Based on the kinematic wave theory, a method has been developed for the determination of road drainage inlet spacing under the continuous grade condition. The input data required are the allowable maximum flood with, the physical characteristics of the roadway, the empirical relationship between the maximum discharge and the intercepted flow, and the rainfall intensity-duration curve. By applying the method to a roadway with the standard drainage inlet used in Singapore, the effect of maximum flood width on the inlet spacing has been assessed. The assessment shows that for a given maximum flood width, the inlet spacing is wider for roads with steeper cross and longitudinal slopes. An increase in maximum flood width gives an exponential increase in inlet spacing.

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