Modern navigation systems warn the user of traffic jams ahead and suggest alternative routes. However, a lemming effect can cause the alternative routes also to become congested, as the system suggests the same route to all users. As such, in an attempt to optimize for the individual driver, the welfare of the traffic network is punished. In this paper we introduce an online and proactive method for collective rerouting recommendations based on real-time data and stochastic optimization. Our system periodically monitors the status of the network to identify potentially congested roads together with vehicles affected by them. The system then uses Uppaal Stratego to perform machine learning and approximate the best rerouting scenarios. As a proof of concept, we build a SUMO model of a representative traffic network. We perform exhaustive experiments considering different traffic loads and different traffic light controllers. Our results are promising, showing considerable improvement in travel times, queue lengths, and CO2 emissions.