Across the world, there is considerable interest in promoting bicycle use. In addition to reducing the climate impact of urban transportation, it will help to improve public health, and reduce traffic congestion, noise and air pollution. Provision of bicycle-friendly infrastructure is a primary means to achieving this. Using a large dataset of observed bicycle trip trajectories and fine-grained network data covering the city of Copenhagen, Denmark, this paper finds a large effect of infrastructure provision on the volume of bicycle traffic.