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The strong expansion of road freight transportation throughout Europe is an important source of congestion and pollution as well as the cause of many accidents. Solving this problem will require the conjunction of many different remedies. One element of a solution would be the promotion and substitution of transportation modes with less negative effects. This paper focuses on this solution. It presents the results of a detailed modeling of the Belgian multimodal freight transportation network inserted within the overall trans-European network. It outlines the results of a simulation of flows over the Belgian network in 1995, which allows for an estimate of some of the costs of several external effects of freight transportation, namely pollution, congestion, accidents, noise, and damage to roads. The paper also presents the simulated impacts on modal choices of a marginal external cost internalization as well as an estimation of the corresponding external cost savings.