Legibility Distances of 9 to 18 Inch Characters For Light-Emitting Diode (LED) Dynamic Message Signs on Arterial Roadways

This report documents the results of a legibility study of 9-inch, 1O.6-inch, and 12-inch high characters on dynamic message signs (DMS) for use on arterial roadways. The study, conducted at Dallas, TX, consisted of 60 Dallas residents (demographically balanced with respect to age and education) who drove a test vehicle as they approached DMSs with one of the above three character heights. Study administrators recorded the distance from the sign at which the participant could correctly read a three-character word. Data were recorded for three trials on each of the three character heights for each participant. Data were collected during daylight (sun overhead) and nighttime conditions. The 85th percentile legibility distances for each character height were used to estimate available viewing times under various approach speeds. These available viewing times dictate the units of information that can then be presented on a DMS of a particular character size. Based on the results of the analysis, researchers recommend that the City of Dallas continue to utilize 12-inch characters for DMS on their arterial roadways. Even then, the amount of information that is presented on the DMS should be limited to 3 units of information or less under nighttime viewing conditions, Agencies should consult other references, as documented within this report, regarding proper message design principles, appropriate abbreviations to use, etc. prior to designing and implementing an arterial street DMS system.