Driver Eye Fixation and Reading Patterns while Using Highway Signs under Dynamic Nighttime Driving Conditions: Effects of Age, Sign Luminance and Environmental Demand

The purpose of this study was to quantify real-world highway sign reading behavior while driving at night. Sign reading behavior was assessed using traditional self-report augmented by an in-vehicle eye tracking system. Good agreement was found between eye movement data (last-look distance) and verbal report measures of sign legibility distance (r = 0.9). Significant reductions in legibility distance were observed as sign reflectivity was reduced from “newly installed” levels down to a level representing proposed FHWA minimum retroreflectivity values. The magnitude of this effect varied as a function of the driving environment. Decreasing sign reflectivity was accompanied by a 17% reduction in legibility distance while driving on a high speed (65 MPH) rural road; while a reduction of 24% was observed for a low speed (35 MPH) road located in a visually cluttered suburban business district. No age differences in sign legibility were observed. Mean duration of the last fixation made while reading exceeded 3 seconds. Total eyes-on-the-sign time exceeded 6 seconds given unrestricted sight distance. Dynamically determined measures of average sign reading distances yielded legibility indices that ranged from 32-36 ft/in on a rural highway at 65 MPH to 30-37 ft/in for a suburban street at 35 MPH, significantly shorter than the 40 ft/in in the MUTCD design recommendation. Taken together, the eye fixation data suggests that reading a missioncritical highway sign requires more attentional resources than expected. Long-range conspicuity afforded by highway signs could not be adequately quantified since measurement reliability of the eye movement data was poor beyond distances of 300 m.