Substituting bicycling for driving is frequently promoted as a means of reducing energy consumption and the associated degradation of the environment. This paper estimates the magnitude of this effect. The analysis takes account of the first-order effects due to the dramatically lower energy requirements of transportation by bicycle relative to automobiles. The environmental benefits of human power are, however, strongly coupled to the environmental costs of increased population, due to increased longevity of those who engage in physical activity. Paradoxically, increased use of human power for transportation is unlikely to reduce substantially the use of energy because of this second-order effect. Human-powered transportation is therefore less an environmental issue and more an issue of public health. The interplay between longevity and environmental impact is a central feature of the conflicting societal objectives of improving human health and increasing environmental sustainability.