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Efficient Transit Solutions Fall 2010 |
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Another Small Step for Man? Peter J. Muller, P.E.
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 | | The Heathrow Pod |
On Tuesday, October 19, 2010, the ULTra personal rapid transit (PRT) system went into 12 hours-a-day, 7 days-a-week service, carrying the public (albeit limited to Heathrow airport employees). The significance of this step is debatable. On the one hand, this may just be another small step in the painfully-slow re-emergence of the PRT concept. It may be no more significant than the opening of the Morgantown PRT system by Trish Nixon, with much fanfare, in 1972 - the system only, truly, coming on line in 1975.
 | | PRT vehicle testing at Masdar |
On the other hand, the Heathrow system is the first ever to meet the definition of modern PRT. It represents a truly new system of transportation, requiring new infrastructure the likes of which has never been seen before. Previous transportation revolutions have always been accompanied by new infrastructure - canals, railroads, roads, interstate highways - the very names describe the infrastructure upon which these systems are dependent. Could the Heathrow PRT system be the harbinger of a transportation revolution?
Until now, public transit has operated on the stagecoach model - passengers wait at a station for the transportation and then travel to their destination station, sharing the ride with many others, and frequently stopping along the way for others to embark or disembark. Now, for the first time, passengers wait less than a minute for their ride (70% at Heathrow do not wait at all), do not need to share their ride with anyone else, always get a seat and have no need to stop or transfer along the way.
 | | PRT Station at Heathrow T5 |
Will this revolution in level of service be enough to result in a transportation revolution? Will future oil shortages and fear of climate change play a role in a revolution leading to this type of automated, electrically-driven, transit that sips energy? Hopefully, the 2getthere PRT system will open to the public, before the end of this year, so that the fate of a potentially revolutionary transit system is not so dependent on what happens in one small application. |
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Why Has Masdar Personal Rapid Transit (PRT) Been Scaled Back? |
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 Masdar City, near Abu Dhabi in the UAE, set out to be a sustainable, zero-carbon, zero-waste community. Part of the premise was to exclude automobiles entirely. The city was going to rely mostly on a personal rapid transit system for internal mobility. A PRT system with some 80 stations and thousands of vehicles was planned. Unfortunately, recent (October 2010) announcements indicate that this plan has now been scaled back, and the PRT system will be confined to a pilot system or a small system serving the area close to the Masdar Institute of Science and Technology. This is a blow to PRT proponents, but is PRT to blame for the setback?
Since PRT does not mix well with street level traffic, most urban plans call for an elevated PRT system with a very small ground-level footprint. This was considered at Masdar, but there was a chicken and egg problem. If the PRT was built first, it would be difficult to integrate it in the buildings to follow. Worse, all the costs would have to be borne by the developer. If the buildings were built first, they could be planned to accommodate and integrate the PRT system, and the building developers could possibly absorb station and other costs.
Building developers were slow in coming forward and so the latter, preferable, option was not feasible, although it could possibly still have been accomplished by imposing carefully-planned building requirements (not easily accomplished in the prevailing culture). This, coupled with a desire to provide a pedestrian-friendly "ground" level, led to the decision to raise the pedestrian level, some 7 meters above the ground level, to form a "podium" level, and to build the PRT system and the utilities in the undercroft formed below. Future buildings could then be constructed on the podium level with few restrictions.
Putting PRT in the undercroft has proven problematic because:
- The PRT routing was constrained to follow surface street routing, which is deliberately discontinuous to help prevent windy conditions.
- The PRT routing is the same as that for special delivery (and, possibly, emergency) vehicles - violating one of the principles of PRT, which is to separate it from other traffic.
- There is no way of enticing building developers to share the costs of the PRT stations and other elements.
The podium decision was made in more prosperous times. It is an expensive way to build a city, and current economic considerations have resulted in the decision being made not to extend the podium concept and the PRT system throughout the city. Thus, the podium/undercroft decision underlies the decision not to extend the PRT system. Hopefully, the PRT system will soon come into successful public service, indicating that it can at least do the first part of what it was acquired to do.
Masdar is apparently considering electric cars and/or buses instead of the PRT system. It is difficult to imagine how this can be accomplished without revisiting the design parameters for the streets, which are narrow and discontinuous. It may make much more sense to take a hard look at how an elevated PRT system could work and what it would take to merge it with the undercroft system. |
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Peter J. Muller, P.E. President
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| Website Updates |
In addition to PRT news updates (which now occur almost daily) we have recently made the following additions:
> New images for 2getthere and the BAA Heathrow and Masdar City PRT projects are now available for viewing in the Picture Galleries.
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Jan 2011 · Washington, DC
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PRT Modeling Concepts and Concerns
PRT modeling involves mathematical and computer models and simulations to figure out many aspects of a PRT application. Each aspect is quite intricate and usually interrelated with one or more other aspects. Models can become so complex that it is difficult to tell if they are working correctly or if hidden flaws are skewing the results. A very large consulting firm was recently reported to have told a client that each PRT vehicle would cost over a million dollars - in this case it was easy to tell that their cost model was seriously flawed!
Much has been written about the pros and cons of synchronous and asynchronous control systems. Synchronous control systems have previously been proven to jam up after four or five merges. Being a proponent of simple solutions (of which synchronous control systems may be one), we investigated and found at least three important factors that previous investigators had ignored. We can now prove synchronous control systems can handle about 40 merges (about a twenty-mile trip). The point is that many aspects of PRT modeling have yet to be thoroughly tested, improved and proven.
We have come across PRT simulators that cause asynchronous merges to back up under overload, both upstream (to be expected) and downstream (not to be expected). In another case, an asynchronous control system broke down to increased headways under stress. It seems that asynchronous control systems do not work as well as advertised or (more likely) these two simulation models are flawed and giving incorrect results.
Garbage in, garbage out. Vectus is safety certified to a minimum headway of 3 seconds at 28mph. Taxi 2000 claims they meet ASCE standards down to 1.1 seconds; a 12-foot-long vehicle can theoretically meet ASCE standards, with a little over 2-second headway at 25 mph, using a leisurely 0.25G deceleration rate. Why are we then seeing consultants use a 5-second headway at 25mph?
When consultants stack very conservative unit costs, on top of very conservative headways, on top of flawed simulations, it is no wonder PRT ends up looking expensive. Just as concerning are situations where PRT is claimed to be the low-cost solution to all problems. Clients may need to consider having key factors independently modeled. |
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