Increasing the default interletter spacing of words can help drivers to read traffic signs at longer distances.

Would an increase in the default interletter spacing improve the legibility of words in traffic signs? Previous evidence on traffic sign design and recent studies on the cognitive processes involved in visual word recognition have provided conflicting results. The present work examined whether an increase in the default interletter spacing would improve the search of a word in direction traffic signs. To achieve this objective, twenty-two drivers participated in a driving simulation experiment. They followed a highway route and indicated whether a target place name was present among a set of distractors shown on direction traffic signs along the route. We compared the default interletter spacing of the Spanish "CC Rige" font (which is based on the internationally-used Transport font) and a 2.5-times expanded interletter spacing. The results revealed that the drivers were able to give a correct response at a distance to the traffic sign that was on average longer in the expanded than in the default spacing condition. This advantage in the legibility distance was observed in the absence of significant differences in reading accuracy, gaze behavior, or driving performance measures. Therefore, the evidence provided supports that drivers can benefit from a slight increase in interletter spacing relative to the standard spacing. Some of the design factors influencing this effect are discussed.
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