Split Zone

The area covered by TDM23 consists of 5,739 transportation analysis zones (TAZs). In dense areas, the TAZs are smaller to better represent the characteristics of each zone. However, certain projects may require a focus on sub-TAZs, and the ability to split a TAZ is a desirable feature. TDM23 facilitates this need with a process outlined as follows:

1. Update Shapefiles:

To split a TAZ, you can use TransCAD, ArcMap, or any GIS software that supports shapefile editing. Ensure that the resulting file maintains the same column structure. Here is a guide on how to split a TAZ specifically in ArcMap:

  • Open ArcMap.
  • Open the shapefile containing the TAZ data. (Source files: "tdm23\inputs\zonal\shp\")
  • Locate the specific zone on the map that needs to be split.
  • On the "Editor" toolbar, choose "Editor > Start Editing."
  • Select the zone you want to split.
  • Choose the "Cut Polygon Tools" option.
  • Opt for a straight segment and draw a line that cuts through the zone.
  • Update the "TAZ_ID" for the new sub-TAZ.
  • Update the "Total_Area" and "Land_Area" accordingly.
  • On the "Editor" toolbar, select "Editor > Stop Editing."
  • To save the changes, right-click on the layer name and go to "Data > Export Data." A window will appear. Choose to export all features and set the save type as "shapefile." Save the changes. Make sure that the exported file has the same fields and field names. This process ensures that the TAZ is successfully split and the updated shapefile reflects the changes with consistent column structure.

2. Update Network File:

Take the following steps to update the network file by adding centroids and connectors for split TAZs, and by updating the node and link attributes appropriately to reflect the changes in TAZ IDs:

  • Begin by making a copy of the network folder to initiate the editing process. (Ensure to retain the turn penalty table and warm starts files within this folder, as they are not subject to editing but should be maintained alongside the highway and transit files.)
  • Open the highway network and the zonal shape file, to identify potential centroids and connectors to add.
  • Add centroid and centroid connectors: ensure the network file includes centroids and centroid connectors for the newly split TAZs.
  • Update link attributes: Identify all links with the old TAZ_ID and update them with the IDs of the split TAZs.

  • Update node attributes: TransCAD does not allow for the direct modification of the ID field of a layer. Therefore, we need to export the highway network to a new folder (e.g., "split_network") with the updated ID fields:

    • Create a new column (e.g., “node_id_2”) in the node layer and copy the ID column to it.
    • Update the value of centroids with the new TAZ IDs and update the “int_zone” column.
    • Select the "links" as the active layer.
    • Go to “File > Export > Geography” and export all features to a standard geography file.
    • For "Node ID Field," select the column you updated.
    • Copy RTS files and the turn-penalty table to the "split_network" folder.

3. Update the Open Street Map (OSM) file:

The walk/bike network is located in "\inputs\network\osm_version." Updating the OSM network accordingly.

4. Update Zonal Files for New TAZs:

Several CSV files are organized by zones and serve as inputs. These files must be updated to accurately represent the new TAZs. If specific information is not available, use the corresponding data from the old zones. This step ensures that the zonal files align with the updated TAZ configuration.

The following is a categorized list of these files based on their location on the Scenario Parameters window.

Primary Input:

  • Parking Cost: The file contains data on the average parking cost for each zone. If information is not provided, use the old parking cost for the new split zones.

    (File: “zonal\Parking_year_version.csv”)

  • Intersection Densities: The data on walking and bicycling conditions reflect intersection densities.

    (File: “zonal\walkbike_year_v.csv”)

  • School and University Enrollment:This file contains school and university enrollment data.

    (File: “zonal\enroll_year_v.csv”)

Supply | Model Geography:

  • Block to TAZ allocation: The 2010 Census blocks are allocated to TAZs.

    (File: “zonal\shp\taz_2010block_allocation_v.csv”)

  • Block to TAZ assignment: A single TAZ is assigned to each 2010 Census block.

    (File: “zonal\shp\taz_2010block_assignment_v.csv”)

  • TAZ to PUMA association: The US Census Bureau’s Public Use Microdata Areas (PUMAs) data are allocated to TAZs.

    (File: "zonal\shp\tazpuma.csv")

Post Process | Equity:

  • park_hwy/park_trn: Access to open spaces using highway or transit.

    (File: “ zonal\equity\open_space_access_points_by_TAZ_v.csv”)

  • hlth_hwy/hlth_trn: Access to healthcare facilities using highways or transit.

    (File: “zonal\equity\healthcare_facilities_by_TAZ_v.csv”)

  • espl_hwy/espl_trn: Access to essential places using highways.

    (File: “zonal\equity\essential_place_presence_by_TAZ_v.csv”)

  • min_pct: Minority population proportion.

    (File: “zonal \equity\TAZ_minority_v.csv”)

  • lowinc_pct: Low-income population proportion.

    (File: “zonal \equity\TAZ_lowincome_v.csv”)

5. Update Scenario Parameters:

In TransCAD, open the CTPS_TDM23.model file and create a scenario for the split zone process. Then, update the parameters and file names for this scenario. Below is a categorized list of the parameters to change based on their location in the user interface (UI):

  • Primary Input | Networks and Zonal Data:

    • Roadway DBD File
    • Transit RTS File
    • Turn Penalties
    • Init Speeds - am
    • Init Speeds - md
    • Init PnR Demand - am
    • Parking Costs
    • NonMotorized DBD File (i.e., the OSM file in Step #3)
    • Intersection Densities

  • Primary Inputs | Socio-Economic and Trips:

    • School and University Enrollment

  • Post Process | Equity | Accessibility Metrics for Environmental Justice

    • park_hwy
    • park_trn
    • hlth_hwy
    • hlth_trn
    • espl_hwy
    • espl_trn

  • Post Process | Equity | Equity Population Proportion by TAZ

    • min_pct
    • lowinc_pct

  • Model Geography

    • Number of Zones
    • TAZ shapefile dbf
    • Block to TAZ allocation
    • Block to TAZ assignment
    • TAZ to PUMA association

6.Check for Validation:

Check your results to validate that the model ran successfully:

  • Population and employment must match before and after splitting.

    (File: “output_network\taz_demogr.csv”)

  • Trip productions must match before and after splitting.

    (File: “output_demand\td”)