THE TRRL ROAD INVESTMENT MODEL FOR DEVELOPING COUNTRIES (RTIM2)

A computer model is described which is designed to aid investment decisions within the roads sector in developing countries. The model calculates the construction cost of a road and predicts the condition of the road as time passes and vehicles travel over it. Having predicted the condition of the road, the model estimates the costs of road maintenance and the cost of operation of the vehicles for each year. All these costs are then discounted back to the base year and summed over the life of the road to obtain the total cost. All estimates are made in terms of physical quantities and costs are obtained by applying unit rates to these quantities. The results of a study in Kenya were used to calibrate a prototype version of the model. This prototype was tested extensively for the appraisal of road projects in developing countries. As a result of this experience, the model has now been reprogrammed to make it easier to use and to fit onto smaller computers. The opportunity has also been taken to include in the model the results of the latest research in developing countries in this field carried out by TRRL. The relationships built into the model allow it to be used to study the interrelationships between road design and construction standard, road maintenance policy, vehicle characteristics, traffic flow and growth rates, environment, and road deterioration. The model can be used to study various aspects of a road investment project such as the optimum maintenance standards for the road, the effects of providing an earth, gravel or bituminous pavement, or the differing benefits that can be obtained by adopting various stage construction options. The model will also allow the planner to study the consequences of uncertainties in traffic forecasts or in the discount rate. (Author/TRRL)