The Cell Transmission Model framework, in this AMS effort, is used to model traffic conditions for both the freeway and arterials. At its most basic, it starts with a unidirectional roadway with one entrance and one exit. The road is divided into cells, or links (note that “cell” and “link” are used interchangeably), representing roadway segments. The cell length is chosen so it is reasonable for the speed limit and the desired time granularity of the results:
At each point in time, the traffic state consists of the number of vehicles located in each cell. The change in the number of vehicles in a cell (the evolution of the traffic state) is a function of the number of vehicles entering and leaving the cell:
The empirical relationship between the number of vehicles occupying a cell of length x (a.k.a. density) and the number of vehicles entering or leaving the cell over time t (a.k.a. flow) is represented by the cell’s fundamental diagram. This AMS study uses the triangular fundamental diagram:
As more vehicles flow into the cell (i.e., as the flow increases), density in the cell also increases, up to a critical point (the peak of the triangle). Up to this critical point, the cell can receive additional vehicles at a rate up to its capacity. If density continues to increase beyond that point, the cell can only receive additional vehicles at a reduced rate—the flow corresponding to its current density according to its fundamental diagram. Traffic slows, congestion develops, and a congestion wave may propagate backward into the upstream cells.
 Show me how a fundamental diagram is constructed from traffic measurements—how loop detector data reveals freeway behavior. |
Note: For more details on the model, see Technical Description of CTM.