Subjective Color Preferences of Common Road Sign Materials Under Headlamp Bulb Illumination

This document reports on a study undertaken to determine subjective color preferences of common road sign materials under headlamp bulb illumination. The authors first review the different types of headlamps that have become available, beginning with conventional halogen lamps that produce the familiar blackbody-like continuous-spectrum output light. In the last ten years, high-intensity discharge (HID) light sources have come onto the market place with their characteristic discrete spectra and higher correlated color temperature (CCT). In addition, coated ""blue'' halogen lamps have become available which reduce the amount of long-wavelength light and shift the light output to higher CCTs. The study incorporated side-by-side and sequential viewing of targets illuminated by headlamp bulbs. Neodymium, HID, ""blue''-coated, and conventional halogen lamps were compared. Common sign materials were used to make up the test targets, because traffic signs are among the most important colored objects along the roadway. The study focused on color viewing properties including color naming, specific color comparison preference, overall color comparison preference, and overall preference. They conclude with a discussion of the implications of the study results for both driving performance and aesthetics.