  To gain essential understandings of traffic flow, four theorems are derived
to establish the kinematics of the basic unit of traffic flow, namely two
consecutive vehicles. The first is to determine the two critical distances of
the vehicle spacing, the second is to derive the spacing margin and the
critical distance difference as well as the relationship they must satisfy, the
third is to complete the mathematical definition and give the necessary and
sufficient condition for the time headway to remain constant, and the fourth is
to derive the governing equation of the spacing margin and determine the two
key constraint spaces of the motion of consecutive vehicles. On this basis, the
composition of the traffic flow phenomenon and that of the traffic flow
property are determined, and the inherent regularity of traffic flow is briefly
elucidated. Thus, a basic theoretical framework specific to the study of
traffic flow is obtained. Based on the framework, three important issues,
namely traffic flow instability, phase, and empirical datasets, are briefly
reviewed.

Physics and Society