The fifteen-minute city (FMC) concept aims to reduce greenhouse gas emissions, improve urban liveability and free up space by promoting a modal shift from car to active modes. According to theory, the proximity of all basic needs should have a maximum travel time of 15 minutes by bike or foot to achieve the FMC goals. However, basic needs are poorly specified and discussed, the effectiveness of the FMC concept is unknown and difficult to measure by current indicators. Therefore, this paper clarifies the FMC concept. It statistically analyses the effect on the promotion of active modes by logistic binary regression, applied to the Rotterdam-The Hague metropolitan region. It introduces FMC indicators to measure and geographically analyse urban regions. It reveals a set of amenities that characterises the FMC. Further, it demonstrates that the FMC concept has a positive effect on the probability of making trips by active modes, but also demonstrates the effects of other relevant variables, such as car ownership or the quality of cycle paths. Moreover, an FMC indicator that combines both the proximity of amenities and the number of amenities within 15 minutes predicts the effect of the FMC concept best. The FMC concept is beneficial to promote active modes, but to be most effective, active modes should also be stimulated via other measures, including the improvement of the quality of cycle paths and discouragement of car ownership for areas with all FMC amenities in close proximity.