Some Questions About Congestion At & Around Dualmode Stations

by Gary Penn 

It appears to me that the dual-mode vs. PRT argument is a case of the "better against the good" that threatens to drain energy away from getting anything out into the marketplace that is an improvement over the sterile "cars vs. trains" set of options. The seductive appeal of dual-mode is the door-to-door trip, otherwise known as "no walking required". Given the widespread doubt that Americans will walk at all, regardless of the benefits, this may seems like a persuasive point. However, it appears that dual-mode advocates get a bit schizophrenic when questions are raised about congestion around dual-mode stations or entrance/exit points are raised.

The argument is that dual-mode's off-guideway mobility allows a coarse grid with widely spaced lines and many fewer stations than a PRT system that depends on walking customers. However, this also means that, given equal volumes of traffic, the traffic in and out of the dual-mode station is much heavier than at any of the much more numerous PRT stations. Furthermore, since each traveler comes encased in a car-sized vehicle probably occupying  4 to 16 times as many square feet of public right of way as the occupant would walking, the traffic impact on the neighborhood lucky(?) enough to get such a station will be even further multiplied.

This may be why design plans of dual-mode stations are so scarce. Since start-up plans typically envision a mix of the rare early adopter dual-mode cars, conventional cars to be loaded on pallets/ferries and guideway captive vehicles running in PRT or GRT mode, these stations look like a difficult design proposition as well as likely to be Locally Unwanted Land Uses to be fiercely opposed by NIMBYS arguing about local street traffic congestion.

Now at this point dual-mode advocates typically say something like: "No problem, the dual-mode/ pallet borne cars need not actually exit the system at congested points, the user will exit the vehicle and the system will park the vehicle in an automated parking facility, not necessarily co-located with the station." The problem with this is that the trip is no longer door-to-door. It is now door-to-station and the station is one of few on a coarse grid that is unlikely to be near the traveler's final destination so he or she must walk a greater distance than on a fine gridded PRT network or change modes to get to the destination door.

Dual-mode advocates seem to imagine that congestion prone station locations would occur only in central business districts and can be regarded as an exception case. Intuitively (because I have no numbers available) I would suspect that most station locations on a coarse dual-mode grid would turn out to be congested, at least from the perspective of the adjacent neighborhoods.

MegaRail and Higherway seem to be seen by their developers as freeway capacity extenders rather than metropolitan area-wide universal transportation systems which is probably why they are not especially concerned with the congestion impact of their entrance and exit points - freeway strips are already congested and will remain so in any imaginable scenario. Some of the more abstract advocates of dual-mode, who sometimes seem to be as much opponents of single-mode PRT as advocates for anything, appear to not have thought much about how entrance/exit points would work, especially at the beginning when purpose-made dual-mode cars are scarce. RUF, with its dual-mode Maxirufs and widely distributed short-term rental dual-mode cars, seems to be making a more serious attempt to deal with urban reality. One does wonder about the economics of the drivers required by the Maxirufs operating as dial-a-ride minibuses and also about what its switch points would actually look like with their elevated road decks.

Another way to look at this might be to ask: How much does it cost to add dual-mode to an elevated, automated guideway system?  If you save money to pay for dual-mode's extra complexity by going to a coarse grid with few stations how much potential traffic do you lose from travelers who will not or cannot buy/rent/drive dual-mode cars but could and would walk to PRT stations in a fine meshed network?

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Last modified: July 29, 2002