In the field of tunnel lighting design, a linear relationship in double logarithmic plots with a slope of minus 0.5 has been widely referred to as the relation between average road surface luminance and the overall uniformity of road surface luminance with keeping object visibility constant. This indicates that if the uniformity of road surface luminance is increased, the average road surface luminance could be lowered while maintaining object visibility at a certain level. Advancements in lighting technology, such as LED, have enabled highly uniform road surface luminance; however, lowering the average road surface luminance is a concern given the increase in elderly drivers. In this study, the relation between average road surface luminance and the uniformity of road surface luminance was investigated for young and elderly observers over a range of low to high uniformity. Our results indicate a non-linear relation in double logarithmic plots, especially in the low road surface luminance and high uniformity region. Furthermore, a marked increase in average road surface luminance is necessary for elderly observers to maintain the same visibility level as that of young observers.