Of the places I’ve lived in the US, Boulder makes car-free living the easiest and most enjoyable. For me, that means riding my bike. Yes, there’s a little snow, and a few times each winter bitter cold will slide down from Canada, and yes there’s a bit of topography coming out of the Boulder Creek floodplain. However, on balance the weather is very manageable with 300+ sunny days a year, and the terrain is varied enough to be interesting without daunting a healthy though unathletic cyclist. The city’s scale is also very accessible, with the longest possible trip taking about 45 minutes, between the northern and southern extrema. Most trips are 15 minutes or less. However, what really sets the city apart is the infrastructure and the burgeoning bicycle culture. Just watch Boulder Goes Bike Platinum from Streetfilms, and A Day in the Life of Community Cycles from Ryan Van Duzer.
I’m not saying it’s perfect, but whereas being a dedicated cyclist in Southern California felt like a heroic or sometimes Sisyphean labor, and often felt lonely, using my bike to get around here mostly just feels wonderful. It’s convenient, fast, cheap, and feels relatively safe. They plow the bike paths when it snows. Something like 10% of commute trips are done by bike. We have climbing lanes paired with downhill sharrows. The separated 13th St. contra-flow bike lane is blissful. There are sometimes (gasp!) signs specifically for bikes, telling you where the path you’re on will take you. This fall we got a couple of bike corrals on Pearl. Our cycling infrastructure can and should continue to be improved, but I think it might actually be more important right now to get more people familiar with using it.
I’ve also talked to people who don’t currently bike for transportation, but would like to. These folks are often outside the usual American cycling subculture demographic, which tends to be skewed toward young to middle-aged athletic and/or rebellious spandex-clad and/or tattooed males without families. In Los Angeles, I never felt I could recommend living car-free without reservations. It was clearly possible — I did it for 11 years — but it wasn’t always enjoyable, at least not in the way I knew it could be from living in Japan and bike touring in Europe. In SoCal, we were happy if we could just get the Powers That Be to recognize bikes ought to be considered transportation instead of (or in addition to) recreation, never mind getting them to make investments of money and space. Here, the City has been making those investments slowly over the past few decades. There, I was only really comfortable advocating the car-free life and its many benefits to people I knew, and who had a temperament to deal with the associated trials and tribulations.  Here, I feel like I can unabashedly recommend utilitarian cycling to just about anyone. Here the personal costs are much lower, and the benefits — economic, bodily, environmental, etc. — are as great as ever.
