Nighttime Legibility of Ground-Mounted Traffic Signs as a Function of Font, Color, and Retroreflective Sheeting Type (0-1796-2)

A closed course nighttime legibility study measured legibility distance for 6 inch letters using Highway Series D and two experimental fonts, Clearview Condensed Road and a D-Modified font. The Clearview font has a thinner stroke width than Series D and was used for white-on-green signs. The D-Modified font has a thicker stroke width than Series D and was used for black letters on white, yellow, and orange backgrounds. Three types of retroreflective sheeting were tested: ASTM Types 111, VIII, and IX. Forty-eight signs were used; all sign blanks were 12 inch x 30 inch with a border. Twenty-four participants, aged 55 - 75, drove a passenger sedan around a closed course at 30 mph while attempting to read ground-mounted signs on the right shoulder. Results showed no difference between drivers aged 55 - 64 and those aged 65 - 75. Overall legibility distances ranged from 143 ft to 206 ft, producing legibility indexes in the range of 24 - 34 ft of legibility per inch of letter height. Color was found to be a significant factor in legibility with yellow and white producing the longest legibility distances followed by green and then orange. Across all colors, retroreflective sheeting type was a significant factor with specific differences among sheetings dependent on color. The font results were surprising in that Highway Series D was better than or equivalent to both alternatives tested and the version of Clearview tested performed slightly worse than the standard font.