TRB Special Report 301: Air Traffic Controller Staffing in the En Route Domain: A Review of the Federal Aviation Administration's Task Load Model examines the structure, empirical basis, and validation methods of a Federal Aviation Administration model that estimates the time controllers spend performing tasks when handling en route traffic. The model's task load output is being used to inform workforce planning. The committee that developed the report concluded that the model is superior to past models because it takes into account traffic complexity when estimating task load. However, the report recommends that more operational and experimental data on task performance be obtained to establish and validate many key model assumptions, relationships, and parameters.