Typically, in analyzing the traffic impacts of a new land use development, the area of greatest uncertainty is in the distribution of the traffic to the roadway network. One of the many capabilities that a properly calibrated model can provide is in determining the trip distribution for a site impact study. There are two methods in determining the trip distribution estimates.

The first approach is for looking at larger developments that have significant impacts on existing travelers' behaviors. In this case, a new zone and any needed links are created for the distribution of site traffic and assignment on the roadway network.

As an example, consider an 85-zone model with zones 1-81 being internal zones and 82-85 being external. The network has 700 total links and 375 nodes. In the screen graphics editor add a new node (376) for the zone centroid and any links needed to provide access to the development. Next, create a correspondence file in the renumbering utility section that would look like the table below.

The new node 376 would be renumbered to be the highest internal zone number (82) and the external zones would be increased by one. The remaining existing nodes could then be kept the same. After renumbering the link and node files, examine them in the screen graphic editor. If the model has turn penalty files and a trip table input file, renumber them if necessary.

Recalculate the origins and destinations using the new LU2 file that contains the development in zone 82. Run the model using the select zone analysis, option 1, for zone 82. The trips that enter/exit at zone 82 can then be viewed in the screen graphics editor by selecting the SZ1 field from the Data Type area.

The second approach is for analyzing a smaller development that wouldn't significantly alter the existing traveler's behaviors. For this approach, either percentage distributions or actual vehicle distributions of site traffic can be determined by using the calibrated trip table. Recalling the previous example, the modifications in the network would be identical for creating a new zone for the site. In order to simulate the distribution of neighboring zones, trips from the zones in the vicinity of the site are added to the new zone in the trip table using the new option 21 in the Trip Table Manipulation section 2.3. For percentage distributions, the trip table for zone 82 is then normalized to totals of 100 trips entering and 100 exiting the site.

Then run the model using assignment only for the modified network and trip table. Use the select zone feature to save the trips to and from zone 82 which will permit the model to report the distribution of the site trip percentages. A note on the results--the total percentage of trips shown entering/exiting the site may be slightly greater or less than 100. This is due to the rounding procedures during the assignment. Judgement can be used to adjust the percentages to equal 100 on the major directional movement to or from the site when manual assignments are performed.

As usual, plots can best display results for your report or for review by others.