Drainage gradient (DG) is a term in road design, defined as the combined slope due to road surface cross slope (CS) and longitudinal slope (hilliness). Although the term may not be used, the concept is also used in roof design and landscape architecture.If the drainage gradient is too low, rain and melt water drainage will be insufficient. This results in water pooling on the road surface, thereby increasing the risk for hydroplaning and wet-pavement vehicle crashes.