Second in a series about opinions held by the "swing voters" of bicycling.

To people who ride bikes in many U.S. cities, Portland and San Francisco might feel like heaven. But for the thousands of people in those cities who own bikes but don't ride frequently, that's far from the case.

Asked to rate the amount at which they were "concerned about safety when cycling in your city" on a scale of 1 to 5, three-quarters of such riders said

    

First in a series.

San Francisco and Portland are celebrated as two of the best U.S. cities for biking. In fact, one in every 25 American bike commuters lives in one of these two cities.

But even in these cities' bike-friendly neighborhoods, hundreds of thousands of people — it's perhaps half the population — have ridden bikes before but rarely use them.

What's stopping them?

Even if they don't personally bike, what images make them