Probabilistic Breakdown Phenomenon at On-Ramp Bottlenecks in Three-Phase Traffic Theory

TL;DR

This paper presents a nucleation model based on three-phase traffic theory explaining the probabilistic breakdown phenomenon at on-ramp bottlenecks, including the conditions and rates of traffic flow transitions from free to synchronized flow.

Contribution

It introduces a novel nucleation model that links random vehicle cluster growth to traffic breakdown, supported by empirical and simulation data within three-phase traffic theory.

Findings

Mean time delay and nucleation rate of breakdown are quantified.

Breakdown boundaries are identified and correlated with microscopic traffic simulations.

The model exhibits different characteristics than previous traffic nucleation models.

Abstract

A nucleation model for the breakdown phenomenon in freeway free traffic flow at an on-ramp bottleneck is presented. This model, which can explain empirical results on the breakdown phenomenon, is based on assumptions of three-phase traffic theory in which the breakdown phenomenon is related to a first-order phase transition from the "free flow" phase to the "synchronized flow" phase. The main idea of this nucleation model is that random synchronized flow nucleation occurs within a metastable inhomogeneous steady state associated with a deterministic local perturbation in free flow, which can be considered "deterministic vehicle cluster" in free flow at the bottleneck. This deterministic vehicle cluster in free flow is motionless and exists permanent at the bottleneck due to the on-ramp inflow. In the nucleation model, traffic breakdown nucleation occurs through a random increase in vehicle number within this deterministic vehicle cluster, if the amplitude of the resulting random vehicle cluster exceeds some critical amplitude. The mean time delay and the associated nucleation rate of speed breakdown at the bottleneck are found and investigated. The nucleation rate of traffic breakdown as a function of the flow rates to the on-ramp and upstream of the bottleneck is studied. The nucleation model and the associated results exhibit qualitative different characteristics than those found earlier in other traffic flow nucleation models. Boundaries for speed breakdown in the diagram of congested patterns at the bottleneck are found. These boundaries are qualitatively correlated with numerical results of simulation of microscopic traffic flow models in the context of three-phase traffic theory.