  Multitasking with a touch screen user-interface while driving is known to
impact negatively driving performance and safety. Literature shows that list
scrolling interfaces generate more visual-manual distraction than structured
menus and sequential navigation. Depth and breadth trade-offs for structured
navigation have been studied. However, little is known on how secondary task
characteristics interact with those trade-offs. In this study, we make the
hypothesis that both menu's depth and task complexity interact in generating
visual-manual distraction. Using a driving simulation setup, we collected
telemetry and eye-tracking data to evaluate driving performance. Participants
were multitasking with a mobile app, presenting a range of eight depth and
breadth trade-offs under three types of secondary tasks, involving different
cognitive operations (Systematic reading, Search for an item, Memorize items'
state). The results confirm our hypothesis. Systematic interaction with menu
items generated a visual demand that increased with menu's depth, while visual
demand reach an optimum for Search and Memory tasks. We discuss implications
for design: In a multitasking context, display design effectiveness must be
assessed while considering menu's layout but also cognitive processes involved.
