Influence of letter size on word reading performance during walking

Mobile devices like smartphones facilitate parallel activities of IT-interaction and other tasks. Especially smartphone usage during walking can often be observed. This can lead to distracted walking which frequently results in serious consequences for the user and others. As people will probably go on using mobile devices in this way, e.g., for navigation, we have to provide interaction interfaces which are adapted to the special needs of secondary usage while walking. One way to adapt apps to walking is to reduce time for content perception. A substantial part of content is given in written language, and therefore, reading is of high importance. Based on this, we pursued the reduction of reading time on a mobile device during walking. We conducted a study which provides information about the minimum acceptable letter sizes for reading single words while walking. We carried out an experiment in which we combined short presentation times of commonly used words with different letter sizes and analyzed the outcome by means of psychometric functions. We administered these functions for the independent variables presentation time, walking speed vs. standing and length of words (number of letters) and analyzed the outcome statistically in respect to the minimal visual angle. We found highly significant influences of all three conditions on the legibility of the words. We recommend the adaptation of applications for walking according to our findings.