Defining effective policies for managing traffic in large cities, particularly near major logistics hubs such as ports, is a challenging problem due to the critical interaction between mobility and freight flows. In this work, we combine a traffic simulator with a change-detection test to assess a priori whether a specific policy would have an impact on city mobility. More specifically, we propose a general methodology to identify the expected number of days/monitoring samples before gaining evidence that the policy has introduced a detectable change in the traffic data acquired after enforcing the policy. Our experiments, conducted on simulated traffic, focused on the port-city context of Genova, showcase that our proposed methodology can provide outcomes that are consistent with the kind and expected effectiveness of policies under evaluation.