The freeway performance measurement system (PeMS) is a system for all of California. It processes 2 gigabytes/day of 30-s loop detector data in real time to produce useful information. Managers at any time can have a uniform and comprehensive assessment of freeway performance. Traffic engineers can base their operational decisions on knowledge of the current state of the freeway network. Planners can determine whether congestion bottlenecks can be alleviated by improving operations or by minor capital improvements. Travelers can obtain the current shortest route and travel time estimates. Researchers can validate their theory and calibrate simulation models. The use of PeMS in conducting operational analysis, planning, and research studies is described here. The advantages of PeMS over conventional study approaches are demonstrated from case studies on conducting freeway operational analyses, bottleneck identification, level of service determination, assessment of incident impacts, and evaluation of advanced control strategies.