CTRE Home
Parent Page
Search/Browse

CONTACT INFO
CTRE
ISU Research Park
2901 S. Loop Dr.
Suite 3100
Ames, IA 50010

Phone
515-294-8103
Fax
515-294-0467

Webmaster:
mregenol@iastate.edu

Establish Network

After collecting the data into the GIS, the next step is to establish the traffic analysis zones and select the major streets in the network which will appear in the model. This step is one where lots of personal judgment enters the modeling process. The decision on zone location, number of zones, size and shape for the zones, are all things that come with practice. If the Census block groups provide adequate coverage of the study area, they make a relatively good first cut for zones. If they don't provide a good coverage for the area, using streets with higher traffic volumes for edges of zones can provide a good guideline.

The user at this point should develop a new table called TAZ.TAB with columns for zone number, and all the variables required to perform the trip generation calculations. As an example, if the NCHRP equations are used for trip generation then the minimal columns required to calculate production and attraction include households, retail employment, service employment, and other employment (of these, the standard industry codes for the types of employment can be found in the NCHRP report). If the PhoneDisk CD-ROM is used with a single trip purpose, then the TAZ table needs to have columns for total households and total businesses.








After developing the table, the user needs to make the table editable and use the polygon tool and define the traffic analysis zones. Note that it is important to locate the zones and number the zones in the same fashion as the centroids (which will be placed later). An example of traffic analysis zones developed for Waverly are shown.





















While the traffic analysis zones are being defined, the user needs to select the major streets in the network which will become part of the model. This information will be taken from the Coordinated Management Database through queries to determine appropriate roadways and through user judgment. At this point, the user should developed a table called NETWORK.TAB that doesn't need any attributes other than a key. After making this table editable, the user should select the line drawing tools and make lines over the important streets in the network (or the streets that the user wants to keep in the model). The example for Waverly is shown.







At this point, with the traffic analysis zones and network streets open, the user should continue to edit the network table by drawing in the location for all the centroid connectors in the network. Note that this step is only here for future visualization of the network and the user is not actually entering the centroid connectors at this point.

The user should either go back to the main bypass page or to the Site Map to continue.