Mesopic vision corresponds to luminances in which both the rods and cones contribute to vision. Many light levels for roadways, parking lots, and other exterior locations result in luminances within the mesopic vision region. Research is summarized that shows that the potentially large magnitude of spectral effects under mesopic conditions cannot be predicted from orthodox models of visual response without additional information about the contrast, size, and context of a visual stimulus. If peripheral detection is a relevant visual task for a roadway lighting installation that operates at mesopic conditions, then characterizing light levels in terms of photopic luminous efficiency, as is conventionally done, will not properly characterize the effectiveness of those light levels for this response. Consideration of the visual periphery can result in different selection of light sources than would result from a consideration of on-axis vision alone.