Performance Comparison of Three Freeway Guide Sign Alphabets

Abstract

Modern fully retroreflective legends can create a blooming effect that reduces legibility. In this research project three alphabets were evaluated to determine whether the legibility of a white high-intensity legend on a green high-intensity background could be improved by reducing the blooming effect. The three alphabets were: Series E(Modified), Clearview, and British Transport Medium. Overhead and ground-mounted sign positions were evaluated in both daytime and nighttime conditions. There was no sign illumination for the nighttime conditions other than the vehicle headlights. Both legibility (unknown word) and recognition (known word) distances were measured. A total of 54 subjects participated in both daytime and nighttime trials. There was significant variability in the results of the various experimental conditions. In general, the results indicated that Clearview was slightly more legible than Series E(Modified) in the overhead position in both daytime and nighttime conditions. The extent of improvement over Series E(Modified) was generally in the range 2 to 8 percent. The greatest improvement was achieved for worst-case drivers. Clearview ground-mounted signs were less legible than Series E(Modified) in daytime conditions. In nighttime conditions, the ground-mounted Clearview signs did not demonstrate a consistently better performance than Series E(Modified). A greater degree of improvement was realized in the recognition of Clearview in the overhead position for both day and night conditions. British Transport Medium was generally less legible than Series E(Modified).