Methods and systems for motion detection can be used with groups of elements, such as groups of people. Motion detecting includes acquiring a series of images including a current image and a previous image, and determining multiple optical flow vectors. The optical flow vectors each represent movement of one of several visual elements from a first location in the older image to a second location in the current image. Average velocities are determined and stored for the optical flow vectors for different time points. A motion index is calculated using the average velocities. The average velocities can be positive or negative. Filters can be applied to exclude selected images from the motion detection field.