It is analyzed what the effect is of the exact network structure on the shape of the Network Fundamental Diagram (NFD). In order to do so, a tool is developed which can create random networks for which the aggregated properties are the same. Using this tool, different urban networks with the same properties are designed. They show different NFDs. Also, networks with extra arterial roads are designed. They do not increase the road capacity, but also increase the network inhomogeneity so the overall effect on network capacity is limited. Variations with the location of the extra arterials show that an asymmetrical arterial reduces network performance. Therefore, it is required that for each network an NFD is determined.