A study was conducted to determine the legibility impacts of freeway guide signs when encapsulated retroreflective sheeting is replaced with microprismatic retroreflective sheeting. The study included freeway guide signs mounted in an overhead position and exclusively illuminated with vehicle headlamps. A total of 60 subjects divided into three age groups participated in this nighttime study. All 60 subjects drove two vehicles, a modern sport utility vehicle (SUV) and a late-model passenger car. The findings show that microprismatic sheeting does provide statistically longer legibility distances than encapsulated sheeting. Overall, the improvement was 53 ft, or 9.5%. However, for the modern SUV, the improvement was much greater (78 ft) compared with the late-model passenger car (28 ft). The main differences are related to the evolution of vehicle design and specifications. Today’s United States citizens prefer large vehicles such as an SUV, pickup, and minivan. These vehicles also meet recently revised headlamp specifications. These two issues inherently reduce the amount of headlamp light retroreflected from the sign back to the driver. Unfortunately, these counterproductive trends show signs of continuing. Considering the increasing proportion of older drivers in the United States, it becomes even more critical that transportation agencies do all they can to increase overhead-sign luminance. The findings show that increasing overhead-sign luminance by switching from encapsulated retroreflective sheeting to microprismatic retro-reflective sheeting results in significantly longer legibility distances. The magnitude of the difference will continue to increase as long as the SUV-like proportion of the U.S. fleet continues to grow and headlamp specifications continue to direct less light toward overhead signs.
