The requirements of the forecasting methods which underlie the evaluation procedures described in this book can be very stringent, requiring estimates of the demand curves for total trips (over the relevant ranges) and for the various available modes and of the interrelationships between them. Such estimates are expensive and timeconsuming to prepare or they may require the analyst to go beyond the limits of established research results. Very frequently the necessary time and money will not be available nor will the option be open of getting the information on which to base them. Further, as we see in more detail in Chapters 11 and 12, the organisational and institutional constraints under which the analyst has to work are sometimes severe. For these reasons, the analyst must often adopt a limited framework from the very start.