The primary outcome was cycling prevalence as measured by numbers or rates of people cycling, including active commuting or leisure cycling. Studies where overall exercise or physical activity were measured were included if the effect on bicycling or cycling was reported separately. Study inclusion was conditional on having an objective measure of the physical environment. Policy reports or surveys exclusively investigating changes in municipal, regional or national policy rather than measuring cycling or exercise rates were excluded. Studies with potential for conflict of interest, for example, commissioned by construction companies or bicycle manufacturers were excluded.