The document discusses the positive and negative externalities of transportation networks. It notes that negative externalities like accidents, congestion, pollution, and road damage impose costs on society beyond what is borne by transport users. These external costs should be factored into transportation decisions to avoid misallocation of resources. The main negative externalities are accidents which result in medical, legal and lost time costs, as well as congestion which slows traffic flows and imposes delays. Positive externalities include increased land values and business benefits from proximity to transport, along with social benefits like accessibility, emergency response, and personal mobility.