This paper describes a method for the real-time measurement of vehicle queue parameters in a video-based traffic monitoring experimental system. The method proposed here is based on vehicle presence detection and movement analysis in video sequences acquired by a stationary camera. Queues are detected and characterized by a severity index. Intensive experiments show the robustness of the method under varying illumination and weather conditions. The system is presently undergoing an on-field testing phase in a double ways road near Trento, Italy, where queues frequently occur.