This paper analyses the determinants of peoples’ decision to purchase public transport season tickets. Based on the Deutschlandticket, it focuses on identifying primary customer groups, the role of public transport irregularities such as delays and cancellations, and behavioural inertia of repetitive purchase decisions. It builds on a panel dataset covering survey answers and GPS tracking data from 2920 participants between March and December 2023 and applies a series of static and dynamic binary logit models. The results confirm that the season ticket does not significantly contribute to a modal shift towards public transport, as it is primarily purchased by people who were frequent public transport users or even owners of a season ticket already in the past, whereas car owners refrain from purchasing the ticket. It cannot be confirmed that an increase in public transport reliability could boost season ticket ownership, as the impact of irregularities on the purchase decision is small and limited to specific subgroups. The purchase decision shows strong inertia, indicating that current season ticket owners are likely to keep their ticket in the future. Policy recommendations highlight the need to lower access barriers and introduce tickets variants targeting less frequent users of public transport to attract additional users.