But now if we expect a low-density city, say 16 houses per block, then the subway grid spacing should be wide, since there isn’t going to be much traffic justifying many lines. If the city is very compact, then the subway could even be a grid, at least locally. Let’s say blocks are 100*100 meters, and the planners can figure out the target density in advance when designing the subway network. To see this a bit more formally, think of an idealized city on a grid. The intuitive relationship between line spacing and density is that denser cities need more capacity, which requires them to build more rail lines. So why would denser cities have tighter line spacing? With so many streetcar lines extending well past Midtown, it is not too surprising that there had to be frequent subway stops. In the other direction, there were important streetcars on so many important cross-streets that it was desirable to intersect most or ideally all of them with transfers. If the line spacing is tight – say this is Midtown Manhattan and there is a subway line underneath Lex/Park, Broadway, 6th, 7th, and 8th – then crossing lines have to have tight stop spacing in order to connect to all of these parallel lines. The upshot is that the optimal stop spacing depends on the line spacing. This isn’t always achieved in practice, though Paris has just one missed connection on the Métro (not the RER), M5/M14 near Bastille New York has dozens, possibly as many as all other cities combined, but the lines built before 1930 only have one or two, the 3/L in East New York and maybe the 1/4-5 around South Ferry. The reason is that public transportation works as a combined network, which requires every intersection between two lines to have a transfer. The spacing between transit stops interacts with that between transit lines. This can potentially help explain why Paris has such tight stop spacing on the Métro and why New York has such tight stop spacing on the local subway lines. However, there is one exception to the rule that absolute density does not matter, coming from line spacing and transfer placement. there should be more stops in areas that along a line have higher density, for example city centers with high commercial density, but absolute density does not. To resurrect past models, higher overall density means that there are more people near a potential transit stop, but also that there are more people on the train going through it, so overall it doesn’t influence the decision of whether the stop should be included or deleted. Normally, the best interstation distance between subway or bus stops does not depend on population density.
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