A Few Thoughts on Palletized Dualmode
[Comments like this added by J. Schneider]
Is the concept of palleted dualmode the answer to our transportation problems? So it seems if the dualmode debate pages of recent are any indicator. Pallets have gained new friends among debaters and newcomers are brought into line as pallet backers apparently unaware of the serious problems associated with palleted dualmode, while active defenders of true dualmode are few and far apart. I resent that development, as the decision "to pallet or not to pallet" is a crucial one not to be taken lightly.
Pallets have advantages as well as disadvantages, however this comment is not a balanced one counting pros and cons with equal zealousness. In fact only the problematic side to pallets is given due attention here, as my thesis is that these problems are so many and so immense that possible advantages are rendered utterly irrelevant. While some of the problems will simply reduce the performance of the system, or increase cost, or reduce user acceptance; others are crippling for the pallet case. Many have already been brought forward by others, but I believe most readers will be able to find yet another reason or two to reject the pallet idea.
Routing "empties" to where they are needed when they are needed is a serious management problem to which no perfect solution exist. Routing is essentially done according to where and when demand is expected to appear and therefore the systems management (computerized as it is) needs to know as much as possible about traffic patterns, so they can make qualified guesses about upcoming demand. Even if good guesses are made, a small proportion of the pallets will be sent to locations where they are not needed after all, and a somewhat larger proportion will arrive at points of need only after patrons have waited some time at the loading facility.
Waiting time is a serious obstacle to the broad user acceptance of a system, so it will have to be minimized as much as possible. This can only be accomplished if a high number of pallets (relative to the numbers of users) are made available, thus increasing the overall system cost. The guideway entry points will also need a high storage capacity for empty pallets waiting for users and will therefore require large land areas at every entry point. [Unless parking structures are utilized].
Given that the demand pattern is within "normal bounds" it will probably be possible to almost eliminate waiting at guideway entry points, again provided that large numbers of pallets are included. But what if demand patterns show abnormalities? An anomaly could be a sports event, any cultural event, a weekend market or a large political rally. Anything where a large number of people are concentrated (getting there by car) at any certain time. In order to prepare for the upcoming demand, the system management will need to know where an event takes place, how many people are expected to attend, when it starts and crucially; when it ends.
All available information has to be fed continuously into the routing control system, as the normal distribution of pallets throughout the guideway network cant handle sharp local surges in demand. The system always has to prepare for anomalous situations, but what if an unexpected surge in demand appears. There is no way to avoid chaos then. If a major cultural event for some reason ends an hour early (e.g.due to an unexpected rainstorm), everyone would want to go home immediately, but pallets wouldnt be ready to handle this sudden demand from thousands of users. Pallets wouldnt be ready as the control system expected the surge to happen an hour later. The result would be long waiting time for patrons and everyone would be dissatisfied.
Another effect of routing is that "empties" will use up part of the systems capacity, so that fewer loaded pallets (with paying customers) fit on the guideway. As entry to and exit from the guideway are almost always locally unbalanced if the amount of routing is significant, probably constituting somewhere between 25% and 50% of the total traffic on the guideway (according to the few very loose estimates I have been able to find).
The best and most realistic study is a Swedish computer simulation for Gothenburg using a large PRT-network (same routing problem) by Ingmar Andréasson. The simulation found that 42,7% of traffic was "empties" being moved to and from stations. It was also revealed that the average waiting time for users was around 1 min. 20 seconds and for every 100 loaded PRT vehicles on the guideway there would also be 69 empty ones, plus 87 waiting for costumers. These figures are of interest when considering pallet supporters common claim that only a few pallets would be needed. If up to 20% of all users are expected to be on the guideway at any one time (during rush hours), we can deduce that one pallet is needed for approximately every two users. If this percentage is expected to be higher, then more pallets will be needed. If average waiting time should be shorter, then more pallets will be needed.
Considering the percentage of routing traffic to be high one has to remember that if many more arrivals occur than do departures in any given location, it can be necessary to send a pallet back on the guideway immediately after unloading, even if empty and no useful destination can be assigned for it. The actual amount of routing traffic can be estimated from computer modelling but will remain unproven until a large network is in real operation. It is however more than likely that extra miles of guideways will have to be built to compensate for the loss of capacity due to the routing problem.[Storage depots, as utilized in the Gothenburg simulation study, offer another possibility].
The process of loading a car onto a pallet at an entry point and unloading it again at an exit point will be time consuming. The duration of the loading procedure will decide the capacity of a loading berth, and thus the number of loading berths needed at each entry point. First the pallet would need to get into the loading berth before the user could drive his car onto it. Some drivers will be slow and careful while others will need several attempts to align their cars correctly. When aligned the car will be locked to the pallet by some mechanical means and the driver can insert his desired destination into the control system. Then there could be a few seconds wait for an empty space on the guideway. The pallet would then have to completely clear the loading berth before another pallet could enter to repeat the procedure.
If the entire procedure could be completed in 20 seconds on average (an optimistic assumption I believe) every loading berth would have a modest capacity of 180 pallets/hour. As it is impossible to drastically cut this loading time, most entry points would need many loading berths. Furthermore if waiting time is included, assuming that the average would be only half of what the Swedish study found to be realistic, capacity would drop to a meagre 60 pallets/hour per loading berth (40 seconds wait, 20 seconds loading). Even 10 berths would have a modest capacity of 600 pallets/hour only, and this inherent low entry capacity will make it impossible to deal with the aforementioned sharp local surges in demand, even if they were anticipated.
A difference between a pallettized dualmode system and a true dualmode system is that pallets require entry and exit facilities to be located in connection with each other, while true dualmode can have entry and exit independently of each other. Not only does entry and exit need to be connected but it must also be possible to convey empty pallets from unloading berths to a storage area [or structure] and from there to loading berths. So, an access facility for a pallettized dualmode system would require a deceleration ramp (from the guideway), an acceleration ramp (to the guideway), maybe 10 unloading berths and 10 loading berths, a storage area [or structure] for pallets (waiting for cars) and a storage area for cars (waiting for pallets).
All of this would require so much land, that it would be more than difficult and prohibitively expensive to fit it into an existing city. This is certainly the case for all the European cities I can think of, and Im sure something similar applies in Asia and the U.S. In my opinion this is a devastating problem for the palletized dualmode concept, and would restrict dualmode to applications mainly outside existing urban areas. Contrary to this true dualmode need only two short entry lanes merging into an acceleration ramp to provide high capacity entry, and this modest facility can easily be fitted into existing cities.
The energy requirements will, all things equal, be higher for a pallettized dualmode system than for a true dualmode system. First, because of the widely used movement of empty pallets, which will consume lots of energy yielding no productive transportation work in return. Secondly, a pallet plus a car will be heavier and more bulky than a true dualmode vehicle and will therefore use more energy for the same work.
When considering the widespread claim from developers that their pallets will be light and cheap adding little extra energy usage, one has to remember that developers face a choice between a one-size-fits-all pallet accommodating the heaviest vehicles allowed on the guideway or several types of pallets for vehicles of varying weights and sizes. While several different pallets will create impossible management problems, a standard pallet would need to be heavy, probably expensive and will increase energy consumption over true dualmode significantly. A heavy pallet plus the weight of the heaviest vehicle will also dictate the need for a stronger, more expensive guideway than would be the case for a true dualmode system.
Prospective users will evaluate the advantages and potential risks associated with a palletized dualmode guideway, comparing it with the conventional roadway. Having experienced long waits for pallets at entry points on a few occasions, users will conclude that the guideways are only advantageous for longer trips. Considering the combined effect of possible waiting time and the time loading and unloading, trips would certainly have to be longer than the 3-5 miles constituting a very large proportion of all trips. If a dualmode system can only attract longer journeys it will be much less useful to society and much less profitable. It also means that it cannot be implemented by the yard, but will have to be built in larger increments, as hardly anyone would use a pallet operating guideway that was only, say, two miles long. True dualmode guideways could be very short and still be advantageous for users.
Another point considered by prospective users is the question of cost. What will the price for using the palleted dualmode system be? The cost per mile on the guideway can easily be compared with the cost per mile using the road (usually only the price of fuel). Therefore pricing will have to be competitive and this constitutes a problem for palletized dualmode. The users will have to rent a pallet, pay for the use of somewhat heavier guideways as well as large, expensive access facilities plus the somewhat larger energy consumption of the pallet system.
The variable cost will be high compared to true dualmode where customers only pay for use of the cheaper guideways and a somewhat smaller energy usage. The challenge of true dualmode is to provide cheap vehicles to prospective users, while palletized dualmode will always struggle to keep prices competitive. I believe the former is achievable while palleted dualmode will provide services of lesser convenience at higher prices.
A few more points to consider: In case of a power outage a pallet will be immobilized on the guideway with its load on top unless it is equipped with a back-up power supply and (depending on propulsion principle) maybe even an auxiliary propulsion system. This will make pallets heavier and more expensive. During an evacuation pallets would need to be directed to the nearest exit point for unloading and then leave the unloading berth again as more pallets will follow. This procedure will complicate the control system considerably as evacuation can probably not be controlled by drivers from their conventional cars. Evacuation in a true dualmode system on the other hand will be easy (for a suspended-vehicles system only), as all vehicles already have the means to drive off the guideway under their own power and control.
A useful feature of automated guideway transit is the prospect of automatic parking. Patrons will leave their car at some sort of station, from where it will be sent to a parking facility (possibly several miles away) under system control. It will also be brought back under system control upon request. However palletized dualmode would have little scope for automatic parking as the car would have to stay on the pallet for the duration of the parking. Renting a parking space as well as a pallet for the entire period will most likely render automatic parking too expensive for most users.
Which came first? The chicken or the egg? And what will come first? The TDMVs (true dualmode vehicles) or the guideways for them? This dilemma has been used by most pallet proponents to justify the pallet idea. If a dualmode system could accommodate existing cars it would be much easier to implement, as lots of prospective customers would be able to access the system immediately. But as has already been repeatedly pointed out by Francis Reynolds, this advantage is more apparent than real, as most of our present cars will be gone by the time a dualmode system is up and running. Thus there will be adequate time to produce vehicles with special provisions for dualmode (TDMVs). I agree and will amplify the argument even further.
In order for existing cars to use a pallet operating dualmode system they would need extensive modifications. Not only would they need some control interface for communication with the system, but they would also need a power connection for on-board uses and auxiliary electric heaters and air-conditioners (existing systems depend on the internal combustion engine to run). These modifications will be expensive and car owners will be reluctant to have them done on their halfway worn out vehicles. Conversation will be slow and a pallet operating guideway will ,in effect, face the same problem as a true dualmode guideway. No customers on opening day!
All these observations prompt me to add a few comments concerning the HiLoMag concept, which in my eyes has evolved into the most illogical concept imaginable. Francis Reynolds has repeatedly expressed his dislike of the pallet concept, but nevertheless intends to include not one but two or more standard pallets per vehicle, as though that will make the problems go away. It will not. Quite the contrary. HiLoMag faces the problem of getting the pallets to where they are needed, when they are needed and on top of that, in the correct numbers. Furthermore HiLoMag gets the worst of both worlds, as it has opted for pallets requiring special vehicles (with special roof-mounted sockets), instead of trying to get at least some of the existing cars onto the guideway. In that way HiLoMag is burdened with all the pallet-associated problems and the full implementation problem of true dualmode. I dont get it!
Note! Time is a scarce resource, and Im unable to keep up. So I hope that those who have commented on some of my earlier contributions can forgive me my slow responses. Their patience will be honoured as soon as I catch my breath.
Last modified: September 09, 2000