Fundamental Diagram on Urban Roads: Myth or Truth?

There is an ongoing discussion about the suitability of fundamental diagrams in urban networks. The authors have argued in favor of such a fundamental diagram, while some researchers deny that there is such a thing as a fundamental diagram on urban roads, because for any level of demand any travel time is possible. This contribution tries to sort this out by referring to new data sources as well as to simulation approaches. The authors hypothesize that in fact there is a fundamental diagram in urban roads; however, there is no one-to-one correspondence to the fundamental diagram on freeways.