Modeling Atlanta Traffic – Part 1

Atlanta has suffered a series of out-of-the-ordinary traffic crises recently. In addition to the opening of a new baseball stadium and pavement buckling in I20, the collapse of an elevated section of highway I85 after a fire broke a major connection between the northern suburbs and central Atlanta, including the Georgia Tech campus.  Intercax fortuitously moved its offices that same week north of the break, making the commute easier for most of its employees (sorry, Rose). Still, this event offers us the opportunity to apply the power of Model-Based Engineering to important aspects of modern life.


Figure 1

Atlanta has always been a transportation hub. Its first name was Terminus, as a major stop on the Savannah-Chattanooga railroad line. Today, in addition to the world’s busiest airport, it is the crossing point of three major interstate highways

  • I85 – Northeast to Southwest
  • I75 – Northwest to Southeast
  • I20 – East to West

with a peripheral beltway, I285, as indicated in Figure 1.


Figure 2

We begin by building a model of the highway structure using a system of highway segments and junctions. The composition of the Metro Highway System is shown in the SysML block definition diagram in Figure 2. A junction where two highways cross is called a 4-way junction. Our system has seven of these, six are where the interstates cross the perimeter and one is at the center. Atlanta also has two 3-way junctions, one north of the center where I75 and I85 join to form the Downtown Connector and one below the center where they split.

We also, for convenience sake, define two types of highway segments, identical in structure, but different in nomenclature. Segment block flows are labeled inbound (IB) and outbound (OB). SegmentPeripheral block flows are labeled clockwise (CW) and counterclockwise (CCW). Good labeling makes assembling the connections diagrams (IBD and PAR) later much easier.


Figure 3

We assign proxy ports to our blocks to clarify the network structure. Segments have two inputs (sinks), typed by the interface block Traffic, and two outputs (sources), typed by Traffic conjugated. Junctions have six (3-way) or eight (4-way) proxy ports, equal numbers of sinks and sources, similarly typed. With these, we can build an IBD (SysML internal block diagram) for the MetroHighwaySystem block, whose own ports represent inbound and outbound traffic from outside Atlanta’s metro boundaries (Figure 3). Those familiar with Atlanta will note that this is a simplified representation. Several shorter highways, not to mention local roads, have been left out, but it serves the purpose of illustrating the principles.


Figure 4

To look closely at the network, we zoom in on the Northeast Quadrant in Figure 4. Here we see I85 coming in from the northeast, crossing I285 (locally known as Spaghetti Junction because of the interchange complexity), joining I75 at the Brookwood Merge and passing south by Georgia Tech to its junction with I20 downtown.


Figure 5

This model changed dramatically on Thursday, March 30, 2017 after the bridge collapse (Figure 5). The segment between Spaghetti Junction and the Brookwood Merge was split in two by the collapse. The disconnected stubs in this model are typed by a new block SegmentStub with a different set of proxy ports and parametrics.

In a later part of this blog series, we will make these SysML models available for download for multiple SysML authoring tools, and see how these models form the basis for simulation and analysis. However, we next turn our attention to modeling behavior, specifically, the behavior of an Atlanta driver faced with this difficult situation.

Dirk Zwemer

Dr. Dirk Zwemer ( is President of Intercax LLC (Atlanta, GA), a supplier of MBE engineering software platforms like Syndeia and ParaMagic. He is an active teacher and consultant in the field and holds Level 4 Model Builder-Advanced certification as an OMG System Modeling Professional.