2001 - Swaziland - Comparison of HDM-III and HDM-4

The Highway Design and Maintenance Standards Model (HDM-III), developed by the World Bank, has been used extensively for the economic evaluation of road projects over a period of approximately 15 years. The Highway Development and Management Tools (HDM-4) was released early this year (2000) as the result of a process of upgrading and extension of the HDM-III. Both these models were applied to a case study, the Nsoko-Maloma road in Swaziland, in order to gain insight on the changes and improvements achieved in HDM-4. The paper provides an overview of the development of the  HDM-III and HDM-III Manager (which was used for the analysis), and the improvements implemented in HDM-4. A description of the case study and data required for the economic evaluation process are then addressed. The output of the two models is compared. It is shown that with regard to the sub-models the most important effect is the lowering of vehicle operating costs and the increase in travel speed which result from the adjustment of the sub-models for the improvement in the efficiency of motorised vehicles. The HDM-4 consequently produced lower values for the benefit-cost ratio and internal rate of return on the case study. When compared to the justification level, however, this reduction did not influence the economic viability of the project. In conclusion the improvements of HDM-4 as observed in the analysis are summarised. These are a modern-day programming style and wider options with regard to the definition of the vehicle fleet, upgrading choices, pavement types, seal types and other maintenance actions.