The simulation is of a highway paved with a porous asphalt overlay. When it rains, the pavement fills up and water starts moving over the road surface. The left graph shows a cross section of the roadway. The dashed gray line is the pavemevent surface that vehicles travel on and the black line is the bottom of the porous layer (which is 0.05 meters thick). The second graph shows the rainfall in green and the amount of water flowing off the road in blue. The main point is that the porous layer greatly reduces the time that water ponds on the road.