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160 I N N O VAT I O N S I N T R AV E L D E M A N D M O D E L I N G , V O L U M E 2
tory dependency in time allocation and activity partici- information with a view to stimulate discussion among
pation, in-home versus out-of-home activity substitution conference participants on how best to validate and
and generation, induced demand, and travel efficiency assess activity-based model systems vis-ą-vis existing
(say, through chaining of trips) are but a few of the con- four-step models:
cepts that merit recognition.
· Behavioral processes and decision rules: Recent 1. Model validation guidelines for activity-based
work in activitytravel modeling has focused on the travel demand models including information on what
behavioral processes and decision-making rules that peo- base-year traffic conditions activity-based travel demand
ple employ when scheduling and executing activities and models should replicate, the margins of error that are
trips. Rule-based heuristics, in addition to random utility acceptable, and the extent to which adjustments of
theory-based models, are being incorporated into mod- model components and parameters are acceptable;
els to reflect these behavioral processes in microsimula- 2. Model assessment guidelines for activity-based
tion frameworks. Understanding behavioral processes is travel demand models including the range-of-sensitivity
key to developing robust model structures, specifica- tests, policy measures, land use scenarios, and technolo-
tions, and forms. gies to which the activity-based models should be sub-
jected, the model outputs that should be examined, and
the acceptable ranges of responses in model outputs;
Comparisons Between Model Systems 3. Model comparison guidelines for comparing
activity-based travel demand models with existing four-
The discussion so far has dwelled on how one might step travel demand models including the development
assess the performance of an activity-based travel and presentation of a comprehensive matrix that clearly
demand model system. However, the following questions shows how and where four-step models, tour-based
remain: models, and activity-based microsimulation models are
applicable to addressing a range of issues; and
1. How does one know or determine whether the 4. The design of comprehensive experiment(s) for
activity-based travel demand model is giving the right performing controlled comparisons of activity-based
answer or level of sensitivity for a particular scenario? travel model outputs and existing four-step travel model
2. In comparing the outputs (in response to a scenario outputs. A variety of scenarios have played out in the
analysis) between an existing four-step travel demand real world, thus providing real-world data against which
model and an activity-based travel demand model, how model predictions can be assessed. Both activity-based
does one know or determine which one is right or more and existing four-step travel models can be applied to
accurate (in cases in which it is not obvious)? these situations and the outputs can be assessed against
real-world observational data.
CONCLUSIONS AND CONTENT The presentation will also include results of model
OF PRESENTATION validation and assessment exercises that have been
undertaken using the Florida Activity Mobility Simula-
This white paper raises important questions regarding tor and Comprehensive Econometric Microsimulator of
the potential adoption of new and innovative activity- Daily Activity Patterns to illustrate how the guidelines
based travel demand modeling systems in practice. The presented can be used to assess and validate activity-
proposed presentation serves to deliver the following based travel demand models.
