Over more than half a century, traffic flow theorists have been pursuing two goals: (1) simple and efficient models to abstract vehicular traffic flow and (2) a unified framework in which existing traffic flow models fit and relate to each other. Continuing these efforts, we report our humble understanding in a trio of papers. This paper (Part I) introduces a Field Theory with an emphasis on traffic flow modeling at the microscopic level. In this theory, highways and vehicles are perceived as a field by a subject driver whose driving strategy is to navigate through the field along its valley. The second paper (Part II) uses the Field Theory as a framework to relate existing microscopic and macroscopic models to each other and such relations are represented in a Unified Diagram. The third paper (Part III) provides numerical and empirical results of the Field Theory and benchmarks some traffic flow models presented in the Unified Diagram.