Redefining empty trips through occupancy and transferability of cross-modal strategies: A review

Empty trips, vehicle movements without payload, generate unsustainable externalities. This study addresses these challenges by identifying shortcomings in the literature on empty trip generation across all modes through a combined narrative and systematic review. The review revealed that the conventional definition of empty trips adopts a binary empty/non-empty perspective, which fails to capture the extent of the problem. In response, we propose a continuous measure based on the proportion of unused vehicle capacity. Our review also reveals that mitigation strategies are often case-specific, which limits their impact and transferability across modes. To address this limitation, we introduce a framework that facilitates the transferability of strategies by highlighting cross-modal similarities in generating empty trips. Finally, we reviewed these strategies, their methodologies, and the associated problems related to empty trips using the systematic review in driver-provided mobility, which includes public transportation, carpooling, taxi, and ride-hailing to support sustainable mobility through strategy generalization.