Sustainable development has become a central objective of policy
worldwide. Although the term is widely used, there is little agreement on
what it means in practice and how progress toward it can be measured. The
European Commission, as part of its Programme on Competitive and
Sustainable Growth, commissioned the SUMMA (SUstainable Mobility, policy
Measures and Assessment) project. Among SUMMA's objectives were to
define and operationalize the concept of sustainable transport and
mobility in terms of its environmental, economic, and social dimensions,
and to define outcome indicators from the transport system that can help
policy makers monitor progress toward its achievement. To achieve these
objectives, we used the systems approach, which identifies the
interrelationships among the elements of a complex system and helps in the
design of policies to steer the system toward sustainability. This article
describes the first phase of the systems approach: the set of outcome
indicators that we developed and how we defined the transport system in
terms of three markets—a movement market, a transport market, and a
traffic market—in which choices are made that result in traffic
streams. The traffic streams determine the values of the outcome
indicators, which can then be used to identify good policies. The approach
is illustrated with examples from the SUMMA project.