Christof Spieler atIntermodality, the blog of Houston's Citizen's Transportation Coalition, may have the answer:
San Francisco's BART is on the left; DC's Metrorail is on the right. They're shown at the same scale, which reveals both the similarities and differences between the two systems. Says Spieler:
Rail transit projects don’t come with control groups — we can’t clone a section of a city, build two different rail lines, and compare the results. In this case, though, there’s an interesting comparison to be made between two remarkably similar rail systems.Spieler explains the difference:
San Francisco is in the 5th largest metropolitan area in the country. Washington DC is in the 4th largest. Both cities have old, urban cores with major employment centers surrounded by extensive post-war suburban development. In the 1960s, both decided to build a heavy rail system. And not only do those two systems use very similar technology, they are nearly the same length (104 miles vs. 106 miles).
There is one significant difference between San Francisco’s BART and Washington’s Metrorail, though: Washington has 2 1/2 times the ridership (902,100 average weekday boardings compared to 338,100.) Why? I’ve lived in both places, and I’ve ridden both systems. And I think the difference is that BART is primarily a suburban system while Metrorail, even though it serves the suburbs as well, is at its heart a urban system.
BART serves the suburbs. Metrorail serves the suburbs and the urban core.Spieler posts a couple more maps to illustrate this last point:
BART’s furthest station is 25 miles from downtown SF as the crow flies, across two small mountain ranges. Metrorail’s furthest station is 15 miles from the center of DC.
BART has a single line through the city of San Francisco. It serves Downtown and one urban neighborhood, the Mission... Metrorail has 5 lines through Washington, serving many neighborhoods in all parts of DC. Metrorail also serves many more suburban employment centers than BART does...
BART saves money by using existing rights of way; Metrorail maximizes ridership by puting lines where the transit demand is...Only the San Francisco subway, a two-station subway in Downtown Oakland and a three-station subway in Berkeley [don't use existing rights-of-way]. The latter — which serves the University of California along with Downtown Berkeley — was built only because the city contributed money; BART planners wanted to put the station a mile from the edge of the UC campus. Metrorail uses some existing rail lines and a Virginia freeway corridor, but the majority of the system is in subway alignments that serve neighborhoods and employment centers...
BART stations are where the cars are; Metrorail stations are where the people are. The vast majority of BART stations are car-oriented. The “typical” BART station is an elevated structure surrounded by park-and-ride lots in a low-density neighborhood. Over half of Metrorail stations, by contrast, don’t even offer parking. These stations serve employment centers (urban and suburban), universities, neighborhood crossroads, and residential areas...
I've used both systems, and there's just no competition: the Metrorail is a lot more convenient for getting around DC. And it actually extends significantly into the surrounding suburbs, so it's not like it's useless for commuters; but that seems to be the primary function of BART. And that's how you end up with systems of similar lengths, but one of which has 2 1/2 times the ridership of the other: one was built for people, the other was built for cars.
Actually, considering that pretty much everything we've done in the US to develop our cities since WWII has been designed primarily for the use of cars, rather than people, it's sort of miraculous that the DC system got built the way it did. Here's hoping that the designers of future mass transit systems are receptive to the lesson that the differing histories of BART and Metrorail provide.