The joint impact of road density and width on road network performances —a simulation approach based on macroscopic fundamental diagram

The urban transportation system is vital for efficient daily travel. Understanding how urban road network density and width influence traffic performance is crucial for informed decision-making in urban planning. In this study, we designed three homogeneous square road networks with varying densities and conducted simulation experiments. By controlling for heterogeneity in road network structure and variables such as network size, road area ratio, and traffic input, we ensured effective comparison of simulation results. Using the macroscopic fundamental diagram (MFD) model, we analyzed metrics including MFD scatter dispersion, changes in experimental results with different vehicle arrival patterns, distortion of the MFD image with varying traffic volume, MFD slope variations, and maximum outflow rates. This analysis enables comparison of road network performance under different vehicle arrival rates and traffic input variations, exploring the impact of the "narrow and dense network" strategy on reliability, stability, and efficiency. In addition, the results of our proposed scatter fitting method are similar to previous clustering-then-fitting methods and compensate for the challenges in critical point estimation. Results indicate that, while maintaining a constant road area ratio, higher density improves network reliability but may initially decrease stability, while efficiency varies with traffic conditions and examination angles. These findings elucidate the interplay between road network density, width, traffic state, and macroscopic performance, providing a basis for rational urban road network construction.