To serve the needs of the future, transportation systems must undergo a radical transformation, from the centralized paradigm to the asynchronous, distributed paradigm that integrates fast computers and high-performance networks through novel computer algorithms. Intelligent Transportation Systems takes the first step towards meeting that challenge. Focusing on highway and railway systems, the authors stress the need for computer modeling and simulation to help design and validate complex, large-scale systems and the design of new performance metrics to estimate the performance of these systems. They present the basic principles related to travel-related decision making, including coordination, control, and routing.