This document reviews the analytical needs that are explicit or implicit requirements of recent Federal legislation, and translates them into specific design features for future planning tools. The document focuses particularly on the implications of the Intermodal Surface Transportation Efficiency Act and Clean Air Act Amendments for the design of TRANSIMS, a new supercomputer-based planning tool. Particularly important for such tools will be forecasting issues related to infrastructure, congestion, and air quality. Specific model features reviewed include data structures; statistical models to characterize regions; representations of the traveling population, weather and atmospheric effects, and intermodal transportation system, and the simulation process for the transportation system itself.