Deterministic planning is the de facto standard in deployed multi-modal journey planning systems. However, in reality, transportation networks feature many types of uncertainty, including variations of the arrival and the departure times of public transport vehicles. Under uncertainty, deterministic plans could result in missed connections, leading to an arrival time much worse than originally planned. We contribute an empirical study using transportation network data from three European cities. We show that, in the presence of uncertainty, contingent plans can often provide significant savings in terms of travel time. We view our results as important because they advocate adopting uncertainty-aware multi-modal journey plans, shifting from the current practice based on using deterministic planning.