Luminance and contrast requirements for legibility and visibility of highway signs. Paper prepared for the 54th Annual Meeting of the Transportation Research Board TRB, Washington, D.C., January 1975.

The study reported on in this paper took place in 2 parts: laboratory tests and outdoor observations. In the laboratory tests, which were conducted in a darkroom, 51 series of test signs were projected from color slides against 5 backgrounds simulating rural, suburban, and lighted city conditions. Luminances ranged from 0.37 to 4.45 ft-L (1.3 to 15.1 cd/sq m). Legibilities ranged from 20 to 60 ft/in. (2.4 to 7.2 m/cm) and increased approximately linearly with log luminance. Legibility was impaired when contrast was below 50 to 60 percent and was maximum when contrast was between the ratios of 6 and 13 to 1. Optimum stroke width appeared to be narrower for white than for black letter combinations. Color recognition fell below 75 percent correct at lower luminances except for red. The reverse was true for small targets. Outdoor observations of 12-in. (30.5cm) reflective letters (2 color combinations) under headlights confirmed the lower contrast limits for legibility and effects of stroke width. High-beam outdoor legibility curves and laboratory-corrected trends were similar. Apparently, glance legibiilty, which was measured by the laboratory procedure, requires higher luminance. In a previous visibility study, luminance contrasts of sign to background and legend to sign were critical. These results suggest minimum requirements for both legibility and visibility.