The promotion of cycling is essential for city planners aiming at sustainable mobility. Against this backdrop, we evaluate the “Leezenflow” system, a green wave assistant designed specifically for cyclists and is installed 110 meters in front of a traffic light in Münster, Germany. The system indicates the remaining time of the current traffic light phase through an expiring bar. This is intended to help cyclists adjust their speed in order to cross the traffic lights when green, and consequently optimize their cycling flow. We conduct a natural field experiment to analyze the system’s impact on cycling flow and safety, and find a statistically significant impact on cycling flow. The number of cyclists that have to stop at the red lights decreases by 6.6 %. The data also indicate positive effects on traffic safety. The results of the natural field experiment confirm and put into perspective the feedback of an accompanying online survey. The Leezenflow system can thus help city planners to promote cycling, thereby enabling more sustainable mobility.