I lived on the last streetcar line in Pittsburgh, PA. It survived for the simple reason it was faster then going by automobile for it was mostly on its own right of way, this included its own tunnel underneath Mt Washington. The tunnel emptied onto Smithfield Street Bridge and thus right into downtown Pittsburgh.
Various attempts to replace the Streetcars were attempted, but converting to buses was not one of them, for such a conversion meant abandoning the exclusive right of way and mixing with auto traffic, doubling or tripling the time of travel. Most other streetcar lines operated on public roads and thus auto traffic was the same for them as for buses, but not on the last lines in Pittsburgh.
An attempt was made to adopt a people mover in the late 1960s to replace the streetcars, but the technology was not quite there yet (Thus a limited number of stops). The reduction in stops hit the two larger user of the Streetcars the hardest (The plan was for both community to replace their streetcars, with buses that ran from one stop of the people mover to the next stop, instead of the direct loading onto the streetcars both communities had enjoyed since 1905. Both communities went into open revolt, and that delaying the adoption of the people mover, till a actual study was ordered by the Federal Government. In that study, done by engineers, pointed out from a per dollar per passenger basis, the upgrading the streetcars was more cost efficient then adopting the people mover.
Thus the plan came to be upgrading the Streetcar lines. That actually required the streetcar line to be shut down for two years as the old bridges were replaced and the tracks widen to permit use of new LRVs in place of the 1940 era PCC Streetcars.
Anyway, when the new LRV system was adopted, I notice a lot more riders, but riders who did NOT follow the old rules. The main rule was to leave people out of the Streetcar before entering the streetcar themselves. These new riders all tried to get in at the same time people where exiting. Under the old system, by leaving people exit, you had more room in the Streetcar to fit, in the new system you ended up with people going both ways at the same time, slowing loading and unloading. It is still a problem with the LRVs, to many people think that they can fit in as people exit, when it would be better to leave people out and then enter. To many new riders in place of the older riders who did follow the old rule of waiting for people to exit.
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