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Concept
Three-phase traffic theory
Summary
Three-phase traffic theory is a theory of traffic flow developed by Boris Kerner between 1996 and 2002. It focuses mainly on the explanation of the physics of traffic breakdown and resulting congested traffic on highways. Kerner describes three phases of traffic, while the classical theories based on the fundamental diagram of traffic flow have two phases: free flow and congested traffic. Kerner’s theory divides congested traffic into two distinct phases, synchronized flow and wide moving jam, bringing the total number of phases to three:
Free flow (F)
Synchronized flow (S)
Wide moving jam (J)
The word "wide" is used even though it is the length of the traffic jam that is being referred to.
A phase is defined as a state in space and time.
In free traffic flow, empirical data show a positive correlation between the flow rate (in vehicles per unit time) and vehicle density (in vehicles per unit distance). This relationship stops at the maximum free flow with a corresponding critical density . (See Figure 1.)
Data show a weaker relationship between flow and density in congested conditions. Therefore, Kerner argues that the fundamental diagram, as used in classical traffic theory, cannot adequately describe the complex dynamics of vehicular traffic. He instead divides congestion into synchronized flow and wide moving jams.
In congested traffic, the vehicle speed is lower than the lowest vehicle speed encountered in free flow, i.e., the line with the slope of the minimal speed in free flow (dotted line in Figure 2) divides the empirical data on the flow-density plane into two regions: on the left side data points of free flow and on the right side data points corresponding to congested traffic.
In Kerner's theory, the phases J and S in congested traffic are observed outcomes in universal spatial-temporal features of real traffic data. The phases J and S are defined through the definitions [J] and [S] as follows:
A so-called "wide moving jam" moves upstream through any highway bottlenecks.
Official source
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