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154 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
focus group discussion best summarizes DRCOG's DRCOG VALIDATION AND SENSITIVITY
philosophy: TESTING APPROACH
Researchers and practitioners have not thought A plan describing the validation tests to be conducted for
carefully enough about the criteria for validation the ABM components and the overall model system has
of models. Researchers have the habit of asking been developed with the specification of the ABM for the
practitioners to believe that activity-based methods region. It includes the standards by which the tests will
will produce better impact assessment and fore- be evaluated, such as:
casts because such models more appropriately rep-
resent the actual decision process (we plead guilty · Checks to ensure that the model component is pro-
to this charge). There is a good basis for this line of ducing the correct results (i.e., verification of computa-
thought, but researchers need to go beyond this tions);
argument. They need to develop clear validation · Comparisons of model parameters to comparable
criteria and demonstrate the value of activity-based parameters in similar models in other areas;
methods in ways that are easily understood. (2) · Disaggregate validation of all model components
estimated using disaggregate methods and comparing
Because the ABM development process for the Den- the model outputs to the estimation data;
ver region has just begun, this paper focuses on the ini- · Testing of each model's sensitivity to variables
tial plans for the validation and sensitivity testing of the through controlled modification of those input variables;
models. · Comparisons of the model component outputs to
the results from the survey data set; and
· Comparisons, where data are available, of the base
DRCOG ACTIVITY-BASED MODEL APPROACH year outputs from each model component to indepen-
dent observed data (e.g., comparisons of mode choice
The ABM approaches for the Denver region will be model outputs to linked trips estimated from transit
based on those used in other parts of the country, partic- boarding counts).
ularly the San Francisco Transportation Authority. Most
of the components will be nested or multinomial logit The above tests are typical of model validation tests
(MNL) models, sensitive to person and household demo- that have been recommended in documents such as the
graphic variables and transportation level-of-service Model Validation and Reasonableness Checking Manual
variables. Anticipated model components include: (1) and should be performed for all model development
efforts.
· Synthetic population generator; Because there are more components in the proposed
· Regular workplace location choice model for each ABM than in a conventional trip-based model, there will
worker; be significantly more component testing. It will be
· Regular school location choice model for each important to design validation tests that are appropriate
student; for each component. While some tests will be analogous
· Household auto ownership choice model; to those performed for components of trip-based mod-
· Daily activity pattern choice model for each person- els, others will be different. Examples of similar tests
day; include comparisons of modeled trip length frequencies
· Number of tours choice model for each person day; with those from the household survey (tour lengths must
· Work-based sub-tour generation; also be compared) and comparisons of modeled and
· Tour-level destination choice; observed mode shares. Examples of tests to be performed
· Tour-level mode choice; without comparable trip-based tests include the number
· Tour-level time-of-day choice; of trips per tour by purpose, amount of time spent in
· Trip-level destination choice; activities versus traveling on tours by purpose, and the
· Trip-level mode choice (conditional on tour mode number of activities performed by each person. The
choice); and DRCOG model validation plan (3) provides a list of all
· Trip-level time-of-day choice (conditional on time tests to be performed.
windows remaining after all previous choices). One difficulty in performing the tests is the lack of
experience to determine standards. For example, how
Several components will be transferred or adapted close should the modeled number of activities per person
from the existing four-step model for the region. Exam- be to the observed number? In some cases, established
ples include the area-type and parking cost models, and standards for trip-based models may be used to inform
traffic and transit assignment procedures. the choice of standards for the ABM. In other cases, the
