ABSTRACT
In Chapter 11 it was argued that any proposed intervention into the transport system should be properly assessed both before and after its implementation – before, so that the number of unwise interventions may be minimised; after, to establish whether the intervention had been worthwhile. This second analysis may appear to be no more than belatedly identifying regretted mistakes, but it is actually a very important contribution to recognising which sorts of interventions are likely to be the most effective – the popularly named ‘learning curve’. The assessment rests on ten stages:
measuring the existing movement through the system;
forecasting the future movements if things are left as they are;
forecasting future movements once the intervention has been made;
estimating the movement costs, first with and then without the intervention;
hence, estimating the expected net change in movement costs wrought by the intervention; these are reckoned to be the ‘benefits’ to be credited to the intervention;
calculating the likely cost of the intervention;
estimating whether the cost of the intervention is likely to be justified by the estimated benefits;
making a go/no-go decision on whether to put the scheme into effect;
after the intervention has been made and has been working for long enough to have had a significant impact, measuring and costing the actual movement through the altered system and comparing it with the forecasts and estimates made in the original assessment;
striving to learn from the differences and, if necessary, modifying the assessment methodology.
It is important to realise that the differences between the initial forecasts and estimates and the later reality may disclose better-than-predicted benefits as well as worse and that such errors may be just as informative as the evidence of poor decisions. The ‘errors’ in error-actuated feedback may be positive as well as negative.