Menu hierarchies for in-vehicle user-interfaces: Modelling the depth vs. breadth trade-off

• We investigate the design of menu hierarchies for in-vehicle user-interfaces.
• Breadth is favoured over depth for structured menus.
• Breadth and depth are equally favoured for unstructured menus.
• Guidelines are given regarding acceptable combinations of breadth and depth for menus used in a driving context.

Menus are commonly employed within user-interfaces, but are not necessarily a suitable solution for emerging new contexts. For instance, for in-vehicle displays, the use of visually oriented menus creates a clear distraction burden. To investigate how the visual demand of menus varied as a function of their breadth, depth and structure, a study was conducted following the ISO occlusion protocol. Participants were asked to find and select target words on a touchscreen by navigating menus of varying breadths and depths (16 × 3; 8 × 4; 4 × 6; 12 × 2) when options were arranged either in alphabetical order (structured menus) or randomly (unstructured menus). Tasks were achieved either with full vision or whilst wearing occlusion goggles enabling only brief (1.5 s) opportunities to visually access the touchscreen. Preliminary equations were derived from the data indicating fundamentally different relationships for the visual demand of an interface dependent on whether anticipation can be used during the task. For structured menus, the visual demand was a logarithmic function of the breadth of the menus, whereas for unstructured menus, the relationship was quadratic. Moreover, results indicated that for structured menus breadth was favoured over depth, as the lowest visual demand was associated with 16 × 3 menu hierarchies. Conversely, for unstructured menus, compromise hierarchies (e.g. 4 × 6; 8 × 4) were associated with the least visual demand. Conclusions are drawn regarding the setting of boundary (acceptable/unacceptable) conditions for alternative menu structures for use within vehicles.