Effects of Driving Speed and Sign Reflectance on Nighttime Highway Sign Legibility Distance and Reading Time Requirements

Nighttime highway sign legibility distance was evaluated as drivers maintained speeds of 5 versus 60 MPH. Effective legibility distance fell by approximately 30% when driving at high speed (114.9 versus 81.2 m at 5 and 60 MPH, respectively; F(1,9) = 48.6, p < 0.001). This finding suggests that past research, usually conducted under static viewing conditions (i.e., less than 5 MPH), probably overestimates the distance at which drivers can effectively read signs at night. In order to evaluate the appropriateness of new minimum sign reflectance levels proposed by the Federal Highway Administration (FHWA), legibility distance was also evaluated using 100% (new) versus 15% reflectance signs. Results indicate that nighttime legibility distance can be expected to fall by approximately 12% over the life cycle of a microprismatic retroreflective highway sign given the proposed FHWA minimum reflectance specification (104.2 versus 91.9 m for 100% and 15% reflectance, respectively; F(1,9) = 20.6, p < 0.001).