Accot-Zhai Steering Law
Human physiology
Moving a cursor along a long, straight line is physically difficult for humans due to the physiology of our elbows and wrists. As a result, the longer the motion, the greater the chance of error.
When to Use
Steer-friendly design
Ensure the path in which the cursor must travel along dropdown menus, hierarchical menus, sliders and other path-following UI elements is as wide and as short as possible. Avoid hierarchical menus more than two-levels deep, and use a short time delay between mouse hover and reveal of the child menu.
Diagonal movement
Additionally, allow for diagonal movement between parent menu items and the corresponding child menu to ensure they are not inadvertently closed if the cursor strays from a straight path.
Origin
The steering law has been independently discovered and studied three times (Rashevsky, 1959; Drury, 1971; Accot and Zhai, 1997). Within human-computer interaction, the law was rediscovered by Johnny Accot and Shumin Zhai, who mathematically derived it from Fitts' law using integral calculus, experimentally verified it for a class of tasks, and developed the most general mathematical statement of it.