Textbook of Usability

The Steering Law, proposed by Johnny Accot and Shumin Zhai in 1997, extends Fitts's Law to constrained movements — navigating through a tunnel of fixed width rather than pointing to a single target. For a straight tunnel of length $D$ and width $W$:

$$T = a + b \cdot \frac{D}{W}$$

Crucially, the relationship is linear in $D/W$, not logarithmic as in Fitts's Law. Constrained movements are therefore much more sensitive to path length and narrowness: halving the tunnel width doubles the time.

The most important design implication concerns cascading menus. When a user must move the cursor horizontally through a narrow corridor to reach a submenu item, they are performing a steering task. The narrow corridor is slow and error-prone; one slip and the submenu closes.

Design responses include:

• Wider activation zones — generous corridors that don't require precise steering
• Hover delays (Amazon's "triangle" technique) that keep submenus open while the cursor moves toward them
• Mega-menus that avoid cascading entirely by showing all submenu content at once
• Flatter menu structures to reduce depth

The Steering Law also governs drag gestures, path drawing, and navigation through narrow scroll bars — any task where motion must stay within a bounded corridor.