Meeting the Challenge of Serving Large Complex Systems with Group Rapid TransitFebruary 3, 2004
Group Rapid Transit (GRT) systems have been criticized for not being able to handle the complex passenger sorting process inherent in grouping people by destination. It is argued that as system size and complexity increases, the challenge of grouping passengers by destination quickly becomes insurmountable and consequently only Personal Rapid Transit (PRT) can truly provide a sufficiently attractive level of service to become commercially viable. I do not believe that this is an either/or issue. I think that both PRT and GRT will become commercially viable technologies with appropriate applications. I think that in some cases, they will actually share the same track as some such as Kirston Henderson have proposed with his Mega-Rail / Micro-Rail design.
Automated Rapid Transit technologies including GRT, PRT, and Dual Mode systems are being proposed as a response to the breakdown of the transportation system resulting from the spatial inefficiency of the automobile. This spatial inefficiency has two costs: traffic congestion and the dominant wasteful role that meeting parking requirements has on the urban form. But regardless of what any of us thinks of the automobile, its role in society is so closely intertwined with the perceived attractiveness of suburban living that it will doubtless continue to play a large and probably dominant role in our transportation system. The ability to cost-effectively serve remote locations coupled with the truly personalized service stemming from owning the vehicle has attractions that no mass transit device can match.Consider also that all transportation projects are expensive and difficult. Consequently any initiative to build new transportation capacity will necessarily be the result of an overwhelming desire to relieve a major problem with congestion in the existing network. The goal of the transportation planners will be to get the biggest bang for the buck. Since the enthusiasm grand schemes is somewhat quiescent these days, my guess is that the new systems will be relatively small, relatively simple, and designed to siphon off as much volume as possible from known choke points. (This may not be the most cost-effective long term solution and may upset the purists and planners, but until there is a wholesale change in the tenor of the political discussion and the political players, I fear that incrementalism and caution are the watch-words of the day.)This argues that initial systems will be small and simple and characterized by having relatively sharp peaks of passenger volume. Such a system may be more cost-effectively handled by a slightly larger vehicle. As I have said many times before, as systems grow in size and complexity, the average volume of demand between any given origin and destination at any point in time will tend to drop. However, there will still be some high volume origins and destinations (primary or central activity centers) which will still maintain relatively high demands even as average demand over the entire system drops. Finally there is the consideration that since it only takes a small reduction in traffic volumes to produce a significant improvement in highway traffic flow, local complacency will quickly re-emerge as the dominant innovative strategy.In this world, the first systems may very well turn out to be GRT systems and if this industry is going to succeed, these initial GRT systems must be successful. Here is an examination of some approaches to making GRT successful.As mentioned above, as system size and complexity grows, the problem of organizing and grouping passengers by destination quickly begins to boggle the mind.However, just as PRT has the analogue of the personal automobile, GRT has, as its analogue, the elevator. In high rise office buildings, individual banks of elevators are dedicated to selected destinations and passengers quickly learn to follow signage to the correct elevator bank. Then they travel in express mode to a small number of floors where they travel in local mode. However, unlike traditional transit, the elevators only stop at those floors where there is a request for a passenger stop. In all but the highest of the high rises, this method works very quickly and efficiently, transporting large numbers of passengers quickly to their specific destinations.The only significant difference between the elevator operations and GRT is that each elevator has its own shaft whereas the individual GRT vehicles each share common main-line tracks. This makes the GRT operation considerably more efficient, if a bit more complex than the elevator operation.In very tall buildings with 80 plus stories, the need to restrict elevator shaft space results in a series of compromises. One technique, employed effectively in the old World Trade Center, was to double up on the elevators. Each elevator opened on two floors simultaneously and passengers traveling to odd numbered floors would depart from the first floor while passengers traveling to even numbered floors would board at the second floor. This approach has no relevance for group rapid transit. Another technique, also employed in the World Trade Center buildings, the Empire State Building and many other very tall buildings involves the use of sky lobbies. A sky lobby is a floor about 50 stories up which would serve as a staging area for floors above. Passengers would take the express to the sky lobby and then would transfer to locals for their particular destination. This approach would double the usage of the elevator shafts since a single shaft could serve local passengers in the 1-49 range as well as in the 50-99 range.The difficulty of using the elevator analogy for GRT is that it presupposes a situation where the vast majority of travelers start or end their journey at a single destination i.e. the ground floor. This may be a viable model for a GRT system functioning as a commuter rail system serving a central downtown, however, in a true regional GRT application, while there may be a primary flow of travel during the two rush hours, the actual numbers of origins and destinations is large and the pairings of origins and destinations is complex.Nevertheless, there is a precedent for this pattern as well. Here the precedent is the national airline industry. In the airline industry the problem of grouping passengers by destination efficiently is solved through the use of a small number of regional or national hubs. This hub and spoke system design is similar to the sky lobby model but with the express cars reaching the sky lobby from a large number of remote destinations.So once again we find ourselves visiting the sky lobby.Transit traditionalists are quick to reject this model citing statistical models showing a decline in ridership for each "seat change". In this research, the quickest way to lose a passenger is to ask him or her to change vehicles in the middle of a trip. There are weaknesses to this reasoning, however. Much of the work documenting ridership reductions to required seat changes has been done in reference to changes in mode i.e. between train and bus without allowing for factors such as exposure to weather and uncertainty regarding the arrival of the new vehicle. Ridership reductions stemming from poorly timed modal interfaces which leave passengers shivering the night seem very reasonable.On the other hand, any traveler on the New York City subway system or several other major international transit systems will witness a fluid exchange of passengers between subway lines. I have even observed a significant amount of activity between express and local trains in search of mere moments of time savings. (Riding the northbound IRT #6 train - the Lexington Avenue line, I have seen the same passenger boarding at 59th street who left the train for the express at 42nd street.) The key to this fluidity is the fast, frequent service provided by the Transit Authority. This fluidity declines markedly during off-peak hours when the perceived delay of waiting for an express which has not reached the station exceeds the potential time savings. Conversely, during peak periods, one can change from a local to an express in the hopes of catching the previous local at the next express stop. This is not the case during the off-peak hours, so passengers tend to grab a seat on the local and sit.Since the large number of small vehicles in the GRT system design maximizes the likelihood of fast, frequent service, the applicability of the peak hour NYCTA passenger fluidity model seems realistic.A possible manifestation of this model could be a corridor oriented GRT serving a series of PRT's acting as local feeder/distributors as well as circulators within regional activity centers. In the northeast, where there is a well established commuter rail network utilizing very high capacity heavy rail cars, the GRT could then act as a feeder/distributor to the commuter rail line. Thus a hierarchy of vehicle sizes could be matched with a corresponding hierarchy of corridor capacity requirements.Another factor to consider is the ready availability of intelligent software which will allow the vehicle control system to "learn" the travel patterns of large groups of its regular passengers. As travel patterns emerge over time, specific trips could be formally or informally built into the vehicle routing software. For example, if it appears that a group of passengers tends to travel to between a small group of origins and a particular destination every day during the morning rush hour, a vehicle could be formally scheduled for that route. A schedule could be published showing that a vehicle will leave station N at T time, making scheduled stops at stations n1...nn and arriving at station D at time T2 or before. The advantage of the schedule is that it helps the passengers minimize their wait time at the originating stations, they can time their arrival at the station to allow time to make the T train. In the absence of a schedule, vehicle departing and loading times could be adjusted to ensure the optimal load for passengers traveling to station D.The difficulties of developing and operating this software in a way such that vehicle occupancy rates are high during peak periods and overall passenger wait times are minimized with no passenger encountering undue delay should not be minimized but they are well within the scope of existing scheduling algorithms.The sum and substance of this argument is that in an imperfect world where automobile drivers face the hassle of traffic and inconvenient parking spaces and even PRT passengers will have to face the compromises inherent in getting to and from stations, GRT systems may play a vital role in meeting regional mobility requirements.
Last modified: February 19, 2004