A series of projects at PATH have addressed many aspects of the use of cell phone or probe data for traffic estimation. This data (primarily from fleet vehicles equipped with transmitters) is available from a number of commercial vendors. However, the data must be filtered, processed, and fitted, in order for it to be used by Caltrans’ Performance Measurement System (PeMS). As the number of cars providing mobile data increases, its value for use in estimation, prediction, and traffic management also increases. Researchers have created prototype software to analyze probe data and improve traffic operations managers’ ability to use the information garnered from it.