Environmental Engineering Reference
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Trips
Reference case
With scheme
Induced
travel
Without
scheme
Years
Base Year
Opening year
Forecast year
Figure 21.1
Illustration of variable demand forecasting (source: TAG Unit 2.9.2)
response of particular kinds of traveller to particular kinds of change in the transport
regime to be represented more accurately.
As demand management assumes more prominence in transport policy, so
the technical issue of being able to forecast behavioural responses becomes more
significant. Conventional modelling assumes that the response of a particular kind
of traveller to a particular type of opportunity will be the same in future as it is today.
Hence for example if car drivers in general exhibit a very weak response to current
opportunities for cycling or using public transport then they will be forecast to make
little use of them in future, even if these modes are improved. In practice this can make
it very difficult to demonstrate an economic case for such investments, even though it
is a policy aim to do so!
This is a particular example of a more general difficulty with conventional
modelling, i.e. of forecasting travel outcomes in situations which are different in
kind - not merely quantity - from those prevailing today. For these situations 'stated
preference' techniques have been developed. These use evidence from social research
in laboratory conditions which elicits from different types of traveller the choices
they would make under various hypothetical future conditions. These responses
rather than those observed in current 'real-world' situations (where they exist) can
be incorporated into the modelling of future behaviour. However such refinements
present their own technical difficulties. This highlights the value of various kinds
of pilot or demonstration project which enable evidence to be utilised of people's
actual
response to innovations. The question of possible synergy between new types
of measure is particularly important - hence interest in the results of the Sustainable
Towns Initiative discussed in Chapter 16.
21.5 'Value for money'
As noted earlier the New Approach to Appraisal was originally developed to present
information about scheme impacts comprehensively and impartially and to counter
a situation in which the results of 'narrow' exercises in cost-benefit analysis were
thought to have too dominant an influence on decision-making. However assimilating
and weighing the variety of impacts listed in the AST presents a daunting task. From
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