The process of transportation system management (TSM), the nature of its impacts, impact measures, and analysis techniques are described. The use of basic measures such as capacity, travel time, vehicle occupancy, accidents, transit ridership, and costs is emphasized, and it is shown how each can be estimated on the basis of analogy, published relationships, or analytical models. Impact measures are relatively few for any project, not universally required, and have specific interrelationships. Once the primary measures are computed, the secondary ones can be derived as necessary. Most TSM actions deal with localized improvements whose impacts are small in scale and difficult to estimate. Therefore impact assessment techniques should be direct, simple, and in scale with the problems involved, degree of accuracy required, and resources of the community. Impact assessment is a means, not an end. The main goal of TSM is improvement, not analysis.