A runoff model from porous pavements is presented that links the hydraulic conductivity (kD) of porous pavement with the geometric characteristics of the road section and rainfall intensity (I). The theoretical model has been tested in the laboratory with an original device that simulates rainfall on porous pavements and, at the same time, measures the depth of water over the impervious layer during the seepage motion. Based on the experimental data, a useful chart for porous pavement design is also presented. For every inclination (i) of the impervious layer, the graph provides the value of the nondimensional ratio Hmax/L between the maximum depth (Hmax) of the water table over the impervious layer and the length (L) of the seepage path, in the function of the nondimensional ratio 4I/k between rainfall intensity (I) and permeability (k). Data provided with this graph could improve porous pavement design (e.g., to assess the minimum thickness of an open-graded friction course needed to avoid surface runoff) usually based only on mechanical resistance determinations. If the chart is not used, to ensure the best flow conditions, the relation i2 > 4I/k should be always verified.