Transportation projects, such as those for maritime ports, airports, passenger rail and freight rail are typically large investments and they can be a complex system that includes multiple modes, new technologies, operating concepts, and operators. These projects are initiated by metropolitan planning organizations, city port authorities, and statewide department of transportation agencies and others. Architectural or engineering services firms are often contracted to assist with the project planning steps. The planning processes include evaluating and ranking alternatives with subjective and quantitative criteria. Additionally, as projects evolve, there is a need to gain stakeholder approval and conduct environmental impact reviews. Simulation modeling provides a unique and flexible framework to assist with concept development, stakeholder consensus building, and public approval during environmental impact reviews. Modeling provides the ability to quantify concept performance differences between alternatives and demonstrate that a concept provides operational sustainability for each of the stakeholders and modal flows. This paper describes several transportation planning projects and how simulation modeling can provide quantitative performance metrics to support the planning processes to enable the project to move towards advanced design and engineering stages.