The Not-So-Fundamental Relationship Between Traffic Flow and Speed?

Kösters, Till; Specht, Sebastian; Wessel, Jan

Abstract

The fundamental diagram of traffic congestion states that driving speed generally decreases with traffic flow, and that marginal decreases become more pronounced for higher flows. We find, however, that this seemingly fundamental relationship breaks down when only very few cars are on the road, and speed actually increases with traffic flow. To reveal this surprising finding, we use a unique large-scale real-world dataset with per-minute traffic observations from the German Autobahn, and control for confounders of the speed-flow relationship in a fixed-effects regression model. By linking our robust results to psychological research on social interaction effects in traffic, we then discuss potential reasons for this behaviour.