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allow stakeholders to evaluate alternative urban futures, the interface provides a large collection
of
indicators
: variables that distill some attribute of interest about the results (Gallopin, 1997).
(Examples of indicators are the number of acres of rural land converted to urban use each year, the
degree of poverty segregation, or the mode share between autos and transit.) These categories and
indicators draw on a variety of sources, including empirical research on people's environmental
concepts and values (Kahn, 1999; Kahn and Kellert, 2002), community-based indicator projects
(Palmer, 1998; Hart, 1999), and the policy literature. Stakeholders can then use the interface to select
indicators that speak to values that are important to them from among these categories.
This interface illustrates the interplay among conceptual, technical, and empirical investiga-
tions. The indicators are chosen to speak to different stakeholder values—responding to our dis-
tinction between explicitly supported values and stakeholder values in the initial conceptual
investigation. The value categories are rooted empirically in both human psychology and policy
studies, not just philosophy—and then embodied in a technical artifact (the Web-based interface),
which is in turn evaluated empirically.
Technical Choices Driven by Initial and Emergent Value Considerations
Most of the technical choices in the design of the UrbanSim software are in response to the need
to generate indicators and other evaluation measures that respond to different strongly held stake-
holder values. For example, for some stakeholders, walkable, pedestrian-friendly neighborhoods
are very important. But being able to model walking as a transportation mode makes difficult
demands on the underlying simulation, requiring a finer-grained spatial scale than is needed for
modeling automobile transportation alone. In turn, being able to answer questions about walking
as a transportation mode is important for two explicitly supported values: fairness (not to privi-
lege one transportation mode over another), and democracy (being able to answer questions about
a value that is important to a significant number of stakeholders). As a second example of techni-
cal choices being driven by value considerations, UrbanSim's software architecture is designed to
support rapid evolution in response to changed or additional requirements. For instance, the soft-
ware architecture decouples the individual component models as much as possible, allowing them
to evolve and new ones to be added in a modular fashion. Also, the system writes the simulation
results into an SQL database, making it easy to write queries that produce new indicators quickly
and as needed, rather than embedding the indicator computation code in the component models
themselves. For similar reasons, the UrbanSim team uses the YP agile software development
methodology (Freeman-Benson and Borning, 2003), which allows the system to evolve and
respond quickly to emerging stakeholder values and policy considerations.
Designing for Credibility, Openness, and Accountability
Credibility of the system is of great importance, particularly when the system is being used in a
politically charged situation and is thus the subject of intense scrutiny. The research group has
undertaken a variety of activities to help foster credibility, including using behaviorally transpar-
ent simulation techniques (i.e., simulating agents in the urban environment, such as households,
businesses, and real estate developers, rather than using some more abstract and opaque simula-
tion technique), and performing sensitivity analyses (Franklin et al., 2002) and a historical vali-
dation. In the historical validation, for example, the group started the model with 1980 data from
Eugene/Springfield, simulated through 1994, and compared the simulation output with what
actually happened. One of these comparisons is shown in Figure 16.3. In addition, our techniques
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