The quality of a road is usually gauged by a group of trained raters; the resulting numerical value is known as the Present Serviceability Index (PSI). There are, however, two problems with this approach. First, while it is practical to use trained raters to gauge the quality of major highways, there are also numerous not-so-major roads, and there is not enough trained raters to gauge the quality of all of them. Second, even for skilled raters, their estimates are somewhat subjective: different groups of raters may estimate the quality of the same road segment somewhat differently. Because of these two problems, it is desirable to be able to estimate PSI based on objective measurable characteristics. There exists a formula for such estimation recommended by the current standards. Its limitation is that this formula is purely empirical. In this paper, we provide a common-sense-based theoretical explanation for this formula.