The effect of touch-key size on the usability of In-Vehicle Information Systems and driving safety during simulated driving

• The effect of touch-key sizes on the usability of IVIS's and driving safety was experimentally investigated.
• Both the measurements for usability of IVIS’s and driving safety increased with increased touch-key sizes.
• However, the measurements for usability and driving safety reached an asymptote beyond certain touch-key sizes.
• The appropriated touch-key size considering both the usability and the safety was derived.
• Fitts’ law analysis was performed under dual-task (pointing task while simulated driving).

Investigating the effect of touch-key size on usability of In-Vehicle Information Systems (IVISs) is one of the most important research issues since it is closely related to safety issues besides its usability. This study investigated the effects of the touch-key size of IVISs with respect to safety issues (the standard deviation of lane position, the speed variation, the total glance time, the mean glance time, the mean time between glances, and the mean number of glances) and the usability of IVISs (the task completion time, error rate, subjective preference, and NASA-TLX) through a driving simulation. A total of 30 drivers participated in the task of entering 5-digit numbers with various touch-key sizes while performing simulated driving. The size of the touch-key was 7.5 mm, 12.5 mm, 17.5 mm, 22.5 mm and 27.5 mm, and the speed of driving was set to 0 km/h (stationary state), 50 km/h and 100 km/h. As a result, both the driving safety and the usability of the IVISs increased as the touch-key size increased up to a certain size (17.5 mm in this study), at which they reached asymptotes. We performed Fitts' law analysis of our data, and this revealed that the data from the dual task experiment did not follow Fitts' law.