Effect of Luminance and Text Size on Information Acquisition Time from Traffic Signs

Although visual performance was measured in terms of visual response time in many psychophysical studies, such an approach has not been used in evaluating the effect of luminance on traffic sign legibility performance. Traffic sign and retroreflective sign sheeting performance at night have been historically identified with the threshold (farthest) distance for legibility and in many cases from stationary vehicles with no restrictions on viewing time. Because traffic signs are not always read at threshold distances or threshold luminances and because the time available to read traffic signs is usually limited in the real world, a proper assessment of sign legibility performance requires determining information acquisition times above threshold conditions. This study investigated the effect of (legend) luminance and letter size on the information acquisition time and transfer accuracy from simulated traffic signs. Luminances ranged from 3.2 cd/m² to 80 cd/m² on positive-contrast textual traffic sign stimuli with contrast ratios of 6:1 and 10:1, positioned at 33 ft/in. and 40 ft/in. legibility indices, and viewed under conditions simulating a nighttime driving environment. The findings suggest that increasing the sign luminance significantly reduces the time to acquire information. Similarly, increasing the sign size (or reducing the legibility index) also reduces the information acquisition time. These findings suggest that larger and brighter signs are more efficient in transferring their message to the driver by reducing information acquisition time, or alternatively, by increasing the transfer accuracy. In return, reduced sign viewing durations and increased reading accuracy are likely to improve roadway safety.