As an example, I have lived in Houston in the past for some time. It is huge - I commuted roughly 20 miles each way. That's not "bikeable". And not everyone goes to the same place. Some work downtown, but not nearly as many as you'd think. The energy corridor is in an entirely separate area. Many people work in the Woodlands. Some people have jobs out in Katy. Some work in the industrial areas or near the ship channel.
And don't even get me started on the heat. Nobody wants to bike when it's 95 degrees out, even if you can shower once you reach your destination.
Many of the proposed solutions are centered around cities like New York: small metropolitan areas to which everyone commutes with at least reasonable weather and with reasonable distances. It's also very difficult to design good public transportation for such a city as I described, especially since many of the places I listed are technically different cities.
With that said, I would be interested in hearing ideas for how to solve a problem like this. I and friends of mine have debated how to solve such an issue before, and come away without an answer. If someone has an idea, let me know.