In this study, we carried the spirit of Fitts' law a step forward and explored the possible existence of other robust regularities in interaction tasks. We first demonstrated that the logarithmic relationship between movement time and tangential width of target in a pointing task also exists between movement time and normal width of the target in a "goal passing'' task. A thought experiment of placing infinite numbers of goals along a movement trajectory lead us to hypothesize that there is a simple linear relationship between movement time and the ``tunnel'' width in steering tasks. We then confirmed such a "steering law" with three types of ``tunnels'': rectangle, cone, and spiral, all produced greater than 0.96 fitness. We then generalized the steering law in both integral and local forms. The integral form states that the steering time is linearly related to the index of difficulty, which is defined as the integral of the inverse of the width along the path; the local form states that the speed of movement is linearly related to the normal constraint.
The regularities presented in this study may enrich the small repertoire
of quantitative tools in HCI research and design. They can be applied to
many HCI problems. For example, input device research in the past was typically
based on Fitts' law, which is only meaningful for pointing tasks. Increasingly,
computer input devices are used not only for pointing to targets but also
for producing trajectories, such as in drawing, in writing, and in steering
in 3D space. The steering law provides a new experimental paradigm and
a performance metric for these tasks. Steering law can also be used for
designing hierarchical menus.