perhaps 5 times as many vehicles as an old-style lot.
  • In the future, robocar-only parking garages could have 5' ceilings, more than doubling capacity.
  • Alternately, cars like MIT's proposed city-car, which can stand up vertically when parked via stacking or some jacks, could further increase density.

All that adds up to a lot of parking. So much that I would advocate giving up free parking, and instead selling parking in live auctions. I think it would be cheap, and if it got too expensive, robocars would just move outward to where it was cheaper. City owned parking could be sold with 2nd price auctions. 2nd price auctions have the interesting result that if there is only one bidder, the 2nd bidder's price is thus zero, and the uncontested resource is provided for free, or some base monthly fee. As soon as capacity is reached, however, people pay, and the price goes up quickly to a fair market price for the contested results.

We'll buy software for our cars to let them shop the market for parking. And if there is not enough parking, everybody will be getting in the game. People will rent out their driveways -- why not, it's not exactly doing anything when you're not there -- to vehicles that will swish out of the way before you even get home.

Not only that, lots will rent out their spare spots in a "spot market." Today, each parking lot tries to be sized to meet the peak demand of the buildings it is servicing. Robocars can spread out the demand over a very wide area, so now you have a pool of parking lots all serving the entire area. Vastly more efficient than today's system.

Finally, I think a lot of people will switch to robotaxis. Which don't need to park at all in the traditional sense, though they do need places to wait during low demand. All this removes the parking demand from those who give up owning a car or robocar.

I think with all this, there's so much parking available that we'll see parking lots being demolished and turned into other uses, until the market price for parking goes up enough to justify using land that way again.

I do agree that robocars will more effectively use parking space, but it will not solve the problem completely because you still need space to park all those tens of thousands of privately owned robocars. In a uniformly dense city you'd have to bring the robocars all the way back out to the edges of the city or into the suburbs to find space - or keep a significant portion of those lots in the city core.

Parking today is pretty efficient in terms of space usage per vehicle, and I don't think robocars will significantly improve on that. Parallel parking probably only wastes 10-20% of vehicle space; for lots and ramps, it's probably more like 40%. Even if a significant portion of robocars are half-sized vehicles and they are able to use 100% of the space, I don't think you could do much better than perhaps doubling parking capacity in the currently allocated space.

And I don't see double and triple parking as workable for privately owned vehicles, because every time someone leaves you'd have to reshuffle the outer vehicles. That means potentially 2-4 robocars moving out of their spaces to allow an interior private robocar out, and those 2-4 robocars will now have to find another spot, or circle around and re-park.

Now consider the beginnings of the afternoon rush: those on 6am-3pm shifts will require their cars right at 3pm, but their robocars are buried 3 cars deep behind robocars from later shifts (7-4, 8-5), so you'll have a large amount of vehicle shuffling at the very time when you need the most capacity - during the rush.

For these reasons, I think private ownership of robocars should be, at best, a premium option with much higher usage and parking fees. If private vehicles are to be allowed at all, they must be priced to reflect the additional burden they place on the city relative to public vehicles which can each make 6-10 trips during the rush (thereby reducing the volume of parked cars by up to 90%). I have this same view of privately owned PRT/dual-mode vehicles: it should be discouraged or forbidden.

Take New York City as an example: you yourself said (on the TI group) that many rely on taxis for transportation in New York. Taxis are today's equivalent of tomorrow's publicly owned robocar from a functional standpoint (not an ownership standpoint, but we'll get to that in a minute). The reason taxis work in New York is that (a) the streets are largely kept relatively clear by underground transit, and (b) they don't require parking. Imagine if all those taxi riders in New York required private vehicle parking - it couldn't work.

And when I say "publicly owned", realize that I mean "shared, not individually owned". There could be privately owned "robocar companies" like cab companies that lease out vehicles and maintain them. This would remove the individual passenger-to-vehicle link that prevents vehicle sharing and complicates parking. With robocars, true door-to-door service with shared vehicles would be possible, as with taxis today.

So perhaps a robocar future with a high percentage of shared vehicles would alleviate the parking crunch, but not if most vehicles are privately owned.

But, again I reiterate, this is all 30-years in the future at least, so PRT investment today is worthwhile.

Yes, on-street parking is more efficient than parking lots because the street is the access, but it's not as efficient as you think. In particular, the presence of driveways, hydrants and the rest constrain things, leaving some gaps no modern car fits in, and other gaps not quite long enough for two. On average, every inter-driveway gap will waste half a short car of space, but those gaps are wasting much more.

Valet lots always get a rough idea of when people will want their cars. They arrange the cars so that those cars leaving first are on the outside, no matter when they arrived. Triple-parked robocars will do this, but actually do it a lot better as it is no "work" to do rearrangements when needed, just a relatively tiny amount of energy.

A large fraction of parked cars have a roughly scheduled time they will be used again, especially when parked for work, or meetings, or movies or even meals. "I have no idea how long" is a minority of the cars. I expect clever grad students to work out wonderful optimal algorithms for arranging them.

However, the other reason there is not a problem is that we already have more than enough parking for the current car load, by definition. (Yes, the price of parking is enough to drive some to transit or carpools, and those people will be the first to switch to robotaxis or keep using transit.) So when you start with enough parking for today's cars, and you add the huge increase that robocars can bring, I am not worried. If areas get denser (instead of less dense as I am tentatively predicting) the market for parking will sort itself out, especially if cities stop providing free parking.

New York is the most challenging place in the USA, but I think it will work. All those taxis don't vanish when not doing fares. They are either cruising for fares or standing somewhere during lower demand times. A car cruising for fares takes more space on the roads than one sitting standing in a roadside space. Inherently you can handle more cars on the road stopped than you can moving. Again, if there is no demand for the vehicles, they just take over a new lane of a road, a double parked lane. It does not hurt that they took away that lane of capacity because obviously we are at a low demand time or these taxis would be at work moving people.

The subways in New York won't go away, of course, at least not the tunnels. At the very least they will be turned into express robocar conduits, or may well still run at rush hour.

I think it comes down to a basic disconnect between our assumptions: you believe that automation will have a large effect on capacity (both street and parking capacity), whereas I believe that the effects will be limited.

Do you have simulation experience? It would be interesting to see a robocar world in action using simple 2d grid animation. My own personal skepticism is based on a belief that the combination of limited space and the requirement to mix with pedestrians will ultimately limit what you can do in the street, especially considering that some of the most difficult issues are most prominent at the busiest times. I would be less skeptical if you could demonstrate these gains in simulation (which should be achievable, since I'm concerned only with dynamic movement, not detection and identification of obstacles).

But my gut feeling is that privately owned robocars will not solve the capacity problem, in traffic or parking. A few other points in direct response to your last post:

  • vehicle arrangement - people are unpredictable. I think it will be difficult to predictively arrange double-parked vehicles in a way that minimizes the need to move outer vehicles to release inner ones. Again consider 3pm, the beginning of the rush: you have high pedestrian movement, a large amount of vehicles parked, and a traffic surge, all at the same time that the need to get "buried" vehicles is at a peak. I think this worst case scenario will ultimately limit capacity. I also disagree that the vehicle movement will not be disruptive - if they are really packed in, vehicles will frequently enter traffic to allow an interior vehicle out - again, a simulation would help to resolve this question
  • "clever grad students designing algorithms" - algorithms do well when input is predictable, but humans are inherently unpredictable. Perhaps you could make it part of the human-robocar interaction, where the human actually tells the robocar how long he expects to be? That could work, I guess, but people change their minds. A system couldn't rely on these predictions (what happens when the football game goes to OT and 40,000 people are 1.5 hours later than expected?)
  • "we already have more than enough parking for the current car load, by definition" - yes, but at what expense? How many old city buildings have been torn down for parking? I don't view the status quo as acceptable here. Our cities have been decimated and pockmarked by the automobile - we need to work to reverse that trend, not introduce new technologies that will maintain or enhance the trend. And I remain skeptical that robocars will provide anything more than incremental capacity gains using only the currently allocated space to cars, meaning that a robocar world would merely sustain the current trends, albeit at a lower pace (simulation could prove me wrong here)
  • "taxis don't vanish" - actually, for all intents and purposes, taxis do vanish. If a taxi makes 20 trips per day (2 per hour over 10 hours), and assuming people make round trips, that's 10 private autos removed from the street for every single taxi. That makes the taxi 1/10th the parking space cost of a private vehicle. Shared robocars and PRT would get the same benefit. This is the basis for my insistence on shared vehicles.
  • subway tunnels as "express robocar conduits" - this is precisely what I would suggest for PRT guideway - it can become an express robocar conduit in the future, therefore not wasted
  • The bottom line, I think, is that we have differing assumptions. I feel that robocars will not provide all the gains you claim they will, and even if they do, I am concerned that they will continue the current trend of clogged streets and density-destroying parking lots. I'll keep an open mind on robocars, but I won't abandon promising solutions today based on something that won't arrive for 30 years, if at all.

    All we could simulate today would be not very smart algorithms. For parking lots we could probably simulate. I don't think the algorithms used by Valets in valet parking lots show any particular genius. When I got to the valet lot, they ask me when I plan to leave. Even when I am off by quite a bit, it's never much of a problem, and they pack those lights so tight you can barely walk between the cars. Part of the algorithm is to make both the cars leaving soonest and the cars with high uncertainty less blocked than cars with higher certainty.

    Different from the Valet world, I expect the user to signal on their cell phone when they wish to leave, and that this will often take place several minutes before they get to the street. Most people have this sort of advance warning leaving offices, restaurants, parties and shops. Morning commuters know when they are finishing their breakfasts, and they usually have a pretty close idea of departure time in any event. In many cases events will be on the person's digital calendar. Even if the calendar is wrong and the event runs late, as is often the case, it just means their car is more available than it needs to be for a while, no big whoop.

    On a street, first we would see a lane of cars that fills the street. When it is time to add a 2nd, double parked lane, the parking controller computer on that street (just a local computer though it no doubt talks to the a more general parking controller) would look at the estimates and uncertainties and do the double parking as follows. It would find the car in the curb lane which is leaving soonest, and tell that car to move to a double parked position. The newly arriving car, which is most likely leaving later, would then take the vacated spot. As new cars arrive to double park, in most cases a car leaving sooner would vacate a curb spot and move to the double parked spot. If somebody needs their car in a hurry, then at most they just have to wait for the car that is blocking them to move out temporarily.

    In fact, in the vast majority of cases, the double park lane will not be full. It will have at least one gap in it. In fact, the parking algorithms might decide to always leave one gap in all lanes, except the curb lane, even if doing quadruple or triple parking.

    If there is a gap (long enough for the longest car stored inwards) then a whole chain of cars can move together, forward or back one space, to move the gap so it is positioned for the car that needs to leave. Then they can move again so that the car with the longest predicted stay can move into the vacated spot, it can be kept vacant and another car can enter.

    This algorithm allows very dense parking and in fact if you leave these gaps you don't even need to prioritize the cars by departure time because any car can get out in just a few seconds no matter how deep. It took me just a few seconds to think of this, and I am sure that people looking at the problem harder will find solutions even faster.

    I am not endorsing the status quo. I am just saying we have enough, and robocars will give us more, and as they do, we can convert old parking lots to something nicer, or add more pedestrian areas to streets and so on.

    PRT guideway can only be a robocar conduit if it's more general, like ULTra. Many planned PRT guideways, such as suspended monorail guideways, steel truss rails etc. would require very special robocars for use. While people would probably buy such cars if it got them a faster trip, there would be extra cost.

    For intra city transport I think that the key is to use a hub based transport system. Every type of transportation technology has distance range over witch it best suited. I would like to suggest that for intra city travel 3 distinct modes of movement should be used. Walking is ideal for distances of 1 to couple of 100 meters ( now of course walking for much longer distances is easily done just more costly in terms of time.) Next comes bikes / electric bikes / electric scooters / segways? which are good over the distances of ~100 meters to ~ 5 kilometers (~ 3 miles) (again you can go further but it becomes more costly in terms of time) Finally you get buses which would be used for distances of ~ 5 kilometers to ~ 100 kilometers (the width of the city).

    Depending on density you have a transport hub in the center of a circle with a two to five kilometer radius. You have enough hubs so that everyone in the city is at most 5 kilometers from the nearest hub. Buses only travel between hubs and they only pick up and drop off people at hubs. To get from from the hub to your destination you are a member of a bike / scooter / segway share program that has big depots at every hub and little depots on just about every block.

    So a commute would go something like this; you walk out of your house to the e-bike depot at the end of the block. You hop on a e-bike and ride the mile to the nearest hub (time from house to hub less than 5 minutes). You drop off the e-bike and hop on the bus to the hub nearest your place of work ( with fairly regular travel patterns it might be possible to optimize the the bus system so that you go directly to your destination hub without stopping at other hubs along the way). At the destination hub you pick up a different e-bike and ride to your work place which has an e-bike depot.

    I think this type of system would be efficient, fast, flexible and inexpensive.

    You could not get a transit system like this today (even if it were a good idea, which I am not sure it is) because it doesn't deal with the disabled, or others unable to walk to the e-bike depot or ride the e-bike.

    It also does not work for shopping. To work in bad weather, you would need not bikes, but some sort of sealed vehicles.

    Wait.. so a technology not yet available should be seen as a stepping stone to one that is?

    And why exactly will RoboCars be easier to sell than PRT? Nobody is worried about PRT jumping a curb and killing a bunch of people because of a software bug.

    Yes, that is the leap. A not yet ready technology could indeed spell doom for a simpler, but also unimplemented technology. Happens all the time in high-tech.

    Robocars will not get on the road until they demonstrate their safety of course. And while that's something that seems, and is, further away technologically than dedicated ROW vehicles like PRT, PRT faces all these non-technological obstacles. They are outlined in detail in the relevant article.

    The first working (and certified safe and legal) robocar can be bought by any early-adopter individual. And on day one it can go everywhere that has accepted the legality, which would certainly mean everywhere in a city, for example. The first PRT has to be bought by an innovation-averse city transportation board, and on day one it can take you only between PRT stops.

    Thus the difference.

    I share your sentiments that robocars are inevitable, but diverge from you in the short term implications, and think the final model you propose makes too many sacrifices for the sake of simplicity.

    A long term solution that includes narrow, cheap, elevated guideways is preferable for a number of causes, with only minor drawbacks. You mentioned many of these advantages, but seemed to have dismissed them.

    • Energy Efficiency - via lower rolling resistance.
    • Energy Efficiency - via no stopping due to pedestrian interference.
    • Energy Efficiency - via lighter weight vehicles due to battery size reduction.
    • Energy Efficiency - via lighter weight vehicles due simpler suspension.
    • Cost - via cheaper $/mile for additional capacity.
    • Cost - via cheaper $/mile where elevated track is necessary/desirable.
    • Visual appearance - via narrower guideways where elevated track is necessary/desirable.
    • Parking - if everyone owns a vehicle that includes it's own propulsion, storage space is large.

    So, the solution I would advocate long term is a mixed system with the following components:

    • A rubber wheeled robocar "sled".
    • Both public and privately owned "compartments".
    • A PRT guideway network, probably a suspended system.
    • Some single mode PRT vehicles.
    • Some walk-up single mode PRT stations.
    • Some dual mode PRT "carriers".
    • Some automated dual mode PRT stations.
    • The existing road infrastructure.

    In this system, in urban areas the sleds would probably be operated by a public utility, or via groups of private service companies. Private sled ownership is feasible, but would generally be discouraged in urban areas due to parking. In rural areas it may be common to have plain robocars, since guideway may not be common.

    The benefits of this system confer mostly to urban environments but in addition to those mentioned before would also include:

    • Walkability - Too much robocar would exacerbate the unhealthy non-walking culture. Single Mode PRT stations however would preserve walkability while also providing cheap, efficient, fast urban transport.
    • Road reclamation - Some local streets could be converted from thoroughfares, to grass avenues, and only rarely used for local delivery access.
    • Road reclamation - Some elevated highways could be removed after replaced with guideway, allowing broken neighborhoods to reunite.
    • Reliability - Suspension/engine/navigation ownership requires personal maintenance, which if neglected could pose risks others.
    • Reliability - Suspension/engine/navigation ownership requires personal maintenance, which could be a personal hassle.
    • Cost - Suspension/engine/navigation ownership would be more expensive than a share infrastructure.
    • Parking - Personally owned compartments could be stacked in ultra-compact automated garages. Full robocars would be more difficult by requiring larger amounts of space, special care in handling wheels, and vastly larger weight (most of a vehicles weight is in engine and suspension).

    For all these reason this is a more logical long term model. In addition because a Single Mode system is a component of this system, investing in it in the short term while robocars are completed is still a logical step. The dual mode components I would imagine entering the system around the same time as the robotic control itself.

    You could read more about my early musings on such a system here:

    I was thinking that hybrid robocars might be a better idea than road-only robocars because it is not one infrastructural system, but two, that are commonplace - not just roads, but also rail. Furthermore, the robocar would be able to extend passenger railroads' reach by essentially eliminating the train station (or at best rendering it redundant), as well as sidling up promptly to the train stations (for the traditionalists, anyhow). This measure would require building tracks on the sides of rail mainlines, and a method for connecting robocars to trains so that passengers would be able to go between vehicles safely. (If there are too many pax or if they are too slow, they could slow down the vehicles, or detach from the train and chase after it to try again on the next spur. Warning would be provided and the doors would shut before detachment.) The idea would be for robocars to sidle onto the railroads and run down the rails, deposit passengers, pick up passengers, and go either to the destinations or to home base, presumably the old train station. I furthermore suppose that this opens up the possibility of new highway vehicles of a similar nature to railroad trains, in which case the robocars attach to what is essentially a large robobus and allow passengers to get on and off. Once aboard, people would be able to shop, chat, socialize, do business, relax, and have fun. Once they near their destination, they would disembark to a robocar, whereupon they would head off to their desired doorstop. Robocars would then go either hunting after new passengers or back to the robobus or robotrain areas.

    This sort of system might help make so called boondoggles such as Amtrak more user-friendly by improving flexibility and allowing rental cars to show up at far-flung stations without any need for someone to drive them.

    But the management of railroad tracks is currently quite archaic. Headways (allowed distances between vehicles) are massive, many minutes, and all designed around trains which can't stop in time if the engineer sees something on the track.

    Small light vehicles of course could stop quickly (even more quickly than cars can if they can squeeze the rails) and also accelerate quickly. And if designed for it, leave the rails if there is a place for them to go. But there is still the issue of a vehicle stopped on the rails and what to do about it if a train comes, and getting rail lines to adapt to the idea of small vehicles using the rails.

    Lots of discussion about parking, but in a robocar world, why do you *need* a personal vehicle? Why not just rent one from a central facility?

    Indeed, "personal vehicle ownership" could become an expression of ultimate luxury--"he's so rich he can afford his own car!" That would solve the problems of working out parking for personal vehicles by making it be entirely the owner's responsibility.

    Sort of like horses. Nobody worries about where to put the hitching posts and watering troughs outside a modern building. Pretty soon, people might feel the same way about parking lots. Why worry about where to park? The car just goes back to the garage when we're done with it. And people who own their own cars can make their own arrangements, with the understanding that not everywhere is going to be equipped to support personal vehicles--just like not everywhere, these days, is equipped to support horses.

    There are cities where car ownership is a sign of great wealth, like Hong Kong. But while half the time I get comments like this (who would bother to own?) the other half of the time I get the reverse (people will never give up their cars.)

    Only the future will answer this. It will go both ways. I think a lot of people will give up their cars, but many won't. The reasons they won't give up their cars include:

    • They like to customize it, fill it with their own stuff (not just in a locker in the trunk.)
    • They like to use it as expression of their style
    • They are absolutely assured to have just the car they like
    • Storing it is no longer so expensive
    • They can rent it out (if they keep it clean) to others and thus afford a better car
    • They are developers, or companies.
    • They have kids and trust only their own car with their kids most of the time.

    It will be more expensive. But we already are willing to spend a lot to get a car these days.

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