Transportation planning concerns the planning, functional design, operation, and management of facilities of any mode of transportation used to provide movement of people and goods. Traffic engineers are primarily concerned with planning for the flow of traffic on urban and rural streets. They usually have degrees in civil engineering, with specialization in traffic engineering. Highway engineers design streets, bridges, interchanges, and street drainage facilities; they prepare construction plans and specifications for them, and supervise their construction. Knowledge of present land uses and present socioeconomic conditions are needed as input to a traffic model; they are a source of the data that is used to test the ability of the model to replicate existing traffic flows. If the model assigns trips to the existing street and transit networks that appear to replicate traffic flows with reasonable accuracy, it is probable that the model is working reasonably well.