Specifica of Fundamental Diagram in Urban Traffic

Summary The relation between the fundamental quantities flow, density, and speed is well studied empirically in “undistorted” traffic, i.e. traffic on highways or major non-urban roads. The situation on urban roads is more complicated since urban traffic is influenced by a number of effects. Especially, the situation at intersections (traffic lights, merging traffic) is of striking importance. While the flow between two intersections can be considered as roughly constant, the density and the speed are quantities that vary rapidly along the road. By analyzing data from stationary sensors (measuring flow and local speed) as well as Floating Car Data (measuring travel time, i.e. averaged speed) we found that the speed on many urban roads does not depend on the corresponding flow (respectively the density) alone. For a given flow, the speeds at different times of the day may differ. This effect can be observed for local speeds as well as for speeds averaged along edges or over an area. Very often the maximum in the flow at morning peak precedes the speed minimum. The time difference between these two events is up to one hour varying from road to road. Similar observations have been found in averages over an urban area. A possible explanation for the observed relation between flow and speed is — besides the influence of traffic signalling — that a more heterogeneous traffic (with a higher fraction of vehicles merging into different directions) leads to slower speeds.