Abstract
Recent studies have proposed using well-defined relationships between network productivity and accumulation—otherwise known as Network or Macroscopic Fundamental Diagrams (network MFDs)—to model the dynamics of large-scale urban traffic networks. Network MFDs have been used to develop a variety of network-wide traffic control policies to improve a network’s operational efficiency. However, the relationship between a network’s MFD and its safety performance has not been well explored. This study proposes the existence of a Macroscopic Safety Diagram (MSD) that relates safety performance (e.g., likelihood of a crash occurring or number of vehicle conflicts observed) with the current network state (i.e., average density) in an urban traffic network. We theoretically posit a relationship between a network’s MSD and its MFD based on the average maneuver envelop of vehicles traveling within the network. Based on this model, we show that the density associated with maximum crash propensity is always expected to be larger than the density associated with maximum network productivity. This finding suggests that congested states are not only inefficient, but they might also be associated with more crashes, which can be both more unsafe and lead to decreased network reliability. These theoretical results are validated using surrogate safety assessment metrics in microsimulation and limited field empirical data from a small arterial network in Riyadh, Kingdom of Saudi Arabia. The existence of such MSDs can be used to develop more comprehensive network-wide control policies that can ensure both safe, efficient and reliable network operations.
Highlights
Introduction and motivationThere has been decades of work focused on aggregate models to describe urban traffic network performance at a regional level
This study proposes the existence of a Macroscopic Safety Diagram (MSD) that relates safety performance with the current network state in an urban traffic network
The goal of this paper is to explore the existence of Macroscopic Safety Diagrams (MSDs) of urban networks that relate safety performance and operational performance in urban traffic networks in a way that is consistent with
Summary
There has been decades of work focused on aggregate models to describe urban traffic network performance at a regional level (e.g., across small neighborhoods or networks). A simple theoretical model is proposed to relate the risk of multi-vehicle rear-end collisions and operational traffic state considering the average space a vehicle has to maneuver in the time-space plane This model is used to unveil the existence of network MSDs and identify some of their pertinent features. We propose here a simple theoretical model to explore the potential relationship between safety performance (i.e., propensity of a multi-vehicle rear-end collision) and operational performance (i.e., current traffic state) in dynamic urban networks. The validity of this model will be tested using simulated and empirical data in subsequent sections. The remainder of this paper will validate the applicability of this model and the resulting trends using both simulation and empirical data
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