Legibility distances of smaller letters in changeable message signs with light-emitting diodes
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This paper presents the results of a legibility study of 9-in. and 10.6-in. letters on changeable message signs (CMSs) with light-emitting diodes. The study, conducted in Dallas, Texas, consisted of 60 subjects, demographically balanced with respect to age, gender, and education. The subjects drove a test vehicle as they approached CMSs with one of the above letter heights. Study administrators recorded the distance from the sign at which the participant could correctly read a three-letter word. Data were recorded for three trials on each of the letter heights for each participant. Data were collected during daylight (sun overhead) and nighttime conditions. The 85th percentile legibility distance for the 9-in. letter height was 228 ft for daytime conditions and 114 ft for nighttime conditions. For the 10.6-in. letter height, the 85th percentile legibility distances were 324 ft for daytime conditions and 203 ft for night-time conditions, respectively. The 85th percentile legibility distances for each letter height were used to estimate available viewing times under various approach speeds. These available viewing times dictate the units of information that can be presented on a CMS of a particular letter size.
