1. Source: NHI course on Travel Demand Forecasting, Ch. 8 (152054A)

2. Objectives Define external trip purposes
Identify the trip distribution inputs for each external trip purpose
Identify the trip distribution outputs for each external trip purpose
Apply a Fratar distribution to a sample data set

3. Terminology

4. Study area boundary location
Encompass current urban area plus forecasted urban area (20-25 years)
Extend boundary to include commute trips into the area for work, etc. (practical?)
Have no more than 15% of VMT from trips that begin or end outside of study area
Cover non-attainment area
Trip length characteristics
Trip characteristics differ depending on whether the trip maker lives within or outside of the study area (so different friction factor curve, right? Really?)

5. Key Concepts IE is usually modeled as a separate trip purpose
Q: what about II trips made by IE travelers?

6. Inputs and Outputs
Inputs for external-internal trips (from external survey)
Productions for each external station TAZ
Trip length frequency distribution
Travel time impedance matrix
Scaled NHB attractions by TAZ
Inputs for external-external trips
Counts at each external station TAZ
Origin and destination trip table (base-year)
Outputs
24-hr external-internal trip table (P&A)
24-hr external-external trip table (O&D)

7. Data Collection Vehicle Counting and Classification
24-hr traffic counts for each external station TAZ (to expand survey data)
Manual counts used to correct automatic counts (vehicle classification)

8. External Station Survey Methodology…
Have an approved traffic control plan
Survey in outbound direction (why?)
Move vehicles out of traffic stream
Conduct interviews or give postcards
Interview in daylight
hours only
Survey maximum
number of vehicles
Sample all high volume
locations
Take random sample
of low volume locations

9. Data Elements to Collect
Number of persons in vehicle
Highway name/number (for entry and departure)
Time entered study area
Time departed study area
Address of last stop in study area
Trip purpose of last stop and next stop
Home address (to see if they live in the study area)

10. Data analysis Expand survey data (count to survey ratio)
Separate trips between external-internal and external-external
Develop TLFD for external-internal trips (to calibrate gravity model)
Distribute external-internal trips
Use scaled NHB trips as a proxy
Apply the gravity model

11. Data Analysis
Diagonal
Always zero

12. Forecasting Future Year Trip Tables
Forecast total volume for each external station
Assume split between external-internal and external-external is same as base year
Forecast external-internal trip tables
Forecast external-external trip tables

13. Fratar Example