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10 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 1
While progress is being made in many areas, it is slow. ther, the committee found that point estimates provide
There is also wide diversity among MPOs in the use of very poor support of policy making. Existing models are
models, available resources, and staff capabilities. A difficult to apply to new policy issues, such as evacuation
total of 11 out of 228 responding agencies reported using scenarios addressing terrorism or hurricanes. The com-
destination choice models. Fewer than 50% of large mittee also found poor representation of nonresident
MPOs distribute person trips rather than vehicle trips. travel, such as conventions and tourism, which are a
Approximately 75% of smaller MPOs and one-third of growing percentage of the trips made in some cities.
large MPOs do not use impedances in trip distribution These individuals have different travel patterns, use dif-
that vary by time of day. These results indicate there is ferent modes, and stay in different areas than residents.
still a long way to go to implement what most of us We do a poor job of modeling the travel of these groups,
regard as the state of the practice. which are very important to the economy of those areas.
Only about half of the responding MPOs reported Despite these findings, most of the agencies respond-
using K-factors or any zone-specific adjustments to trip ing to the survey rated their performance as acceptable
distribution models. The lack of independent data was and their models as adequate to their tasks. The com-
frequently noted as the reason for not using K-factors or mittee even thought that the modeling community has
other zone-specific adjustments. A total of 22 MPOs performed reasonably well with the resources at hand.
reported being engaged in New Start transit planning, Most MPOs reported that they have too few staff and
but do not have mode choice modeling capabilities. not enough staff time to consider the development of
Approximately 80% of large MPOs, 40% of midsized model improvements. If MPOs do not advocate for
MPOs, and a few small MPOs reported feeding back model improvements as a priority, who will?
travel times from assignment into distribution. Only Given this situation, the committee considered
40% of large MPOs reported feeding back travel times approaches by which to enhance travel models and the
to land use or automobile ownership models. modeling process. There was agreement that better data,
Goods movement is emerging as a very important better use of existing models, and new model develop-
public policy issue. The survey results indicate that ment are all needed. As a modeling community, we may
approximately half of the small and medium-sized be guilty of resting on our laurels. There is a need for fed-
MPOs and 80% of large MPOs model truck trips. Fur- eral leadership beyond TMIP. The possibility of MPO
ther, 20% of the respondents reported using synthetic pooled-funding efforts is another approach. More MPOs
trip tables and 30% reported using factoring procedures. and university partnerships represent another approach
Some 50% reported using other methods, including bor- for advancing the state of the art and state of the practice.
rowing coefficients from other regions. Approximately
25% of the MPOs reported using truck models that are
more than 10 years old. MODELING NEEDS
Despite decades of discussions in the literature, only
one MPO reported using an activity-based model set and Keith Lawton
two reported trying tour-based modeling but abandoned
that approach. The vast majority of MPOs stated they My comments focus on some of the limitations with cur-
have no interest in trying those approaches. Most of the rent models and possible approaches to address these
MPOs specifically reported seeing no reason to consider issues. There are both structural problems and problems
changing their current practice. This result suggested of practice with current travel demand models. Exam-
there is still a wide gap between the state of the art and ples of structural issues include the use of aggregate trip-
the state of the practice. based models with a matrix-based approach and the
The survey results indicated that validation is not con- application of static assignment using volume and delay
ducted at all by most agencies. Where something called functions. An example of a problem of practice is the
validation was performed, it usually consisted of com- limited use of integrated or linked transport and land use
paring model outputs across a screenline with ground models.
counts, but often using the same data that were used to Trip-based aggregate travel modes do not address the
calibrate the model. Statisticians would tell us that this concept of trip sequencing in a tour. This concept is
approach is flawed as method of validation. Fewer than important because it affects location choice and mode
10 agencies nationally demonstrated statistically appro- choice. The needs of certain activities in a sequence affect
priate validation procedures. earlier or later mode choices. Trip-based models cannot
The committee identified a number of pressing issues. address questions related to the impact of variable pric-
These included error propagation through chains of ing or dynamic pricing. Aggregate matrix processing lim-
models, poor representation of the price or the cost of its the ability to examine multiple market segmentation,
travel, and poor representation of goods movement. Fur- such as effects of household composition, income, and
