Legibility distances of fluorescent traffic signs and their normal color counterparts

It is well known that fluorescent traffic signs have a daytime conspicuity that is far superior to that of normal color signs. This study was conducted to test the hypothesis whether adding the property of fluorescence, while leaving all (or in practical terms most) other sign features the same, does increase the legibility distance. Six diamond-shaped signs were fabricated and laminated with the normal colors on one side and their fluorescent color counterparts on the other. The investigated sign background colors included yellow, fluorescent yellow, fluorescent yellow green, pink, fluorescent pink, purple, and fluorescent purple. Four of the six signs used Landolt rings to avoid word familiarity issues. A pedestrian-crossing symbol sign and a "Plant Entrance" warning sign were used in addition to the Landolt ring signs. It was found that fluorescence actually did statistically significantly increase the legibility distances. However, the actual increases were fairly small from a practical point of view, ranging from 5.3 to 15.9 percent when adding the property of fluorescence. In general, for negative and positive contrast signs where both the legend and the background are fluorescent, a slight increase in the legibility distance can be expected. Note that the illumination conditions used (on a fair day) represent worst-case scenarios for fluorescent colors. Larger percent increases in legibility distance may be expected when using fluorescent colors during the time of twilight, overcast days, and daytime in inclement weather.