In this report, the authors present new algorithms that use transit vehicles as probes to determine traffic speeds and travel times along freeways and other primary arterials. They describe a mass transit tracking system based on automatic vehicle location (AVL) data and a Kalman filter to estimate vehicle position and speed. They also describe a system of "virtual" probe sensors that measure transit vehicle speeds using the track data. Examples showing the correlation between probe data and inductance loop speed trap data are presented. They also present a method that uses probe sensor data to define vehicle speed along an arbitrary roadway as a function of space and time, a speed function. They present the use of this speed function to estimate travel time given an arbitrary starting time. Finally, they introduce a graphical application for viewing real-time speed measurements from a set of virtual sensors that can be located throughout King County on arterials and freeways.