A wide variety of light sources is used for externally illuminated highway signs. Some of these light sources change the color appearance of signs at night. This study evaluates acceptable alternative light sources for illuminating highway signs. Light sources investigated included incandescent, fluorescent, metal halide, mercury, high pressure sodium, and low pressure sodium lamps. The metal halide lamp performed best overall and is recommended to illuminate a broad range of highway sign colors. This also could include the use of metal halide lamps in future automobile headlights. Mercury lamps that are economical and provide good color rendition on green, blue, and white are recommended for overhead signs. With some compromise on the color rendition, high pressure sodium is another cost-saving alternative for overhead signs. High pressure sodium is also the best choice to illuminate construction and maintenance signs.