ALTERNATIVES TO THE RULE OF A HALF IN MATRIX-BASED APPRAISAL

Abstract

Alternatives to the of a are needed if there is to be truly general appraisal methods. However, substantial progress can be made by extending the logic of the rule of half and using the technique of 'numerical integration' in selected cases where the RoH breaks down. For large cost changes (>33%) in matrix based appraisal, the authors recommend that numerical integration be used with 1-3 additional points. It appears that with 1 point the NI error is about 1/4 that of the RoH, whilst with 3 points it is 1/16 that of the RoH. For new modes, the authors recommend that numerical integration be used with 3-5 additional points. This should be sufficient to obtain results accurate to +/-10-20%, a substantial improvement over the other techniques tested. With regard to new generators and attractors, the RoH and NI should be applied as usual, with the land use strategy in place in the do-something if it forms part of an integrated regional strategy. Whatever the land use benefits are, it appears that the transport benefits can be estimated separately using TUBA/NI methods. Similarly, even when the price of the own mode affects the demand on another mode, it is legitimate to add the benefits of one mode to another. Some refinement of the rules for application may be desirable in the light of experience. Nevertheless, application of these methods should help to ensure that we do not overestimate the benefits of new modes, including LRTs, and conversely that we do not overestimate the disbenefits from large cost increases - eg. due to road user charging - in the future. For the covering abstract see ITRD E115303.

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