A closed course nighttime legibility study measured legibility distance for 6-in. 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 III, VIII, and IX. Forty-eight signs were used; all sign blanks were 12 in. x 30 in. 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.
Samenvatting