A TRAFFIC SIGN REQUIREMENTS STUDY WAS CONDUCTED ON FACTORS AFFECTING ABILITY AND ATTENTION VALUE. OUTDOOR OBSERVATIONS WERE MADE BY SUBJECTS RIDING OVER A STANDARDIZED COURSE. MATHEMATICAL MODELS BASED ON KNOWN VISUAL AND LOGICAL RELATIONSHIPS WERE TESTED AGAINST EACH SET OF LABORATORY RESULTS. IT WAS DETERMINED THAT AVERAGE VISIBILITY AND ATTENTION VALUE OF HIGHWAY SIGNS CAN BE ESTIMATED FROM CHARACTERISTICS OF LEGEND, SIGN AND ENVIRONMENTAL BACKGROUND. RELATIVE BRIGHTNESS AND CONTRAST OF SIGN-TO- BACKGROUND AND OF LEGEND-TO-SIGN ARE OF PRIMARY IMPORTANCE FOR VISIBILITY AND THE ATTENTION VALUE. IN VISIBILITY EFFECTS FROM COLORS, RELATIVE BRIGHTNESS IS OF MOST IMPORTANCE, BUT HUE CONTRAST ENHANCES THE BRIGHTNESS EFFECTS IN SOME CASES. FOR BEST VISIBILITY, A SIGN SHOULD BE DARKER AGAINST A BRIGHT DAY BACKGROUND BUT BRIGHTER AGAINST A DARK DAY OR NIGHT BACKGROUND. A FORMULA FOR ESTIMATING VISIBILITY EFFECTS IS SUGGESTED ON AN APPROXIMATE BASIS AND WITHIN LIMITS.