Probabilistic journey planners accounting for uncertainty inherent to travel time are becoming more widely available. One common difficulty associated with such sophisticated planners is communicating the risk in the proposed plan to the end users. In this research, we have empirically investigated how different representations of journey time distributions affect people's understanding. Specifically, we have compared three graphical representations of continuous probability distributions representing journey times: a continuous function graph, a histogram, and a new representation we propose, based on the cumulative distribution function. We evaluated both reading and comparison understanding and explored whether a short tutorial was useful to increase performance. Based on our findings, we provide recommendations to developers of applications involving the visualization of probabilistic journey times.