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I was going to comment on the watered-down BRT post, but this one may be more appropriate. It is very true that light rail / trams are vulnerable to the same kind of compromise as BRT. Here in Helsinki trams are so commonplace that bad design decisions are common. Mixed running, winding alignments and short dead end branches are all being planned and built. Internationally the risk of compromise does seem to be larger for BRT. I think this is not purely for cultural reasons. BRT solutions are often sold on lower costs. The high cost of rail means that it is taken more seriously. Buses also suffer less from mixed running. After all they can bypass obstructions and have similar driving characteristics to cars. As there is less (perceived) damage done, compromise is more attractive. On the cultural side the idea of rail vehicles mixing with car traffic is often foreign while buses mixing with cars are perfectly normal. So there are some technical or financial differences related to this wrong kind of flexibility even if cultural differences are probably more significant.
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I would argue that rail is probably not justified if the rideship in a corrider does not justify frequent off peak rail service. Of course this depends on many factors in each specific case. On the other hand, marginal off peak operating costs for a tram, LRV or even short metro train (if using coupled units) shouldn't necessarily be that much more expensive than the operating costs for a bus. After all the infrastructure and vehicle investments are there already and the driver cost should be similar. The vehicle running cost may be 1.5 to 3 times as much, but that is rarely a dominant cost. But if you do have to run a lot of feeder services as well, this will significantly increase the cost. In any case the Brisbane BRT looks like an excellent fit for local needs. The flexibility is undeniable especially with the city structure you describe. The possibility of running both fast and stopping services is also very efficient. Rail often tends to need two systems (or four tracks) for this. Of course the space requirements would be unacceptable in most European cities, but not in many parts of the New World as you rightly mention. One challenge for the future that I do see in the general case is just this ability to run fast and slow services. If conversion to rail is ever needed, one service group may have to be removed. This might even preclude the conversion from actually taking place or would probably push the stopping services back into the streets. Not a problem, if conversion isn't going to be needed though.
Toggle Commented Jan 3, 2010 on brisbane: bus rapid transit soars at Human Transit
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I broadly agree with your analysis, but as you mentioned it is somewhat generalized. You do not need to have all trip destinations in the central area as long as enough of them are there. The benefits to the majority just have to outweigh the inconvenience caused to the minority. Also many of the trips with transfers may be better in this scenario depending on the structure of the city. Even if you lose time coming into the central area, your final destination may be significantly closer than from the railway station. I believe this is the case for many destinations in Karlsruhe. Another possible factor is overall network design and timetable coordination. In Karlsruhe careful timetable planning at Durlach and other transfer stations may significantly alleviate the problem for passengers going past the central station, although I do not know whether this is the case. They will have to transfer of course, but again more people will benefit. It should also be noted that the light tram train vehicles have significantly more stops than the old heavy rail services, so there are also less transfers at that end of trips. In any case you are naturally correct in that tram trains are only optimal for specific scenarios. I generally agree with our description of these scenarios, but I would also like to add that tram-trains can have useful applications in larger urban areas as additions to larger systems. You might for instance extend an existing low volume local train line into a satellite town, which is conceptually close to the Long Beach case. Or you might use tram trains to alleviate crowding at a central station by moving some low volume services into city streets when space can't be found for more tracks.
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You are underestimating the size of the Bern tram a bit. A seven section Combino will be around 40 m long. At 2.3 m wide it is a bit narrower than a bus though, but still equivalent to at least two articulated 18 m buses. Your argument regarding capacity is still valid of course. By way of comparison Portland's streetcars are 20 m long.
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