"Transportation asset management (TAM) enters a new risk-based era with the enactment of the moving ahead for Progress in the 21st Century Act, also known as MAP-21. The legislation requires a risk-and-performance based asset management plan for bridges and pavements on the national Highway system (NHS). MAP-21 also requires development of a national freight network that consists of critical corridors, facilities and connectors. As transportation agencies develop their asset management plans, it is likely that they will stratify their risks based upon the importance of the key networks, corridors and facilities. This report, the fourth in a series of five reports on risk management, examines how agencies may apply risk-based asset management practices to key networks, corridors and facilities, particularly ones moving freight. The first three reports covered an overview of risk management, multi-level management of risks and achievement of policy objectives through risk management." -- Introduction