The extensive adoption of intelligent travel cards has significantly enhanced the flexibility of pricing mechanisms in public transport. The incorporation of this technology has streamlined the process of targeting subsidies toward the most vulnerable commuters. In this study, we analyze the impact of a monthly voucher, which is loaded onto the travel cards of frequent users, on the utilization of public transport services. A controlled field experiment was conducted within the Integrated Public Transport System in Bogotá, Colombia, encompassing a participant pool of 1,607 individuals. We present empirical evidence of the presence of Mental Accounting Bias within the context of transport voucher utilization. Specifically, we show that individuals who received the transport voucher increased their usage of public transport, subsequently leading to a higher ridership rate. This use increment can be attributed not solely to the added income resulting from the voucher’s value but also to a response generated by a perceived increase of the available transport budget. This phenomenon underscores the prevalence of mental accounting bias among the participants. To demonstrate the presence of this cognitive bias, our research employs three distinct methodological approaches, each leveraging different sets of evidence collected during the experiment. This paper is the first empirical exploration of the Mental Accounting Bias within the domain of public transport. The potential for inducing Mental Accounting Bias through transport vouchers can be crucial for policies aimed at increasing public transport ridership.