This paper investigates the existence of a regional-scale density de-lay function using simulations of the Dublin urban road network. This study divides the network into five zones and simulates traffic flows using the Simulation of Urban Mobility (SUMO). Data from the simulation construct regional density delay diagrams for each zone, revealing a clear relationship between traffic density and delay. The analysis applies the Bureau of Public Roads (BPR) volume delay function, substituting volume with density to fit the collected data. The results demonstrate a strong correlation and provide a robust model for predicting delays based on regional density. This newly identified density delay function has significant implications for regional congestion pricing and vehicle routing control, offering a practical tool for urban traffic management and policy-making.