The Four Step Model
The history of travel demand modeling has been dominated by the four step model. The application of this modeling approach is near universal as in large measure are its criticisms. While the approach has been moderately successful in the aggregate it has failed to perform in most relevant policy tests on both the demand and supply sides. This paper begins with an overview of the four step model (4SM) in the broader context of transportation systems analysis. This discussion is followed by problem and study area definitions, model applications and data requirements. Finally, the most commonly used models in the four step process are illustrated through a sample application. I argue that the widespread use of the 4SM is not due to its analytical superiority but simply because it is the only approach available given current institutional requirements and financial limitations.