Time in a constrained channel is determined by its length and width
Aliases: constrained path · tunnel task
What it is
The steering law describes movement in which a cursor or finger must remain inside a boundary continuously. Time to traverse a constrained channel depends on path length and width: a longer or narrower path, or a narrow bottleneck within it, is slower and more error-prone. Unlike acquiring one target, it concerns whether the entire trajectory remains constrained.
Why it happens
At each small portion of the movement, lateral error must stay inside an allowed range. Narrow channels demand slower motion and repeated visual correction; long channels accumulate more sections that require control. If width varies, a bottleneck is costly because the trajectory must be tightened before it is reached, not merely corrected at the endpoint.
Studying it
Tunnel tasks ask participants to drag a pointer from an entrance to an exit through paths of varied length and width, measuring traversal time, boundary crossings, deviation, and speed. Manipulate constant or varying widths, bends, device, feedback, and gain. Keep the trajectory time series: total time alone cannot locate whether the bottleneck is a narrow point, a turn, or initial alignment.
Where it stops holding
The model suits continuous control with a clear meaning for crossing a boundary. Discrete clicks, drags that can be released and restarted, automatic snapping, and tolerant paths alter the constraint. Finger occlusion, latency, and gesture recognition add costs on touch; mouse-tunnel parameters do not transfer directly to every drag interface.
Applying it
- Inspect narrowest width and total travel for drag, drawing, sliders, and hover paths, not only endpoint size.
- Remove unnecessary thin gaps and sharp turns; where unavoidable, use wider hit areas, snapping, or explicit stage changes.
- Record trajectories and mark where people slow, correct repeatedly, or cross a boundary to locate real channel bottlenecks.