| Get On Board! PRT |
| Home | |
| |
©2001 Get On Board!PRT
By D.S. Gow, MPA
Aug. 8, 2001. Dual Mode is a concept which, in its outward appearance, might be said to out-Jetson PRT. It is a system in which vehicles that can be driven on conventional streets will also be able to operate on a guideway under computer control. In DualMode, the Transportation of the Future, Francis D. Reynolds describes such systems thusly--
...we will have even more private cars, yet we will avoid the problems we now have-- by using our cars in two distinct modes. The cars may be standard or specialized, depending upon the details of the system. They will be driven in the normal manner on the streets, and they will travel automatically on high-speed dedicated guideways-but their drivers won't be driving. Trips of more than several miles will be on these guideways rather than on the highways. A dualmode system will be even more "personal" than "Personal Rapid Transit," since we will be using our own cars. And it will also be more "rapid," than PRT since we won't have to walk to and from stations. We will travel from door to door as we now do, without getting out of our cars, but we will travel safer, cheaper, faster, with less stress, and with neither gasoline nor pollution.
Dual Mode comes in several flavors, some of which are:
Pure Dual Mode, in which road vehicles
would have built-in components allowing them to be used on special
guideways. Rapid, Urban, Flexible (RUF) is a Danish system that has a
prototype vehicle and test track in operation;
'Mixed' Dual Mode, also with guideway
designed for Dual Mode vehicles, but standard autos could have access
too, carried on automated pallets or 'ferries'; examples are Megarail and Autran;
Other Mode, in which road vehicles access
some other mode which is not an elevated guideway; examples are Carbus
(cars are transported on freeways using special trucks) and Flexitrain
(cars which can link into 'trains' guided by the lead vehicle)
Dual Mode has been designed to combine the grade separation and automation advantages of PRT with the 'irresistable' attractiveness of the private automobile. At this time, however, I see this combination to be problematic.
| Issue | Dual Mode, per Reynolds | Comment |
|---|---|---|
| Vehicle weight I | "The cars... will be driven in the normal manner on the streets, and... on high-speed dedicated guideways" | Dual Mode guideway would have to be over-engineered to allow for weight of the largest passenger vehicles-- in a pallet system (see below) this would include the engines, transmissions, drive shafts, suspensions, axles, cargo, etc. of conventional autos. A weight limit would require installation of scales at all access points, not to mention reduce the system's appeal. SUVs might be a fad, but then again they might not. |
| Vehicle weight II | "In some systems "true dualmode cars" would be equipped to run on the guideways directly, and in other systems there would be interfacing pallets between conventional cars and the guideway." | In the first case it appears people will be expected to buy a new Dual Mode-compatible car. And again, in the latter the guideway must carry the weight of a vehicle and a pallette equipped with its own propulsion system, as proposed by several systems. |
| Types of vehicles I | "Most of the traffic will consist of private cars, but there will also be transit buses, Greyhound-type intercity buses, rental cars, taxis, school buses, delivery trucks, cross-county freight traffic, and possibly personal-rapid-transit vehicles." | This means the guideway must be engineered to carry heavy, large-capacity
vehicles and their cargos. This means more time-consuming construction, more
materials, and higher costs compared to PRT.
What is the obsession anyway with vehicles that carry 8, 10, or more passengers? Over 95% of all passenger miles in the U.S. average less than 2 occupants per vehicle. To maximize efficiency of such vehicles would require them to do what trains do-- run on schedules, not on-demand, and force strangers to wait to travel in groups. |
| Types of vehicles II | "Most dualmode cars will be battery-electric or fuel cell powered for street use" | Which means some won't? In the case of a pallet system like Megarail, gasoline-powered cars would be expected to idle their engines (!) if they wanted use of their radio, heat, A/C, etc. But again the assumption is most persons wanting to use the system are expected to buy a new Dual Mode car. This seems to assume that (a) they will be produced widely by the auto industry, or (b) the guideway manufacturer will also build cars. How much capital does it take these days to set up an automobile manufacturing business? |
| Cars? Or trains? | "The bankrupt passenger railroads were taken over by the government in 1971... Yet we talk about more trains as a solution to our traffic problems." | Then why does Dual Mode bother to mimic trains by proposing the option of linking cars together into trains? This is central to operation of Flexitrain, is portrayed as a part of RUF, and Megarail's "Microrail" variant offers a "train mode" which would offer light rail-like service as an option (although the company says it is a sort of a transitional step after guideway completion and before certification of its PRT component). Zero headway would maximize the number of vehicles that could be handled per unit of time, but really-- how many people travel from exactly Point A to exactly Point B, at the same exact time? A disincentive of trains is the need for riders to gather together at designated locations & times and travel with others-- PRT recognizes this disincentive, does Dual Mode? |
| Critical systems | "The different dualmode system inventors have proposed "true dualmode cars"... interfacing pallets... maglev... linear electric motors... synchronous motors [and] HiLoMag | A person might find his Dual Mode car confined to the pavement if traveling to/through an area with an incompatible system. A national or continental Dual Mode Standard would be needed to avoid this. Good luck. |
| Ancillary facilities | "...a high percentage of guideway cars going to the cities will not exit to the downtown streets at all. These cars will be parked directly from the guideways. (Automatic valet parking with no one to tip.) So not only will street traffic be reduced, so will street parking pressures." | He's talking about parking garages. These might be part of a large-footprint
interchange facility as proposed in the Danish system.
And while cheapskates won't have to tip, it's a good bet the parking won't be
free. These are unlikely to be effective-- how close together could these be
placed? Not very, making access less convenient and inviting the same
station-traffic problems as train stations. The acreage required by such a facility means only a few could be
sited within a built-up city: thus very few destinations would be within walking
distance, so the claim of reduced street traffic is questionable.
Another wrinkle: If a dualmode system includes Group Rapid Transit vehicles that never leave the guideway, it will need large stations for walk-on passengers (since large capacity vehicles need to be full to be efficient, riders must have a place to wait for scheduled runs), leading to higher costs. A station which combined such a depot along with onramps and offramps for dualmode cars will be very large and expensive indeed. It would probably not be affordable to build the number of stations needed to provide walking-distance service to all areas covered by the guideway network. Fewer stations means less access for walk-ons, feeder transit, station traffic problems, and less than optimum ridership. And such a system could not provide both GRT and PRT components: stations would have to be even larger to provide separate sidings and platforms so GRT vehicles don't block PRT vehicles; stations would have to be built to accommodate waiting areas for GRT, meaning the number of stations needed by PRT would not be affordable. |
But perhaps my biggest problem with Dual Mode is that it is not mass transit for the public, but rather private transportation that achieves high volume through automation. This characteristic means Say's Law-- supply creates its own demand-- is a part of the Dual Mode equation. It is a basic concept in transportation-- if traffic is reduced on a congested road by means of car pools or transit, they are replaced by people making trips that congestion previously discouraged. Dual Mode introduces this Catch-22 into its system by allowing private cars. As is demonstrated every day at rush hour (and other times), people would rather drive than take mass transit. Therefore I feel that, by effectively expanding the road system to include automated guideways, Dual Mode will ultimately be undermined by the same economics and personal choices which contribute to our existing congestion problems. Under Dual Mode, "why don't more people take mass transit so I can use the Dual Mode guideway?" could become a common refrain.
Instead, why not...
The author has a degree in Policy Analysis from the University of Washington Graduate School of Public Affairs (now known as The Evans School).
Dual Mode Connections
Prof. Jerry Schneider's Dual Mode page, featuring the Dual Mode Debate