Talk:Personal rapid transit

Gravities vs. m/s^2

I think gravities are more meaningful for an average person, and this is an encyclopedia. So, I plan to put the gravities as the primary unit, with m/s^2 in parenthesis.

misc

Reverted away from copyrighted text from www.lightrailnow.org/features/f_prt_2005-01.htm

---

Braking

P1

The reason that PRT guideways can sustain much higher traffic rate has absolutely nothing to do with stopping distance; in fact, their stopping distance is generally longer than an automobile's. Rather, it is their computer-controlled reaction time which is the critical factor. This is why the DMV recommends a 2-second separation between street vehicles (also known as the headway distance); not because a car traveling at 65 mph can stop in two seconds, but because that's the reaction time of the average driver. Once the driver reacts, they can apply the brakes and presumably match the deceleration rate of whatever is in front of them. Because PRT vehicles are computer-controlled, their reaction time is measured in milliseconds instead of seconds. This allows for much shorter headway distances, and much higher traffic flows.

The accellerations originally in the article would still be fatal for most people (except in the case of instantaneous accellerations lasting a few milliseconds at most, which is not pertinent to the current discussion). Moreover, braking does not effect route capacity in any substantial way -- reaction time is essentially the only important factor here. If ATRA has reviewed and approved this text, then I'm frankly dissapointed in them; these sort of physically-impossible claims are precisely the sort of thing which causes conventional-transit advocates to disparage PRT. There's an excellent paper discussing precisely this subject, which is linked from the Innovative Transportation Technologies website; I just need to go through the work of finding it and summarizing it. I'll try to have that done by sometime tomorrow. Skybum 18:49, 4 October 2005 (UTC)

Read the following PDF: Safe Design of Personal Rapid Transit Systems. On the subject of braking, and headway, it has this to say on pages 6 & 7, after first giving the somewhat complex equation for safe minimum headway distances (emphasis mine):

T_min = (L/V) + t_c + k(V/2)(1/a_e - 1/a_f)

The first term is the time required for a vehicle of length L travelling at a velocity V to travel one vehicle length. It is the minimum possible headway. The second term t_c is the time required to detect the malfunction and apply the brakes. The third term is proportional to the available stopping distance after the brakes are fully applied divided by the cruising speed V. a_e is the emergency braking rate and af is the failure braking rate. k is a dimensionless number included to increase safety.

To appreciate the fundamentals of the question of minimum headway, consider some numerical values. For the Taxi 2000 vehicle, L = 3 m and V = 48 kph. Therefore

L/V = 0.224 sec.

Consider t_c. It is the sum of the time interval between failure occurrence and braking initiation, and the time required to fully apply the brakes. In an automobile, the time to react to an emergency depends on human reaction time. Under automation, the time interval needed to detect the failure and to begin applying the brakes depends on the characteristics of the control system. In a PRT system designed for maximum throughput, it is necessary for the zone controller to be able to sense the position of each vehicle at any point, i. e., vehicle-position data must be continuously available. The first part of t_c is about 0.1 sec plus the much smaller time for the computer to react. The second part of t_c must be as short as practical with current technology. With electromagnetic braking, this time is the inductive time constant of the motor, which is of the order of 10 msec. For a state-of-the-art PRT system, I therefore estimate

t_c = 0.12 sec.

Consider the third term in equation (1). [ ... ] The term a_e must be as high as practical. If all passengers are seated, simple experiments show that a 0.5g deceleration will not throw a passenger out of the seat. If passengers are standing, even half this acceleration is too much, so one of the requirements of PRT safety is that the vehicle be designed for all passengers seated.

This passage makes it clear that the reaction time (t_c) is the dominant factor in the determination of headway distances. Moreover, it makes it clear that the emergency braking accleration (a_e), while desireable to be as possible, is acceptable around a level of 0.5g. These values can be found repeated throughout the web, such as here (emphasis mine):

The usual acceleration and deceleration of the vehicle is 1/4 g, or 8 feet/sec^2. In an emergency, the Taxi 2000 system would stop using 4/10 g, or 12.8 feet/sec^2. [ In ] a system that would go 25 mph (37 feet per second) [ ... ] the safe following distance to avoid any possible collisions becomes (37 fps) / 2*(12.8 fpss) = 1.445 seconds, plus the .004 seconds of reaction time, for a total of 1.45 seconds.

Finally, my point about the harm that these kind of false claims (of PRT requiring, or benefiting from, high deceleration rates) can cause was not theoretical. Check out here, or here, which alas uses our PRT page as a source for its criticism.

I believe that this should be the final word on the topic. Unless anybody has some further arguments, I will be heavily revising this info on the main PRT page tomorrow. -- Skybum 04:42, 6 October 2005 (UTC)

Thank-you very much for the excellent work. User:Ray Van De Walker

P2

Why is braking distance so important? When one drives a car, one certainly can not stop in the distance between cars. What am I missing? pstudier 22:18, 2005 Apr 6 (UTC)

I at least, am a more cautious driver than the people in your area. Some of us might say we're willing to take the risk, but some people who say they are willing to take the risk might change their minds after they have an accident. So, in most places, safety laws require public transit vehicles to keep a full stopping distance between themselves and anything they could hit. This lowers the number of vehicles that can fit on the road or guideway or whatever. In turn, this reduces the rate at which the road or guideway can be depreciated. This in turn increases the fares or public costs substantially, because the roads or guideways are the most expensive part of a transport system. User:Ray Van De Walker

Excuse me, but breaking distance is DEFINATELY a factor because the faster a car can break, the closer they can be together. Why is this the case? It is because the law says it is the case. Public transportation vehicals are required to travel far enough behind the vehical in front of it so that it could stop IN THAT DISTANCE. For example, if the vehical in front of a public transportation vehical stopped at infinite deccelleration (immediately), the public vehical would still be able to break in time. Thus, to comply with regulations, cars must be able to stop quickly enough - and the more cars that fit on the track, the larger the track capacity can be. This is a HUGE factor in this sort of thing. The normal road system is failing in large and midsized cities now because of lack of capacity. If PRT wants to solve that, capacity is going to be a main issue. Fresheneesz 00:20, 30 November 2005 (UTC)

Did you read my comments in the rest of this section? They decisively answer the braking question. You are correct that capacity is the main issue, but braking distance is absolutely not what determines capacity. To recap what I've said above, this is because the only thing you'll need to brake to avoid is the vehicle ahead of you, which will either be A.) Gradually braking, or B.) Coasting to a stop. Those are the only two possibilities. Therefore, all that a vehicle needs to do is 1.) Notice that it needs to brake (this is the critical factor: reaction time), then 2.) Match the rate of decelleration of the vehicle ahead of it. As you can hopefully see, the velocity at which you brake is not a factor, so long as it matches the velocity of whatever is in front of you.

Automobiles are different, because they have to deal with cross-traffic. This can create scenarios where there is essentially a "brick wall" in front of the vehicle, and the closer you can get to instantaneous decelleration, the better. PRT doesn't has to deal with this scenario, however. PRT is more akin to a freeway, where the recommend 2-second following distance is absolutely not determined by braking speed (at highway speeds, cars require 6-10 seconds to brake). 1-2 seconds is a typical human reaction time, allowing drivers to notice and match the deceleration of whatever is in front of them. This is essentially the same as PRT. However, in PRT, the reaction time is computer-based rather than human-awareness-based, and this -- and not braking -- is what allows for much closer following distances (as close as a few milliseconds under the most optimistic scenarios), and much greater route capacities. Skybum 07:59, 30 November 2005 (UTC)

Well... you *did* read all of what I wrote right? Because the issue is not saftey here, it is public transit *laws*. Of course, to avoid collision, a vehical must only decellerate about as quickly as the vehical in front of it. However, the law states otherwise - and *that* is the problem. But I do see what you're saying, however I think that both breaking time and reaction time are large factors. Another thing to consider is the semantics of the situation - personally I consider "breaking" to include reaction time in the case of PRT - beacuse it is automatic, integral to breaking, and invariant. For example, in the Skytran system, the vehicals can deccellerate in 1/2 second in an emergency, which I think you can agree contributes to the stopping distance slightly more than the decreased reaction time. Fresheneesz 08:39, 30 November 2005 (UTC)

No, the laws aren't the problem, because the laws for every transportation mode are different, and there are no laws that currently apply to PRT. Trains and busses, for example, can require as much as 30 seconds to come to a full stop when travelling at full speed (because anything much quicker than that would kill their unbelted passengers). They are still allowed to operate. Skybum 17:41, 30 November 2005 (UTC)

They are allowed to operate because they put enough distance between themselves and another train to stop in that distance. Do you think that PRT will be allowed to be built without regulation? Most likely, legislators will be the *same* or *more strict* requirements on safety issues, such as breaking distance, compared to trains, busses or other forms of transportation. There are no laws about PRT now, because it is a miniscule part of our society if anything. Fresheneesz 20:22, 30 November 2005 (UTC)

Yes, legislators will, and should, regulate PRT. It is in everybody's interest to make sure that PRT is as safe as possible. However, it is also important that people not equate safety (or headway distances) with braking speed, because on a closed-track off-grade system, they are almost unrelated. In fact, higher braking speeds can lead to decreases in safetey. Consider a city bus travelling at, say, 45mph. For the sake of its unbelted passengers, it cannot brake at any faster than 1/4g, which means a stopping time of about 8.5 seconds. If it stops much faster than this, passengers will be thrown from their seats and across the bus, severely injuring and possibly even killing them. Thus there is no safety advantage to having a bus stop any more quickly than this (and indeed, they can't do so). Yet busses are allowed to operate at 2-second following distances, because it is the reaction time of the driver which is the important factor. As long as the bus can reliably brake at 1/4g, it does not need to brake any faster.

This principle is universally recognized by transportation planners -- except for those coming from the rail industry. This is because its traffic models have to account for massive frieght trains that have a stopping distance of literally miles -- in other words, beyond the perception & reaction time of the train driver, and reliant upon the signaling system and the stopping distance of the train itself. Because of this, the following distance for trains is determined by its braking speed, and even this is somewhat archaic, given the advent or modern real-time signaling systems. No other land-based transportation system works this way, or should. I have no fear that PRT will be regulated like this, even if the assholes at LightRailNow can't grok the concept.

Anyhow, it is almost certain that PRT headways will be determined in the same fashion that car and bus headways are -- reaction-time-based rather than braking-distance-based. Because regulators are intrinsically conservative, they'll probably start with the same two-second rule that autos use, and gradually reduce this as the system proves itself to be safe and reliable. That's how PRT will achieve a high route capacity. Meanwhile, making claims about its supposedly superior braking velocities is in fact harmful to the PRT cause. This is because if PRT vehicles were to brake more quickly than other vehicles, they would soon begin throwing their occupants through the windshield, in the service of avoiding a collision. That's not an effective safety technique, and isn't in any case how PRT works, so there's no point in talking about it -- except perhaps to dispell the myths surrounding the subject.

By the way, I like the edits that you've been doing. Keep up the good work! Skybum 06:16, 1 December 2005 (UTC)

Thanks! And I agree, breaking speeds has little to do with safety on a basically isolated linear system like PRT systems are. And I hope you're right about future legislation. What does the lightrail now people have against PRT anyway - its not like *they're* making any more money off light rail than anyone else. Anyone smart enough would put their money in a good PRT system rather than pushing it back down. But in any case, I think *this* discussion (i mean a very very consolodated version of it) should be put in the article - rather than skirting the subject entirely. Fresheneesz 06:25, 1 December 2005 (UTC)

I would love to see a condensed, accurate version of the braking facts in the article as well. User:Ray Van De Walker

WVU PRT separate article?

WVU PRT should be seperate article? wikiveterans please advise Codeczero 16:29 Feb 21, 2003 (UTC)

Hi, welcome! As it is, WVU PRT fits well in this article: it is on-topic, and only when an article gets very long we need to split it. - Patrick 20:26 Feb 21, 2003 (UTC)

good info in here but it reads like someones PRO PRT lobbying piece still.. Triptych 02:57, 9 Jan 2004 (UTC)

Not anymore. There are quite a lot of disadvantages listed, and I just put in the guideway choice issue and the dual mode versus single mode debate. Now, the problem is to get the article organized a bit more. Too many of the points mentioned (as advantages or disadvantages or as specs) apply only to one type of guideway but not to another or to a single mode system but not a dual mode system, and this is not made clear.

I wrote the original article, and I'm still one of the biggest contributors (I often fail to log in). I started off pretty ignorant, and very neutral on the topic. I have experience with biomedical software and safety engineering. So I read up on the topic. Unfortunately, most of the people who write books about this thing, and really understand it are enthusiasts (of course). Most of the people who debunk it are clearly doing the capitalization and overhead math wrong. Some of the older advocates say this is happening on purpose to persuade the politicians to buy the wrong transit stuff. User:Ray Van De Walker 2004-01-27 09:20Z

I am always amazed at the naivete of alas too many PRT enthusiasts (and here I am not refering to those who have edited this article, for they seem a level headed lot) and sympathisers . No, PRT systems are not singled out specifically for destruction, yes they will get destroyed mercilessly if they are not nimble. The transit market is all too often a "dog eat dog" world of rather fierce competition between competing kinds of systems, competing companies within the same systems and local financial interests striving for attention. PRT projects get caught in the crossfire most of the time, or brushed aside with a slight shove because of their fragility. Entrenched transit technologies in traditional transport establishments can react in a very brutal way to menacing innovations, and this has happened to much stronger and well prepared projects than what PRT systems are offering. One example is the air cushion aerotrain developped by Jean Bertin in France. It went as far as full scale prototypes and a long test track several kilometers long. The development of the TGV, the Train Grande Vitesse (ultra high speed train) by France's national railways, the SNCF was an effort meant to crush the aerotrain, which it considered as a dangerous competitor. Hundeds of millions of Euros were spent in R and D by the SNCF, dwarfing the aerotrain research budget. Even greater sums were spent on the new dedicated infrastructure for the TGV. The SNCF was successful. Companies which sell trams, buses or or commuter wagons cannot be expected to lie down and say . "OK you have the right stuff and we have the wrong stuff." They can be expected to try every trick in the book to wipe any PRT proposal off the map, as they wipe out other competitors in the traditional bus and tram domain. The only potential allies are car companies, which stand to gain as much by making myriads of PRT vehicles as they are making cars, if personal vehicles are allowed (in single mode as well as in dual modes personal vehicles are possible, though they are rarely discussed) but there is no reason for them to leave the status quo. One should note though that Ford research labs proposed a new PRT system called PRISM, in December 2003.

Some sort of PRT planned in Finland

I can remember reading an article about some sort of a trial of a PRT system being planned in Finland. If I can remember right, it would (at first) be a line from Helsinki-Vantaa airport to some points in Helsinki (or well, they called it a high speed "driverless taxi" moving on a predesigned route, as far as I can remember, but isnt that a PRT system then..). Would anyone have any more info on this, as I couldnt find anything from Google? It would be an interesting addition to the article.--HJV 20:35, 27 August 2005 (UTC)