Decarbonizing transport requires scalable alternatives to private vehicles, yet carpooling adoption remains limited due to coordination failures and weak network density. This paper develops a micro-founded framework of multimodal carpooling adoption under carbon pricing, behavioral nudges, and endogenous network effects. Agents choose between private transport and platform-based carpooling based on generalized costs and a participation-dependent matching probability. This generates nonlinear adoption dynamics, multiple equilibria, and tipping points driven by network externalities and matching efficiency. The model is calibrated using French mobility evidence and simulation-based methods. Results show that carbon pricing increases adoption but is constrained by coordination thresholds. Behavioral nudges significantly reduce these thresholds and amplify pricing effects. Platform efficiency critically determines whether the system converges to high or low adoption equilibria. Mu-CAR digital platform, currently under deployment, will provide future high-frequency mobility data enabling structural estimation and empirical validation. Overall, the framework highlights strong policy complementarities and network effects.
