The economic evaluation of a project in any sector entails the measurement and comparison of cost and benefit streams expected from alternative investments. This paper presents an exposition of the social surplus method of measuring benefits. The exposition is intended to shed light on the nature of benefits to be expected from road transport projects, both with and without various types of market imperfections, and in particular, to show how these benefits relate to changes in the supply and demand of transported commodities. Thus one purpose of the paper is purely expository; to record the economic mechanics behind the social surplus method of benefit evaluation as applied to road projects. In the following chapters, the paper presents a method of measuring benefits which takes traffic responses into account, discusses the factors underlying their relative importance, relates the benefits to associated changes in the production and consumption of the transported commodity, and interprets the impact of market imperfections on benefit measurement.