The Miasma Beach Transportation Model -- Creating and Reporting Screenlines

Screenline Analysis
Screenline analysis compares the results of traffic assignment with actual traffic counts on streets. A screenline is an imaginary line on a map that crosses one or more links. In screenline analysis, the sum of observed link traffic counts that are crossed by the screenline are compared with model estimated volumes, for the same links and directions, from traffic assignment. The output table of screenline analysis labels the sum of a certain direction of traffic counts and flow estimates as IN_COUNT and IN_FLOW. Those in the opposite direction are labeled as OUT_COUNT and OUT_FLOW. The definition of IN and OUT direction is dependent on the way in which the screenline is drawn. The "IN" direction always flows from the left of the screenline to the right of the screenline, where "left" is defined as the left side of the screenline in the direction of its topology, that is, the direction in which the screenline is drawn (refer to the TransCAD Travel Demand Modeling manual for more information on Screenline analysis.

Model Validation/Screenlines
If the joined dataview of Highways/Streets with Traffic Assignment exists, go to Step 2.

1. Open baseasgn.dvw and select Planning / Assignment Utilities / Screenline Analysis. On the Screenline Dataview box, select New Table, then click OK and save this table as screen.scr. The Screenline Analysis Setting dialog box appears. Enter the following:
   
   • Views: Highways/Streets + TASSIGN
   • Check Box Dual Fields
   • Count: AB_FLOW/BA_FLOW
   • Flow: AB_FLOW/BA_FLOW
   
   
2. Click OK and the Screenline editor box is activated. This box resembles the Map editing toolbox (as used in Task 1). Click the "crooked line" button to add a screen line. After screenlines are drawn, use the info button on the screenline editor toolbox to change each screenline name. Lastly, click "Calculate Screenline Analysis Results" button to complete the analysis. Save the resulting file as screena.dbf.

Note: "Count" in the Screenline Analysis Setting box refers to observed forward and reverse traffic counts crossing the screenline. This data was not provided in the Highways/Streets dataview so it must be calculated from the observed link volumes provided, then entered into screena.dbf. Selecting Flows is just a way of avoiding any meaningless calculation (i.e., Dir is 0 for every link).

Examples of Miasma Beach screenlines include splitting:

• zones 1 and 2 to measure traffic in and out of the downtown area
• zones 5 and 6 to measure traffic in and out of the far suburbs
• western Miasma Beach from eastern Miasma Beach (say between 3rd and 4th)
• north and south Miasma Beach between Coast Highway and Miasma Boulevard, eliminating through traffic on Coast Highway from internal flows

The E/W volumes on Mountain Boulevard, Miasma Boulevard, and Coast Highway between 1st and 2nd Streets may individually vary significantly from the observed volumes (provided in Task 7) but the overall corridor volume (the sum of these link volumes, by direction) should closely reflect the observed traffic otherwise the model is simply not reflecting actual travel behavior. Either trip generation, trip distribution, time-of-day adjustments, or external trip rates must be incorrect.