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Norman W. Garrick
Transportation ForecastingWhat 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’
Norman W. Garrick
The Four Step Model
• Trip GenerationEstimates the number of trips from given origins and destinations
• Trip DistributionDetermines the destination for each trip from a given origin
• Mode ChoiceDetermines the mode choice for each trip
• Route AssignmentDetermines the specific route for each trip
Norman W. Garrick
Transportation ForecastingWhat 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.
Norman W. Garrick
Transportation ForecastingScenario 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’.
Norman W. Garrick
Scenario Planning ExampleTolls in Connecticut
What would be the impacts of adding tolls to I-95 and the Merritt Parkway in Fairfield County?
Norman W. Garrick
Scenario Planning ExampleTolls in Connecticut
Why is this information needed?What are some potential impacts?
Norman W. Garrick
Possible ChangesDue to Tolls in Connecticut
Short Term Change Long Term ChangeTrip Generation
Trip Distribution
Mode Share
Trip Assignment
Norman W. Garrick
Possible ChangesDue to Tolls in Connecticut
Short Term Change Long Term ChangeTrip Generation Negligible Substantial
(depending on land use changes)
Trip Distribution
Mode Share
Trip Assignment
Norman W. Garrick
Possible ChangesDue to Tolls in Connecticut
Short Term Change Long Term ChangeTrip Generation Negligible Substantial
(depending on land use changes)
Trip Distribution Negligible Substantial (depending on land use changes)
Mode Share
Trip Assignment
Norman W. Garrick
Possible ChangesDue to Tolls in Connecticut
Short Term Change Long Term ChangeTrip Generation Negligible Substantial
(depending on land use changes)
Trip Distribution Negligible Substantial (depending on land use changes)
Mode Share Some switch to rail
Significant(depending on land use changes)
Trip Assignment
Norman W. Garrick
Possible ChangesDue to Tolls in Connecticut
Short Term Change Long Term ChangeTrip Generation Negligible Substantial
(depending on land use changes)
Trip Distribution Negligible Substantial (depending on land use changes)
Mode Share Some switch to rail
Significant(depending on land use changes)
Trip Assignment SomeSwitch to rail and to toll free roads
Significant(depending on land use changes)
The purpose of Transportation Forecastingis to quantify these changes as accurately as possible