Traffic characteristics intrinsic to contiguous and buffer-separated high-occupancy vehicle (HOV) facilities were explored by examining flow–density–speed relations and an intervening effect from the adjacent general purpose (GP) lane. Detector data were collected from multiple locations and systematically filtered to form well-defined relations between flow, density, and speed that characterize the traffic stream in the HOV lane. Analysis outcomes of the collected data indicated patterns of flow–density–speed relations for contiguous and buffer-separated HOV lanes. The data from the contiguous HOV lanes exhibited relationships as a function of the adjacent GP lane speed, whereas those in the buffer-separated HOV lanes were rather independent of the adjacent GP lane speed. Implications were drawn for capacity and speed of HOV lanes and documented in this paper. These findings were confirmed by comparative analysis of pre- and postconversion HOV configurations, from buffer-separated to contiguous, in a real freeway site.