LESSON 6: INTRODUCTION TO TRAVEL DEMAND FORECASTING

THE FOUR-STEP FORECASTING MODEL

Trip generation

how many trips are generated?

Trip distribution

where do trips go?

Mode Choice

what travel mode is used for each trip?

Trip Assignment

what is the route of each trip?

Trip generation

The first step in the four-step transportation planning process deals with the question of how many trips originate in or are destined for a particular travel analysis zone (TAZ)

TAZ

neighborhoods in the model area and serve as the source or destination for trips

Home-based work (HBW) trip

a trip for which the purpose is to go from home to work or from work to home

Home-based other (HBO) trip

a trip for which the purpose is to go from home to another location other than work ( ex. shopping, school, theater) or from non-work locations to home

Non-Home based (NHB) trip

a trip for which neither trip end is at home

Origin

point at which a trip begins

Destination

point at which a trip ends

Trip distribution

Destination choice is the second component of four-step transportation planning. The trip distribution step matches trip origins with the destination.

Mode Choice

trips between the TAZs are allocated to different transportation modes. Which mode of transport people are using depends on their preferences and aspects of their household or person such as car ownership.

Mode choice/ Mode split

involves separating (splitting) the predicted trips from each origin zone to each destination zone into distinct travel modes ( ex. walking, bicycle, driving, train, bus)

Trip Assignment

the trips between an origin and destination by a particular mode are ‘assigned’ to a specific path. This means that the trip matrices from the prior steps are used as an input to assign route flows to the actual transportation network.

Trip Assignment

“loading the network”; volumes are assigned to links

Free flow speed

speed under no congestion

Free flow travel time

travel time under no congestion

Path loading

loading vehicles to links comprising a path

Level of service

a qualitative measure describing the operation conditions

Capacity restraint

the volume loading process is constrained by the capacity of the link

All or nothing Assignment

link travel times are determined beforehand and trips are assigned at once; fundamental building block in traffic assignment procedures

Incremental Assignment

link travel times are updated through fixed proportions and trips are assigned iteratively; a simple but inconsistent way to account for capacity and congestion effects

Capacity constraint Assignment

link travel times depend on link volumes and final assignment is the average of the last several iterations