It's less about rural population and more about smaller metro populations. Places where there aren't bike lanes - where people on mopeds are really risking their lives trying to go twelve miles. The rural population quote of 17% are legitimately rural counties where population density below a certain point exists. Most smaller metro counties have rural infrastructure supporting a higher population density and simply are not safe for bikes, and a large number of people who are classified as urban are the people making up the population in metro counties. The US Census classifies as rural and urban so it's really hard to know exactly what, but assuming that anyone living in an urban area who is physically capable of doing so could ride a bike is incorrect.