The Road Transport Investment Model for developing countries has been used to examine the effects of different axle loading characterists on the total costs of road transport. It is shown that the sum of vehicle operating costs, road construction costs and road maintenance and rehabilitation costs for a two lane highway initially decrease rapidly as the axle load of the most heavily loaded vehicles increases and passes through a shallow minimum at the optimum axle load. The value of this optimum axle load was found to be strongly dependent on the total freight tonnages carried by the heavily loaded vehicles, the load condition of these vehicles on the return trip, the exponent of the pavement damage - axle load relationship, and the relative prices of the major components of road transport cost. The optimum axle load was found to be virtually independent of the road alignment and strength of the subgrade, but the road construction and strengthening policy and the composition of the vehicle fleet had a small bu significant effect on its value. It is shown that under most conditions there exists a traffic level above which the optimum axle load is above the current legal limits in force in most developing countries. The total road transport costs were usually found to be relatively insensitive to axle load in the region of the minimum total transport cost, changes in axle loads of 10 per cent producing changes in the transport costs of less than 1 per cent. (A)

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