Trajectory-based interactions, such as navigating through nested-menus, drawing curves, and
moving in 3D worlds, are becoming common tasks in modern computer interfaces. Users'
performances in these tasks cannot be successfully modeled with Fitts' law as it has been
applied to pointing tasks. Therefore we explore the possible existence of robust regularities
in trajectory-based tasks. We used ``steering through tunnels'' as our experimental paradigm to
represent such tasks, and found that a simple ``steering law'' indeed exists. The paper
presents the motivation, analysis, a series of four experiments, and the applications of the
steering law.
