Open-Street Programs, also known as Ciclovia, are free multi-sectorial programs for people from different socio-economic backgrounds where public spaces and streets are closed to motorized traffic and open for leisure activities. Over the past three years the expansion rate of such programs worldwide has dramatically increased due to their general benefits to public health and their resource-efficient implementation. Performance indicators of Ciclovia programs allow analyzing the programs' impact on public health. The number of participants is one of the key performance indicators, and thus its reliable estimation is crucial to measuring the cost-effectiveness of the programs for the cities and municipalities. Furthermore, a unified and flexible estimation methodology allows comparisons between programs. In this paper, we propose a discrete-event simulation model to estimate the number of participants in such programs. We apply our approach to a case study in the city of Bogota (Colombia), with the largest program in the world, where we estimate an average of 675,000 participants per day. We also perform a sensitivity analysis on the arrival rate and study its impact on the estimation.