Calibration Report for the TPB Travel Forecasting Model, Version ...

Calibration Report for the TPB Travel Forecasting Model, Version 2.3
Trip generation involves the application of daily trip rates to the number of households, in each of the
64 classes, and to the number of jobs. The trip rates reflect both motorized (i.e., transit and
automobile) and non‐motorized (i.e., bicycle and walk) person travel. The non‐motorized trip‐ends
produced in the trip generation step are not carried forward into trip distribution. Trip attractions are
computed by purpose as a function of zonal land use attributes. External (i.e., external‐to‐internal, X/I,
and internal‐to‐external, I/X) productions and attractions are entered as an exogenous input, by
purpose, into the trip generation process. External travel relates to auto person, commercial vehicle
and truck travel only (transit externals are currently not considered in the model). The home‐based
productions and attractions are developed by the four income levels.
The trip distribution model uses the standard gravity model formulation and makes use of a composite
time function that represents a blending of transit and highway travel times. The gravity model is
doubly constrained for all five trip purposes. The distribution step involves separate gravity model runs
for 30 travel markets, given that home‐based purposes are income stratified, and external travel is
modeled separately by purpose and facility type (interstate travel vs. non‐interstate). However, the trip
distribution process ultimately results in seven daily trip tables corresponding to the basic motorized
person, commercial, and truck purposes.
The mode choice process consists of five models corresponding to the HBW, HBS, HBO, NHW, and NHO
purposes. The models are used to apportion total motorized person trips among SOVs, 2‐occupant
HOVs, 3+occupant HOVs, and 12 combinations of transit mode and access to transit.
The time‐of‐day model apportions daily resident travel among four time periods: AM peak period (6:00
AM to 9:00 AM), midday (9:00 AM to 3:00 PM), PM peak period (3:00 PM to 7:00 PM), and the
nighttime/early morning hours (7:00 PM to 6:00 AM). The time‐of‐day model consists of survey‐based
factors that are applied on the basis of purpose, mode, and directionality (i.e., the home‐to‐non‐home
and non‐home‐to‐home directions). This step also includes provisions for apportioning daily residual
travel 13 and truck travel among the three time periods. The time‐of‐day process ultimately produces
three “total vehicle” trip tables, one for each of the four time periods.
The traffic assignment process addresses six user classes: SOVs, HOV‐2, HOV 3+, commercial vehicles,
trucks, and airport passenger vehicles. Highway link volumes are developed for each of the user classes
by time period. Although one might expect the four time‐of‐day periods to result in four time‐of‐day
traffic assignments, there are actually six traffic assignments conducted, since the AM and PM peak
periods are split into two assignments (HOV3+ and non‐HOV3+, the so‐called “two‐step assignment”).
This is described in more detail in Chapter 8 and Figure 19. Daily transit assignments can also be
produced in the Version 2.3 model, though this capability has yet to be fully calibrated and validated.
13 Residual travel is also referred to as “miscellaneous” travel which represents special travel markets that are
typically not (or not well) represented in home‐interview surveys; it is composed of taxi, school, visitor/tourist, and
air passenger auto driver travel.
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