Drainage gradient (DG) is a term in road technology, defining the resulting vector of a road surface cross slope (CS) and longitudinal gradient (hilliness).
If the DG 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 (tires) and vehicle crash.
Read more about Drainage Gradient: Most Road Design Codes Require Minimum 0.5 % Drainage Gradient, Drainage Gradient Typically Exceeds 1 - 3 %, Short Sections Have Insufficient DG But Are Vehicle Crash Hot Spots, How To Avoid or Minimize Skid Risk Caused By Insufficient Drainage Gradient