Usability of Menu-Based Interfaces
for Motor Vehicle Secondary Functions

Minoru Sumie, Charmian Li, and Paul Green

January, 1998

Three experiments were conducted to develop methods and data for examining the usability of secondary controls for accessing hierarchical menus system while driving. Hierarchical menu interfaces may be used in future vehicles for the operation of radio and navigation systems.

In the first experiment, eight drivers performed a secondary task in an instrumented car and the UMTRI driving simulator. The real and simulated roads were not matched. There were no differences in secondary task time-related measures between contexts, but absolute validity of the primary task was not high. As in previous studies, speed variance was lower and lane variance was greater in the simulator. This experiment established that the UMTRI simulator was a reasonable context for examining the use of secondary controls.

In the second experiment, two subjects drove the simulator for brief periods of time over 20 successive workdays. Secondary task performance continued to improve across the entire period, with most of the performance gains being evident by the fifth day. There were few interactions of time with test conditions, suggesting that testing across a large number of days was not necessary.

In the third experiment, 16 subjects drove the simulator while retrieving information using a menu system. In terms of task completion times, the number keypad yielded the best performance, with the rotary knob and the trackball tied for second, and the touchpad yielding the worst performance. As for the menu structures, the order from best to worst was 8 x 2 (8 items, 2 levels deep), 4 x 3, and 2 x 6, though the differences between 8 x 2 and 4 x 3 were small.

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