  Collaborative Vehicle Routing is where delivery companies cooperate by
sharing their delivery information and performing delivery requests on behalf
of each other. This achieves economies of scale and thus reduces cost,
greenhouse gas emissions, and road congestion. But which company should partner
with whom, and how much should each company be compensated? Traditional game
theoretic solution concepts, such as the Shapley value or nucleolus, are
difficult to calculate for the real-world problem of Collaborative Vehicle
Routing due to the characteristic function scaling exponentially with the
number of agents. This would require solving the Vehicle Routing Problem (an
NP-Hard problem) an exponential number of times. We therefore propose to model
this problem as a coalitional bargaining game where - crucially - agents are
not given access to the characteristic function. Instead, we implicitly reason
about the characteristic function, and thus eliminate the need to evaluate the
VRP an exponential number of times - we only need to evaluate it once. Our
contribution is that our decentralised approach is both scalable and considers
the self-interested nature of companies. The agents learn using a modified
Independent Proximal Policy Optimisation. Our RL agents outperform a strong
heuristic bot. The agents correctly identify the optimal coalitions 79% of the
time with an average optimality gap of 4.2% and reduction in run-time of 62%.
