Trust me if you can: Practical challenges affecting the integration of carpooling in Mobility-as-a-Service platforms

App-based Mobility-as-a-Service (MaaS) platforms combining public transport, car- and micro-mobility-shared services with real-time dynamic carpooling are emerging as viable alternatives to solo car use for sub-urban contexts. Insights from real-life implementation are however still limited. Which practical conditions may hinder the effectiveness of MaaS platforms leveraging carpooling? We tackle this question from the perspective of potential users of MixMyRide, a Swiss-based MaaS platform, based on co-design workshop sessions performed in Summer-Autumn 2022. We find four elements of practical interest, resonating with limitations already identified for carpooling. First, integrating carpooling in inter-modal trips increases the number of inter-changes potentially affected by delays. This requires real-time traffic information data, re-scheduling tools, and features for quick interaction between platform users. Second, features to create trust between strangers are needed, which require trade-offs between strict identity check and quick registration. Third, carpooling pick-up/drop-off may either endanger safety (if bus stops are used) or require inconvenient prior agreements, negatively affecting the MaaS platform's dynamism. Fourth, carpooling offer is not granted. To accept possible discomfort in sharing personal space with strangers and time-effort to enter trip offers, drivers need specific incentives, such as sharing of travel expenses, reward vouchers by public institutions, or virtual gamification and feedback on saved emissions.