1. Transportation Forecasting The Four Step Model

2. Norman W. Garrick Transportation Forecasting What is it? Transportation Forecasting is used to estimate the number of travelers or vehicles that will use a given transportation facility in the future The conventional approach to transportation forecasting is based on what is commonly know as the ‘four step model’

3. Norman W. Garrick The Four Step Model Trip Generation Estimates the number of trips from given origins and destinations Trip Distribution Determines the destination for each trip from a given origin Mode Choice Determines the mode choice for each trip Route Assignment Determines the specific route for each trip

4. Norman W. Garrick Transportation Forecasting What is it used for? Transportation forecasting models have often been used in a ‘predict and provide’ mode to determine the highway capacity needed in response to predicted growth in traffic volumes. Often, in this case, the full impact of changes in land use is not properly taken into account and the resulting induced traffic is over looked.

5. Norman W. Garrick Transportation Forecasting Scenario Planning If transportation forecasting models are adjusted to properly account for the impact of changes in land use patterns they are useful for evaluating the implications of different transportation choices. In other words, they can be used for so called ‘scenario planning’.

6. Norman W. Garrick Scenario Planning Example Tolls in Connecticut What would be the impacts of adding tolls to I-95 and the Merritt Parkway in Fairfield County?

7. Norman W. Garrick Scenario Planning Example Tolls in Connecticut Why is this information needed? What are some potential impacts?

8. Norman W. Garrick Possible Changes Due to Tolls in Connecticut Short Term ChangeLong Term Change Trip Generation Trip Distribution Mode Share Trip Assignment

9. Norman W. Garrick Possible Changes Due to Tolls in Connecticut Short Term ChangeLong Term Change Trip GenerationNegligibleSubstantial (depending on land use changes) Trip Distribution Mode Share Trip Assignment

10. Norman W. Garrick Possible Changes Due to Tolls in Connecticut Short Term ChangeLong Term Change Trip GenerationNegligibleSubstantial (depending on land use changes) Trip DistributionNegligibleSubstantial (depending on land use changes) Mode Share Trip Assignment

11. Norman W. Garrick Possible Changes Due to Tolls in Connecticut Short Term ChangeLong Term Change Trip GenerationNegligibleSubstantial (depending on land use changes) Trip DistributionNegligibleSubstantial (depending on land use changes) Mode ShareSome switch to rail Significant (depending on land use changes) Trip Assignment

12. Norman W. Garrick Possible Changes Due to Tolls in Connecticut Short Term ChangeLong Term Change Trip GenerationNegligibleSubstantial (depending on land use changes) Trip DistributionNegligibleSubstantial (depending on land use changes) Mode ShareSome switch to rail Significant (depending on land use changes) Trip AssignmentSome Switch to rail and to toll free roads Significant (depending on land use changes) The purpose of Transportation Forecasting is to quantify these changes as accurately as possible