Principles

Scenarios can be defined as projective or prospective (see Becker et al., 1982, Van der Heijden, 1996, Makridakis et al., 1998). A projective scenario's starting point is the current situation; extrapolation of current trends results in future conditions. A prospective scenario's starting point is a possible or desirable future situation, usually described by a set of goals or targets; established by assumed events between the current and future situation. The context for a scenario refers to general "macro developments", such as assumptions about economic development, demography, the stability of the supply of fossil fuels, prices of fossil fuels, international relationships, and behavioural changes (Geurs and Ramjerdi, 1997).

The exogenous variables in a scenario are not totally independent of each other. An example is the change in employment and economic development or changes in car ownership and economic development This is why a future scenario, composed of these variables, has to present a coherent picture of that future. Furthermore, it is not always a very simple task to draw a clear line between an exogenous and an endogenous variable. An example is economic development, which is usually included as an exogenous variable in a scenario, while economic development is usually an important indicator or objective

Uncertainties associated with the development of the exogenous variables will create uncertainty about the state of the future. It is commonly known and empirical evidence suggests that a simple averaging of predictions improves forecasting accuracy and reduces uncertainty (S. G. Makridakis, 1990). Addressing uncertainties about future states can be addressed by a scenario study. The level of uncertainties depends on time scale. Generally, the longer the horizon, the higher the level of uncertainty. A scenario study should address a minimum number of scenarios in order to cover the range of uncertainties in future developments and related to the objectives.

Trends

In an initial survey of six European cities, the following exogenous variables were identified as influencing trends in the development of transport and land use over the previous decade. The numbers identifying them are shown in brackets:
1 Population growth (6)
2 Economic growth (5)
3 Changes in car ownership (4)
4 Changes in job/employment structure (3)
5 Changes in employment location (3)
6 Age structure of population (2)
7 Changes in total employment (2)
8 Changes in relative prices (2)
9 Changes in surface coverage of the city (2)
10 Decentralisation/urban sprawl (2)

It should be noted that these variables are not wholly independent of one another; for example, employment will be correlated with economic and population growth. European cities' views were sought on the importance of the top five from the above list, and their responses are given in the figure below. It shows that 80% of cities feel that employment location has been important (or very important) to them, whilst population growth, economic growth and car ownership have been important to approximately 70% of cities and employment structure has been important to 44%. In addition, the cities suggested a number of other variables which have been important in their city, including location of schools, urban and social structures and the development of environmental legislation. Smaller cities place more emphasis on population growth, less on employment location, and much less on employment structure than do medium and larger cities; medium cities place less importance on employment growth and car ownership than do small and large cities.

Scenarios

In a similar way, the six cities were asked to identify the principal variables which would influence trends in transport and land use demand over the next decade or more. These are listed, with the numbers identifying each, below.
1 Population growth (4)
2 Economic growth (4)
3 Changes in work location (3)
4 Changes in car ownership (2)
5 Changes in surface coverage of city (2)
6 Changes in urban sprawl (2)
7 Changes in employment (2)

European cities' views were sought on the importance of the first five in the list above, and their responses are given below. Over 80% identify economic growth and changes in employment location as important or very important, and 70% population growth and size of the urban area. The lowest score was for car ownership, which only 60% considered important. Smaller cities placed less emphasis on employment and car ownership than medium and large cities. Medium cities place greater emphasis on population growth, and large cities greater emphasis on economic growth. There were few differences in emphasis on size of the urban area.

Importance of principal scenario variables to 54 survey cities