Number fluctuations induce persistent congestion

TL;DR

This paper demonstrates through a mathematical model that fluctuations in vehicle numbers can cause persistent congestion on street segments even when average flow is below capacity, due to a self-reinforcing reduction in vehicle speeds.

Contribution

It introduces a simple mathematical model showing how number fluctuations alone can induce persistent congestion, highlighting a new mechanism beyond average flow considerations.

Findings

Fluctuations can cause congestion below capacity.

Congestion results from self-amplifying velocity reductions.

Fluctuation-induced congestion persists without high average flow.

Abstract

The capacity of a street segment quantifies the maximal density of vehicles before congestion arises. Here we show in a simple mathematical model that fluctuations in the instantaneous number of vehicles entering a street segment are sufficient to induce persistent congestion. Congestion emerges even if the average flow is below the segment's capacity where congestion is absent without fluctuations. We explain how this fluctuation-induced congestion emerges due to a self-amplifying reduction of the average vehicle velocities.