The road network complements the natural drainage network and accelerates the water cycle in the catchment. In this study the methodology enabling for inclusion of a road network into a simple rainfall-runoff routing model was proposed. The road network was included into the surface drainage system and quantified as the Horton’s ratios. The bifurcation (RB), the length (RL) and area (RA) ratios increased when the road network was introduced to the surface drainage system. The Soil Conservation Service-Curve Number (SCS-CN) and Geomorphological Unit Hydrograph (GIUH) models were adapted to evaluate the influence of a road network on the flood wave hydrograph in the Zalasówka catchment (the Carpathian Foothills). Results revealed that the maximum flow is c.a. 20% higher, when the Horton’s ratios include the natural river system and the road network. Moreover, the differences between the simulated and observed maximum flows were lower when the RB, RL, and RA ratios included the road network as an element of the drainage system. It seems that including the road network in Horton’s ratios improves the simulation of flash flood wave parameters in small catchments.