Macroscopic Fundamental Diagram: Measuring flow and density

This research presents a novel, network-wide approach to identify critical links and estimate average traffic flow and density. The proposed model estimates the Macroscopic Fundamental Diagram (MFD) using flow and density measurements from those critical links, which constitute only a small subset of all the links in the network.

The MFD, which exhibits the relationship between average flow and average density of an urban network, is a promising framework for monitoring and controlling urban traffic networks.

Given that monitoring resources (e.g. loop detectors, probe vehicle data, etc.) are limited in real-world networks, acquiring adequate data to estimate an MFD is of crucial importance.

Applying the same method to identify the critical links, the second aim of this research is to combine the loop detector and probe vehicle data, and use this simultaneously to estimate the MFD.

Furthermore, it may be possible to examine new traffic data sources (i.e. vehicle spacing data) in MFD estimation alongside with traditional traffic data.

Participants

• • University of Queensland

Project background

Traffic congestion has been increasing due to population growth and rapid development of metropolitan areas. It has been proven that increasing the supply (e.g. constructing new infrastructure) will not necessarily alleviate traffic congestion.

Additionally, urban space is often limited and does not allow more roadway infrastructure to be built around the cities. Therefore, the efficient use of existing infrastructures seems to be the only way forward to confront increasing urban traffic congestion. Traffic control methods such as road pricing, adaptive signal controls, etc. are essential to alleviate traffic congestion in urban areas and improve mobility at the network level. The first stage towards this final goal is to properly monitor or estimate traffic dynamics.

Large-scale traffic control remains a big challenge due to the complexity and unpredictability of transportation networks. Current traffic control strategies focus on individual intersections or arterial corridors, and they are considered less efficient in over-saturated traffic conditions, as the modelling of traffic underlying these strategies breaks down under such conditions. Given accurate inputs, detailed traffic models (e.g. microscopic simulation models) can provide substantial traffic predictions at the link level.

However, network management based on these models can be quite inefficient due to:

1. Exhaustive number of required inputs (e.g. detailed OD tables, signal settings, etc.)
2. Inaccuracy of behavioural models (e.g. car following, route choice models, etc.)
3. Chaotic behaviour of traffic in over-saturated networks. Hence, efficient management of transportation systems requires parsimonious modelling of traffic. Therefore, the Macroscopic Fundamental Diagram (MFD) is introduced to simplify the complexity of micro-modelling by capturing the average traffic flow dynamics in a large urban network. MFD is a useful tool for designing and implementing monitoring and management strategies.

A non-biased MFD arises only when all the vehicle trajectories are available. However, in practice, for large urban networks, it is unlikely that such data is available. Therefore, researchers have developed mathematical approaches to identify the most important traffic sensors to collect traffic data. Given that most of the previous studies require a priori knowledge of the ground-truth MFD, our aim is to develop a method for estimating the MFD which does not require such information. We use the ground-truth data only to evaluate our estimations.

The proposed method is focusing on finding a subset of links (i.e., critical links) that we assume they are the links that represent average traffic conditions in the network. Therefore, we aim at identifying the different traffic patterns and selecting the critical links out of important patterns. Critical links are the links where loop detectors are supposed to be installed to estimate the MFD and minimise the estimation error between the MFD that is estimated by limited measurements and the true MFD which is estimated from all the network links measurements.

Project objectives

1. Address limited budget constraints

Usually in large-scale urban networks, collecting adequate traffic data is not straightforward since our budget is limited. For instance, it is not efficient to install loop detectors on all the links in the network as they are expensive to implement and maintain. Therefore, we are aiming to find the most important links to install loop detectors on.

2. Incorporate disparate data sources

In the first part of this study, we assume that only loop detector data is available. In the second part; however, we assume that we have access to another source of traffic data (e.g. probe vehicle GPS data). The goal is to combine these two sources of data to estimate a more accurate MFD.

3. Exploit new vehicle technologies

While in the first and second parts of this study we exploit conventional traffic data sources (loop detector and probe vehicle), in this part we are aiming to take advantage of new vehicle technologies and the particular data that they can provide.

For instance, Driver Assistance Systems (DAS) has been becoming more popular. Such technologies can refer to Adaptive Cruise Control (ACC) which is a substitution for conventional cruise control. This new technology controls both speed of the ACC-equipped vehicle and its distance from the leading vehicle. Hence, spacing and headway data are provided using this technology which can be later applied to measure flow and density of a link, and eventually estimate the MFD.

PhD completed

The student has completed his iMOVE project and scholarship.