Unless these sections have a longitudinal grade of at least 0.4–0.5%, the drainage gradient (resultant to crossfall and longitudinal grade) will be lower than 0.5% so water will not run off the road surface. Storm drains may be installed at regular intervals and modern paving materials are designed to provide high friction in most conditions. Permeable paving allows water to soak through the paving material, reducing slipperiness in very adverse conditions.