The Incredible Shrinking Letter: How Font Size Affects the Legibility of Text Viewed in Brief Glances

As in-vehicle interfaces have become miniature computers with userfacing LCD screens, the complexities of designing for them have increased tremendously. Given their safety-critical nature, designers must carefully consider every aspect of the vehicle’s digital interface. Recent research has suggested that even the typeface used to display the interface’s text can have significant impacts on driver behaviors such as total off-road glance time and secondary task completion time. Here the authors outline a psychophysical method for rapidly assessing the glance-based legibility of two different typefaces (a “humanist” and a “square grotesque”) presented in two different sizes (3mm and 4mm). Consistent with previous research, the authors find that humanist type is more legible than square grotesque. They also find that text is empirically less legible at 3mm compared to 4mm, and that this effect is especially pronounced for the square grotesque typeface. Legibility thresholds were also found to increase linearly with age, more than doubling across the age range studied. The authors hypothesize that the square grotesque’s intrinsic design characteristics cause it to scale poorly at small sizes and lose important details, especially in suboptimal display conditions.