Transportation Forecasting Lecture Notes

This set of vocabulary flashcards covers the traditional four-step transportation forecasting model, including trip generation, distribution, mode choice, and traffic assignment concepts.

Trip Generation

The first step in the traditional transportation forecasting model which develops a relationship between trip ends and land use.

Trip Ends

The two points, consisting of an origin and a destination, of any trip.

Productions

Trip ends that typically originate from a home; they are used as the reference point for balancing trips because they are based on reliable census data.

Home-Based Work (HBW)

A trip purpose classification referring to trips that start or end at home and go to or come from a workplace.

Home-Based Other (HBO)

A trip purpose classification referring to trips from home to a non-work destination, such as a grocery store.

Non-Home-Based (NHB)

Trips where neither the origin nor the destination end is the traveler's home.

Activity Units

Metrics such as the number of employees or floor area used to estimate non-residential trip generation and attractions.

Trip Distribution

The second phase of the forecasting model with the goal of linking origins (productions) to destinations (attractions).

Gravity Model

A distribution model stating that trips between two zones are inversely proportional to travel time.

Friction Factor ($F_{ij}$)

A calibrated value used in the gravity model that is inversely related to travel time.

Singly constrained gravity model

A model variation that ensures trips from each zone add up correctly on the production end only.

Doubly constrained gravity model

A model that iteratively adjusts factors to balance both the production and attraction ends of trip distribution.

Fratar Method

A trip distribution model developed by Thomas J. Fratar that proportions future trips based on current trips and the growth factor of the attracting zone.

Mode Choice Analysis

The third step in the forecasting process used to determine the percentage of trips made by different modes, such as auto versus transit.

Trip End Model

A mode choice model that determines transit percentages before the trip distribution phase.

Urban Travel Factor (UTF)

A value used in the Trip End Model calculated based on households per auto and population density.

Trip Interchange Model

A mode choice model that considers system level-of-service variables such as relative travel time and cost.

Disutility

A mathematical term representing the "unattractiveness" or negative quality of a travel variable, such as time or cost, that causes inconvenience.

Logit Model

A utility-based model used to predict the probability of mode choice; it uses the constant "e" to convert negative utility values into small positive decimals.

Calibration

The process of adjusting model parameters or utility functions to match actual observed traveler choices and data.

Maximum Likelihood Estimation (MLE)

The statistical method used to determine utility coefficients for Logit models using real survey data.

Incremental Logit Model

A variation of the logit model used to predict changes in travel behavior for existing modes when service parameters or system components change.

In-Vehicle Travel Time (IVTT)

The time spent actually traveling inside a vehicle during a trip.

Out-of-Vehicle Travel Time (OVTT)

Time spent waiting, walking, or transferring, which is typically weighted more heavily than IVTT in utility functions.

Travel Value Parameter (TVP)

The ratio of the value of travel time to the hourly employment rate.

Traffic Assignment

The final step in the travel demand forecasting process where O-D trips are assigned to specific physical network segments.

All-or-Nothing Assignment

Also known as Minimum Time Path, this method sends 100% of traffic between a zone pair to the single fastest route.

Diversion Curves

Empirical graphs used to split traffic between two routes based on ratios like time or distance, most appropriate for corridor-level studies.

Capacity Restraint

An assignment method that accounts for the reduction in travel speed as roads reach their capacity and fill with traffic.

Bureau of Public Roads (BPR) Function

The most common formula used to calculate congested travel time based on the volume-to-capacity ratio.

Free-flow travel time ($t_0$)

The time required to traverse a road link when there is no traffic volume.

User Equilibrium

A state where travel times on all active paths between an O-D pair are equal, and no traveler can reduce travel time by switching routes.

Total System Cost Assignment

An assignment method that seeks to achieve the lowest overall travel time for all travelers combined in the network.

Tree Algorithm

A systematic step-by-step process used to determine the minimum path or shortest path through a network.

Network Loading

The process of accumulating trip volumes onto specific road links within the assignment phase.

Traffic Analysis Zone (TAZ)

The geographic unit of analysis used globally for transportation forecasting.

Impedance

A measure of the total "cost" of a trip, incorporating factors such as time and out-of-pocket expenses.

Mode-Specific Constant ($a$)

A constant in a utility function representing factors not captured by time or cost, such as comfort or convenience.

Excel Solver

A spreadsheet tool used to perform iterative optimization for complex networks, often using constraints to find a solution.