The burden of highly subsidized public transportation services necessitates careful planning of operations and resource management. Traditionally, planning involves utilizing costly and cumbersome methods to collect historical data on passenger behavior and travel demand. This study aims to contribute to the use of passive data in the public transportation planning processes by utilizing smart card data from a one-weekday to estimate the potential mode shifts from bus transits following the commencement of the extended metro line in Narlıdere, İzmir. For this purpose, first, the trip chaining algorithm, widely used in literature, is used to estimate the alighting location of passengers. Then, the competitive bus routes are determined, and this process is accompanied by other algorithms developed to create alternative scenarios where the passengers repeat their trips by utilizing the metro. Finally, the mode shift behavior of the passengers is estimated by employing travel time saving, which is calculated both deterministically and considering the passenger’s travel convenience. As a result, 39 bus routes are identified operating inside the service area of the metro extension. Sixteen of them are selected for further analysis based on their competitiveness, which is higher than 55%. The results showed that almost 30 to 55% of the passengers on the competitive bus routes favor the metro opening by lowering travel time and/or increasing travel convenience. Additionally, out of 30,779 passengers using competitive bus routes, it is either not possible or feasible for 5,517 of them to switch to the metro because 76% start and end their journeys outside the metro's service area. Furthermore, the results of the spatial analysis on travel time savings revealed that investments benefit not only those who live nearby but also those who live further away. This emphasizes the significance of enhancing public transportation services, which promotes convenience and accessibility in mobility. Lastly, it is essential to note that the outputs of this study are contingent on assumptions and that varying assumptions will alter the outcomes.