Transportation engineering theory encompasses various models and methodologies for analyzing and optimizing transportation systems' efficiency, safety, and sustainability. Investigate the theoretical basis of traffic flow theory, including fundamental principles such as traffic stream characteristics, flow$-$density relationships, and traffic equilibrium models. Explore advanced concepts such as microscopic and macroscopic traffic flow modeling, stochastic traffic processes, and the application of intelligent transportation systems $($ITS$)$ for traffic management and control. Evaluate the theoretical foundations of transportation network analysis, including graph theory, optimization algorithms, and equilibrium assignment models. Finally, discuss the theoretical challenges and opportunities in addressing emerging issues in transportation engineering, such as connected and autonomous vehicles, shared mobility services, and sustainable transportation planning principles.