separate equations relating road roughness to traffic loading for each integral and half integral value of modified pavement structural number were incorporated in the road transport investment model. it has been found necessary to interpolate between these relationships for intermediate values of modified structural number and so they have been replaced by a single equation relating roughness, traffic loading and modified structural number in a continuous manner. similarly the separate equations which related road surface cracking to traffic loading for each value of modified structural number have also been replaced by a single equation. in addition the data obtained in kenya have been extrapolated to include a wider range of pavement structural numbers and higher levels of traffic loading. the speed estimating equations for medium and heavy goods vehicles on both paved and unpaved roads have been modified to include the effect of vehicle power-to-weight ratio. finally the equations relating vehicle wastage to vehicle age have been improved to ensure that the first derivative of the wastage function is continuous in the range of interest.(a)

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