During extreme rainfall events, urban roads functioning as surface runoff conduits in conjunction with drainage systems represent an economically efficient technique for flood mitigation. However, due to the lack of available calculation tools for flow diversion at road intersections, it is difficult to determine the drainage flow of downstream road sections, resulting in obstacles to the application of this technology. This study focused on T-shaped intersections within urban road networks and conducted hydraulic experiments and Computational Fluid Dynamics (CFD) numerical simulations for three distinct downstream boundary conditions: free outflow, backwater at main road end, and backwater at both road ends. A high-precision and low order intersection flow diversion model was constructed using the obtained data, with a calculation error basically within ± 15%. The flow diversion model provides a direct and straightforward computational tool for engineering applications. Also, it allows seamless integration with the widely used one-dimensional urban hydrological model SWMM (Storm Water Management Model), significantly enhancing the computational accuracy of the SWMM model at intersection nodes, thereby enabling the precise design of the entire large-scale road major drainage system.