I live in Chico, CA; a smaller city of about 100k. Flat terrain, mild weather, good cycling infrastructure, a fairly high population density, and a strong cycling culture all make it easier to live car free. I think one thing that really helps here is that civilization ends. You could live on the edge of town and commute to work at the opposite corner and it still wouldn't be a 10-mile commute. Most people here are only commuting 2-4 miles. That's pretty easy over just about any terrain and especially in the flat as a pancake Central Valley.