PeMS 4 is the latest of four task orders devoted to research, development, and maintenance of the PeMS system. PeMS collects, processes, stores, and makes available online data from the six Caltrans districts (D3, 4, 7, 8, 11, 12), which include the major urban areas in California. The data are obtained from 23,237 loops, grouped into 7,359 vehicle detector stations (VDS). These loops cover 2,812 out of 30,726 miles of interstate and state highways in California. PeMS began as a research project. As the research system evolved, Caltrans determined that the information it provided was very...