  In horizontal collaborations, carriers form coalitions in order to perform
parts of their logistics operations jointly. By exchanging transportation
requests among each other, they can operate more efficiently and in a more
sustainable way. Collaborative vehicle routing has been extensively discussed
in the literature. We identify three major streams of research: (i) centralized
collaborative planning, (ii) decentralized planning without auctions, and (ii)
auction-based decentralized planning. For each of them we give a structured
overview on the state of knowledge and discuss future research directions.
