Heterogeneous perimeter flow distributions and MFD-based traffic simulation

This paper investigates how network and traffic heterogeneities influence the accuracy of a simulation based on the Macroscopic Fundamental Diagram (MFD). To this end, the MFD modeling of a simple grid network is compared to the outputs of a mesoscopic kinematic wave model simulating traffic in the same network. Heterogeneous distributions of demand and supply at the boundaries are set to the local entries and exits of the mesoscopic model to generate heterogeneous network loadings. These boundary conditions challenge the MFD simulation, as significant discrepancies are observed between both modeling approaches in steady state. While the accurate calibration of the MFD and the average trip length can reduce the discrepancies for heterogeneous demand settings, no simple solution exists for heterogeneous supply settings, because they may drive very different internal congestion patterns in the network. We propose a correction method to adjust the MFD model outputs in such a case

Dynamic macroscopic simulation of on-street parking search: a trip based approach

This paper extends a trip-based aggregate dynamic traffic model to account for on-street parking search. The trip-based approach for a road network defined as a reservoir characterizes the internal traffic states by a macroscopic fundamental diagram (MFD) in speed while individualizing all vehicle travel distances. This paper first investigates distances to park for on-street parking based on real data in Lyon (France) and stochastic numerical experiments. An updated formulation compared to the existing literature is proposed for the relation between such distances and the parking occupancy. This new formulation is then incorporated into an event-based numerical scheme that solves the trip-based MFD model

Macroscopic urban dynamics: Analytical and numerical comparisons of existing models

In this paper we compare a single reservoir model and a trip-based model under piecewise linear MFD and a piecewise constant demand. These assumptions allow to establish the exact solution of the accumulation-based model, and continuous approximations of the trip-based model at any order using Taylor series

Modélisation dynamique des grands réseaux de transports

Congestion in urban areas has become a major issue in terms of economic, social or environmental impact. For short or mid term, using dynamic road traffic simulation can help analyzing and providing guidelines to optimization policies of existing infrastructures. Today, because of the complexity of transport systems, classical modeling tools are limited to small geographical areas (of a district size). Computational time, together with simulation calibration, are notably very constraining at large scales. However, a new generation of models designed for metropolitan areas has arisen over the past decades. These models are based on a phenomenological relationship between travel production and the number of vehicles in a given spatial area of a road network, known as the Macroscopic Fundamental Diagram (MFD). This relationship, supported by empirical evidences from several cities around the world, has allowed the study of different traffic control schemes at a whole city scale, but was rarely used for traffic state forecasting. The aim of this PhD is to propose an efficient modeling tool, based upon the concept of MFD, to simulate and analyze traffic states in large metropolitan areas. The theoretical framework of this tool must be consistent and applicable for traffic state forecasting, development of new control policies, traffic emission estimation, etc. There are two major contributions in this PhD. The first one is analyzing the mathematical and physical properties of existing models, and formalizing the dynamics of several trip lengths inside the same urban zone. In particular, this formalization distinguishes between internal trips and trips crossing the zone. Flow merging and diverging issues are also addressed when congestion propagates from one zone to another. The second contribution is proposing a new trip-based model based on individual traveled distance. This approach allows to treat users independently (previously represented with continuous flows), and thus to define their characteristics more precisely to couple their trips with assignment models on different paths. Finally, examples of application from various collaborations are given in the last part of this thesis. It includes a simulation study of the Grand Lyon urban area (France), as well as new modules to simulate search-for-parking or perimeter control. This PhD is part of a European ERC project entitled MAGnUM: Multiscale and Multimodal Traffic Modeling Approach for Sustainable Management of Urban Mobility.La congestion en milieu urbain est un enjeu majeur que ce soit d’un point vue économique, social ou environnemental. À court et moyen terme, l’utilisation de la simulation dynamique du trafic routier peut permettre d’analyser et de guider des politiques d’optimisation des infrastructures existantes. Aujourd’hui, du fait de la complexité des systèmes de transport, les outils de modélisation classiques sont limités à des échelles géographiques peu étendues (de l’ordre du quartier). À grande échelle, le temps de calcul devient rapidement un facteur limitant tout comme le calibrage et la scénarisation. Néanmoins les dernières décennies ont vu l’apparition d’une nouvelle génération de modèles bien adaptés aux métropoles urbaines. Ceux-ci sont basés sur une relation phénoménologique entre la production de déplacements et le nombre de véhicules dans une zone spatiale d’un réseau routier, appelée Diagramme Fondamental de Zone (Macroscopic Fundamental Diagram, MFD). Cette relation, validée empiriquement sur de nombreuses villes, a permis d’étudier différentes méthodes de contrôle du trafic pour une ville entière, mais a été peu utilisée à des fins de prévision de la congestion. L’objectif de cette thèse est de proposer un premier outil opérationnel de simulation et d’analyse des grands réseaux de métropoles, en utilisant et développant les modèles de trafic basés sur la relation MFD. Cet outil doit posséder un cadre théorique cohérent qui puisse convenir à des applications telles que la prévision d’états de trafic, le développement de nouvelles politiques de contrôle, l’estimation de pollutions liées au trafic, etc. Les contributions de la thèse portent sur deux aspects. Le premier est l’analyse des propriétés mathématiques et physiques des modèles existants, en incluant une formalisation complète de la gestion de plusieurs longueurs de parcours au sein d’une même zone urbaine. En particulier, cette formalisation traite de la distinction des trajets internes à la zone et des problèmes de flux convergents et divergents pour les trajets traversant la zone lorsque la congestion se propage d’une zone à l’autre. Le deuxième aspect est la proposition d’un nouveau modèle basé sur la distance individuelle parcourue à l’intérieur d’une zone urbaine (trip-based). Cette approche permet d’individualiser les usagers (auparavant représentés sous forme de flux continus) et donc de définir plus finement leurs caractéristiques, en vue de coupler leurs déplacements à des modèles d’affectations sur différentes routes. Enfin, des exemples d’application illustrant diverses collaborations sont donnés en dernière partie de la thèse. La simulation du trafic sur l’aire urbaine du Grand Lyon (France) y est présentée, ainsi que de nouveaux modules de modélisation de la recherche de parking ou de contrôle périphérique. Cette thèse est partie intégrante d’un projet européen ERC intitulé MAGnUM : Approche multi-échelle et multimodale de la modélisation du trafic pour une gestion durable de la mobilité urbaine

MFD-based simulation: Spillbacks in multi-reservoir networks

97th Transportation Research Board Annual Meeting (TRB), Washington, Etats-Unis, 06-/01/2018 - 11/01/2018In this paper, we propose a thorough analysis of the way congestion is usually handled in the accumulation-based framework. This serves as a basis to implement a proper congestion propagation model in the trip-based approach. Theoretical and simulation studies show that in case of several trip lengths in a zone, there exists only one form of inflow limitation at the reservoir entry that complies with the global constraints on flow and production

Flow exchanges in multi-reservoir systems with spillbacks

Large-scale traffic flow models based on the Network Macroscopic Fundamental Diagram (MFD) are usually grounded on the bathtub analogy and a conservation equation for vehicle accumulation inside a given urban area. Recent studies have proposed a different approach where the MFD defines the spatial mean speed that is shared by all vehicles in a region while their traveling distance is tracked individually. The former approach is also referred to as accumulation-based while the latter is usually named trip-based. While extensive studies of both model properties have been carried out for the single reservoir case (a unique region), the multi-reservoir setting still requires some research effort in particular to clearly understand how inflow merge at a reservoir entry and outflow diverge at exit should be managed. These two components play a significant role in the evolution of the whole system, when flows are exchanged between multiple reservoirs. One of the crucial questions is to ensure that congestion properly propagates backwards through a succession of reservoirs when oversaturated situations are observed. In this paper, we propose a thorough analysis of how to handle congestion propagation in the accumulation-based framework with several trip lengths or categories, e.g. internal and external trips. This allows to derive a congestion propagation model for the trip-based approach in a multi-reservoir setting. Based on theoretical considerations and simulation studies, we develop a consistent framework to restrict the inflow and adapt to oversaturated traffic conditions in a reservoir including several trip lengths. Two inflow merging schemes are investigated. The first one is inspired from the existing literature and shares the available supply based on the demand flow ratio at the entry boundary. It is called exogeneous in contrast to the second endogenous scheme, which shares the supply with respect to the internal accumulation ratio on the different routes. At the reservoir exit, a new outflow diverging scheme is also introduced to better reproduce the effect of queuing vehicles that are prevented from exiting the reservoir when congestion spills back from neighboring reservoirs. Compared to the conventional outflow model, our new approach proves to avoid unrealistic gridlocks when the reservoir becomes oversaturated. Both entry and exit flow models are investigated in details considering the accumulation-based and trip-based frameworks. Finally, the most consistent approach is compared with two other existing MFD models for multiple reservoirs. This demonstrates the importance of properly handling entry and exit flows at boundaries

The MFD trip-based approach applied to multi-reservoir systems.

hEART2016 - 6th symposium arranged by European Association for Research in Transportation, HAIFA, ISRAEL, 12-/09/2017 - 14/09/2017Over the past decade, the Macroscopic Fundamental Diagram (MFD) has appeared to be a powerful tool to describe traffic states at the network level with few implementation and computational efforts. Many studies (e.g. Knoop & Hoogendoorn, 2014, Yildirimoglu & Geroliminis, 2014, Yildirimoglu et al., 2015, Haddad, 2015) have notably used MFD-based traffic simulators for several applications, like traffic state estimation, perimeter control or assessing routing strategies at a large scale. Their modeling approaches take advantage of the multi-reservoir representation of a city, where the dynamics of each urban subregion is described by the single reservoir model of Daganzo (2007). This framework, also referred as the accumulation-based model, assumes that the reservoir outflow is for the simplest model proportional to the total circulating flow inside the zone or that partial outflows are proportional to fractions of the circulating flow for more advanced models. However, while being acceptable in slow-varying conditions, this hypothesis may be too restrictive when the demand evolves too fast as shown by Mariotte et al. (2017). An idea, initially proposed in Arnott (2013), has been exploited in Daganzo & Lehe (2015) and then Lamotte & Geroliminis (2016) to design a 'trip-based' formulation of the MFD model. This approach considers that all users travel at the same speed at a given time, and exit the zone once they have completed their individually assigned trip length. As shown in a thorough comparison with the accumulation-based model by Mariotte et al. (2017), the trip-based approach gives more accurate results during transient phases, and proves to be more flexible to integrate various trip lengths inside the same reservoir

Minimizing network-wide emissions by optimal routing through inner-city gating

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Modeling local flow restriction at boundaries in multi-reservoir systems: An hybrid approach

hEART 2018, 7th Symposium of the European Association for Research in Transportation, Athènes, GRECE, 05-/09/2018 - 07/09/2018Since the early works of Daganzo (2007), Geroliminis & Daganzo (2007), using the Macroscopic Fundamental Diagram (MFD) to simulate traffic states at a city scale has gained more and more interest in the literature. Numerous studies (see e.g., Kouvelas et al., 2017, Sirmatel & Geroliminis, 2017, Yang et al., 2017, Zhong et al., 2017) notably used MFD-based simulation to design promising traffic control frameworks for large-scale networks, where such networks are considered split in several homogeneous reservoirs with well-defined MFD. However, there is still a lack in understanding flow exchanges and limitations at the reservoir boundaries in multi-reservoir systems. More precisely, we identify three research questions which need to be investigated: (i) how to define dynamically the maximum available flow that can enter a reservoir, both for under- and over-saturated conditions? (ii) how to scale up link-level information from the real network to account for local capacity reductions (that may be due to incidents, change in green times, temporarily closure of a major arterial, etc)? (iii) how to manage flow merging and diverging when different demand flows are distinguished (by their origins, destinations, or regional paths)

Multi-reservoir MFD-based simulation: An application to the city network of Lyon

TRISTAN X, The Tenth Triennial Symposium on Transportation Analysis, Hamilton Island, AUSTRALIE, 17-/06/2019 - 21/06/2019In the recent past, the Macroscopic Fundamental Diagram (MFD) proved to be an attractive alternative to describe the traffic states at the network level. But although complex formulations were proposed for MFD-based simulation in the literature, very few detailed validations were carried on real networks. Hence, this work focuses on a thorough validation of MFD simulators on the real field. In particular, we aim at (i) investigating the accuracy of the MFD multi-reservoir trip-based and accumulation-based models for a real large scale network by comparing the simulation results with real traffic data; and (ii) going a step further in defining proper calibration methods for the key parameters of MFD models. The network studied is the city of Lyon, which has the second greatest urban area of France, with more than 2 million inhabitants. This area is manually split into 5 reservoirs exhibiting a well defined MFD. The demand scenario was estimated for a typical weekday in a previous study based on household trip surveys and socio-demographic data. The traffic data consists of GPS trajectories of taxi fleets in Lyon, and all the loop detectors available in the area. The taxi trip data allows to determine the mean speed of each reservoir. The loop data provides a measure of the mean flow of each equipped link, which is then scaled up to each reservoir level by assuming that the measured mean flow is also representative of the non-observed links (homogeneity assumption). The average distances traveled are estimated by using shortest path calculations on the empty network (a method proposed by some of the authors in another study). A first comparison between the accumulation-based MFD simulation and the real data is presented with the evolution of accumulation in each reservoir. While providing a reliable estimation of the accumulation level in reservoir 1, the MFD simulation under-estimates the number of circulating vehicles in the other reservoirs. Different sources of errors are identified, but more investigations are still required for a better calibration of the MFD, the trip lengths and the distribution of flows