The economic and technical appraisal of highway investment in Tanzania: assessment of the application of HDM-4 model in maintenance and upgrading projects

The economic and technical appraisal of various investment strategies is a basic milestone for assisting in highway investment decision-making process. The economic and technical appraisal models simulates the whole life-cycle transport costing of the highway investment by forecasting the pavement cod1tion for the whole analysis period under the prevailing environmental, soil and geometric conditions with different applications of maintenance measures and the resulting road user implications in costs. The net benefits can then be obtained for each investment strategy as compared to the 'Do Minimum' case. This study analysed the use of the 'Highway Development and Management Tools' (HDM-4 system) for appraisal of highway investment strategies in Tanzania and comparison of the results were made with the 'Highway Design and Maintenance Standards model' (HDM-111(1995) system). Data from two case studies of 'Maintenance of Morogoro - Dodoma paved bituminous road' and 'Upgrading of Singida - Nzega unpaved gravel road' feasibility studies projects were input in both models and their respective resulting reports compared. The sensitivity of selected input parameters was evaluated for HDM-4 model and compared to HDM-111(1995) results from other studies as well as the impact of inclusion of accidents costing in HDM-4 model to the resulting economic analysis reports. It was concluded that, there is a marked difference in output results between the HDM-111(1995) and HDM-4 models in NPVs. timing of capital operations, pavement deterioration and sensitivity of the selected input parameters. Also the economic indicators, NPVs, in HDM-4 model differs significantly for 'with' and 'without' accident costs inclusion cases. Further researches are recommended for adapting the HDM-4 system to local environment and classification of all other HDM-4 input parameters on the basis of sensitivity classes. Also there is a necessity of developing the standard methods of data collection for various HDM-4 model input parameters. The inclusion of accidents costing has a negative effect on the improvement projects, especially when the 'Do something' attracts 'diverted' or 'generated' traffic growth. Hence, there is a necessity of applying accidents reduction measures in order to reduce the rate of accident occurrence per vehicle kilometre driven and consequently the benefits of improvement be realised without intolerable future consequences of human losses, injury or property damage.