Horizontal cooperation in logistics refers to how two or more companies can cooperate in order to achieve a common objective, which is usually related to minimization of global distribution costs. This paper discusses backhaul-based horizontal cooperation in road transportation and supports the relevance of this praxis as a way of reducing both routing costs as well as costs due to CO2 emissions. After describing the problem context and reviewing some related work, the paper examines different numerical examples in order to quantify the savings in routing and emissions costs that can be attained throughout backhaul-based horizontal cooperation. Both cooperative and non-cooperative scenarios are discussed and compared against each other. These considerations raise the value of the global objective function, permitting a realistic analysis of the importance of horizontal cooperation in the control of the environmental impact of road transportation.