This column gets it about right. Expanding an urban mass transit system really only is a worthwhile endeavor if it's accompanied by changes in development patterns. We're not talking about turning a suburb into Manhattan, we're talking about taking a place which is very-dense-but-not-dense-enough and encouraging higher density development around stations and transportation corridors.
The myth of LA is that it was the first major city to grow up around the automobile. The truth is, it's the first major city to grow up around the streetcar (1910 map). That's why even decades later it lacks the density that older cities, which grew up around walking and horse-n-buggies, have but is still quite densely developed throughout relative to newer sunbelt cities. The urban highways which were added later as the streetcars were disappearing aren't sufficient to handle the cars, but mass transit options won't really help much unless density is increased in enough places to make it pedestrian friendly. Rail will get you from A to B, but if you've got another mile or so to walk in area which isn't pedestrian friendly it isn't much of a help.
The thing is, to restate, a lot of LA is almost dense enough to be pedestrian friendly, but it's often undermined by car-centric development and policies.
...for a taste of LA density, play around with google's satellite imagery.
