Nowadays, several Apps record real-time data from social networks that enrich users’ decision making. We argue that these data may also be used to better inform policy makers in the absence of monitoring data or as a complementary tool of measurement systems. This is the case of Waze, an App designed to facilitate access to information on real-time traffic conditions reported by users and based on navigation tools through Global Position System in smartphones. We collect traffic speeds reported by Waze and use air quality measurements from the Air Quality Network in Bogotá to assess the impact of car-free days, a social experiment in Bogotá, aimed at promoting the use of alternative transport modes rather than automobile travel, and temporally addressing congestion and air pollution. Our results suggest that car-free days improve traffic speed in 20% and local air quality, measured by particulate matter, in 2%, compared to classical working days.
