The rise of concepts such as the 15-minute-city represents the growing importance of accessibility by active mobility. In order to promote accessibility, accessibility instruments are developed that have substantial potential to assist practitioners in decision making processes. Therefore, this research starts with an up-to-date overview on the suitability of accessibility instruments when planning for active mobility. It was found that accessibility instruments were significantly further developed in the last few years and there is a rising number of tools that contain novel features. However, it was identified that there is a clear lack of tools specifically designed for modeling active mobility that are open source, include interactive scenario building, and can easily be transferred to new study areas.Therefore, an interactive accessibility instrument named GOAT (Geo Open Accessibility Tool) was developed, which is open source, transferable, and has an easy-to-use web interface. This instrument has been developed following an iterative software development process in close cooperation with practitioners from three municipalities in the region of Munich, Germany. The practitioners tested the tool independently in numerous workshops in order to provide feedback, which was integrated into the development. Furthermore, the tool was tested and transferred to more than 20 German municipalities, the City of Bogotá (Colombia) and Matosinhos (Portugal). First results show that the collaborative and open development process produced a user-centric solution, which bears the high potential to make planning for active mobility more effective and efficient.