During extreme rainfall events, urban roads can function as surface runoff conduits when integrated with drainage systems, providing a cost-effective flood mitigation solution. However, the lack of reliable computational tools for modeling flow diversion at road intersections makes it challenging to accurately estimate drainage flow, limiting broader application of this technique. This study focuses on T-shaped intersections in urban road networks and conducts both hydraulic experiments and computational fluid dynamics (CFD) simulations under three distinct downstream boundary conditions: free outflow, backwater at the main road end, and backwater at both road ends. Based on the obtained data, a high-precision, low order flow diversion model for intersections is developed, with a calculation error basically within ±15%. The proposed model provides a simple and practical tool for engineering applications and can be seamlessly integrated into the widely used one-dimensional urban hydrological model SWMM (storm water management model). This integration significantly enhances the accuracy of flow distribution calculations at intersection nodes, enabling more precise design of the large-scale road major drainage system.