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Ρύθμιση φωτεινής σηματοδότησης αστικού δικτύου σε πραγματικό χρόνο υπό κορεσμένες συνθήκες κυκλοφορίας
No full textIn recent decades, mitigating the traffic congestion in urban road networks has been a crucial issue for both the research and the practical operations, which calls for the development and implementation of improved traffic signal control methods and techniques. In particular, the development of efficient and practicable real-time signal control strategies under saturated traffic conditions is a major challenge, as widely used strategies like SCOOT and SCATS are deemed less efficient under saturated traffic conditions. A practical tool, frequently employed against over-saturation of significant or sensitive links, arterials or urban network parts, is gating. The idea is to hold traffic back (via prolonged red phases at traffic signals) upstream of the links to be protected from over-saturation, whereby the level or duration of gating may depend on real-time measurements from the protected links. The method is usually employed in an ad hoc way (based on engineering judgment and manual fine-tuning) regarding the specific gating policy and quantitative details, which may lead to insufficient or unnecessarily strong gating actions. Recently, the reproducible relationship between flow and density occurring at the network level under certain conditions (e.g. homogeneous spatial distribution of the congestion) known as macroscopic or network fundamental diagram (MFD or NFD), has gained increased popularity. Although the NFD notion is still under investigation in various aspects, it can be exploited as a fruitful basis for derivation of urban signal control approaches.In this thesis, the notion of NFD for urban networks is exploited to improve mobility in saturated traffic conditions via application of gating measures, based on an appropriatesimple feedback control structure. Different gating control strategies (i.e. single perimeter gating control by exploiting complete and reduced NFD, multiple-concentric gating control, perimeter traffic control via remote feedback gating) have been proposed and tested on realistic simulation scenarios of two urban networks (i.e. Chania, Greece and San Francisco, USA) successfully. In the investigated examples, feedback gating is demonstrated to lead to substantial improvements of travel delays, network throughput and travel time reliability.Στις τελευταίες δεκαετίες, η ανάγκη μετριασμού της κυκλοφοριακής συμφόρησης στα αστικά οδικά δίκτυα έχει γίνει ένα κρίσιμο θέμα τόσο για την έρευνα όσο και την πρακτική εφαρμογή, το οποίο καλεί για την ανάπτυξη και την εφαρμογή βελτιωμένων μεθόδων και τεχνικών ελέγχου φωτεινής σηματοδότησης. Ειδικότερα, η ανάπτυξη ικανοποιητικών και εφαρμόσιμων στρατηγικών ελέγχου φωτεινής σηματοδότησης σε πραγματικό χρόνο υπό κορεσμένες συνθήκες είναι μια μεγάλη πρόκληση, καθώς ευρέως διαδεδομένες στρατηγικές που χρησιμοποιούνται έως τώρα όπως το SCOOT και το SCATS θεωρούνται λιγότερο αποτελεσματικές υπό κορεσμένες συνθήκες. Ένα πρακτικό εργαλείο, συχνά εφαρμόσιμο κατά του υπερκορεσμού σημαντικών ή ευαίσθητων συνδέσμων, αρτηριών ή περιοχών του αστικού δικτύου, είναι η ελεγχόμενη είσοδος (gating). Η ιδέα είναι να κρατηθεί η κυκλοφορία (μέσω παρατεταμένων κόκκινων φάσεων στους φωτεινούς σηματοδότες) ανάντη των συνδέσμων που πρέπει να προστατευτούν από υπερκορεσμό, ενώ το επίπεδο ή η διάρκεια της ελεγχόμενης εισόδου μπορεί να εξαρτάται από τις μετρήσεις σε πραγματικό χρόνο από τους προστατευόμενους συνδέσμους. Η μέθοδος αυτή εφαρμόζεται συνήθως για εξειδικευμένα και όχι γενικά προβλήματα (με βάση την εμπειρία και πειραματισμούς) ανάλογα με τη συγκεκριμένη πολιτική ελέγχου εισόδου και τις ποσοτικές λεπτομέρειες, τα οποία όμως μπορεί να οδηγήσουν σε ανεπαρκείς ή υπερβολικές δράσεις ελεγχόμενης εισόδου.Πρόσφατα, η αναπαραγόμενη σχέση μεταξύ της ροής και της πυκνότητας ενός δικτύου υπό ορισμένες συνθήκες (π.χ. ομογενής χωρική κατανομή της συμφόρησης) γνωστό ως μακροσκοπικό θεμελιώδες διάγραμμα (MFD) ή θεμελιώδες διάγραμμα του δικτύου (NFD), έχει αποτελέσει αντικείμενο έρευνας. Παρόλο που η έννοια και ιδιότητες του NFD είναιακόμα υπό διερεύνηση ως προς διάφορες πτυχές, το NFD μπορεί να αξιοποιηθεί ως μια γόνιμη βάση για την παραγωγή προσεγγίσεων του ελέγχου φωτεινής σηματοδότησης σε αστικά δίκτυα. Στην παρούσα διατριβή, η έννοια του NFD για αστικά δίκτυα χρησιμοποιείται με σκοπό τη βελτίωση της κινητικότητας σε κορεσμένες κυκλοφοριακές συνθήκες μέσω της εφαρμογής της ελεγχόμενης εισόδου, με βάση μια κατάλληλη απλή δομή ελέγχου ανάδρασης. Διάφορες στρατηγικές ελέγχου εισόδου (όπως έλεγχος εισόδου σε μια περίμετρο αξιοποιώντας πλήρες και μειωμένο NFD, έλεγχος εισόδου πολλαπλών ομόκεντρων περιμέτρων, περιμετρικός έλεγχος εισόδου μέσω απομακρυσμένων πυλών ανάδρασης) προτείνονται και διερευνώνται σε ρεαλιστικά σενάρια προσομοίωσης δύο αστικών δικτύων (Χανιά, Ελλάδα, και Σαν Φρανσίσκο, ΗΠΑ) επιτυχώς. Στα παραδείγματα που μελετήθηκαν, η ελεγχόμενη είσοδος με ανατροφοδότηση οδηγεί σε ουσιαστικές βελτιώσεις στις καθυστερήσεις κατά την διάρκεια του ταξιδίου και αύξηση της συνολικής ροής στο δίκτυο
Macroscopic modeling of mixed bi-modal urban networks: A hybrid model of accumulation- and trip-based principles
No full textNetwork-level traffic flow models either assume steady-state urban flows (i.e. accumulation-based models) or track the movement of all vehicles (i.e. trip-based models). The steady-state assumption present in the accumulation-based models may pose a challenge in light of the multi-modal nature of urban flows. It might be indeed a rough assumption for the flow of some transportation modes like buses, cruising-for-parking vehicles, taxis, and on-demand vehicles. Trip-based models address this concern, however, they need significant parameter calibration effort and are not computationally efficient, which substantially reduces the practicality of these models in real-world applications. Nevertheless, despite the critical importance of developing multi-modal traffic flow models, few attempts have been made to investigate these models in network macroscopic fundamental diagram (NMFD)-related literature. This paper bridges this gap by developing a hybrid network-level traffic flow model for mixed bi-modal (i.e. car and bus) networks. The present hybrid model reproduces the dynamics of car flows via accumulation-based model principles while tracking the movement of buses using the trip-based model. This effort also includes the development of a new FIFO-based entrance function to ensure different modes experience the same delay under saturated traffic conditions. Different numerical experiments are conducted to study the hybrid model performance and to compare it with that of accumulation-based and trip-based models in both steady-state and transition periods under different traffic conditions. Our observations reveal that the hybrid model simulates the dynamics of cars and buses by closely following the behavior of its components under free-flow conditions. The model also outperforms the accumulation-based model under saturated traffic conditions while being considerably less demanding than the trip-based model. A further investigation of the model performance is performed for networks with different bus shares in both free-flow and saturated traffic conditions, confirming the results of the initial numerical experiments. The hybrid model’s computational efficiency is demonstrated. The potential real-world applications of the hybrid model in development of bi-modal network-level simulation models, NMFD-based control strategies along with bus space allocation policies, public transport operation problems, modeling of cruising-for-parking vehicles, taxis, and on-demand vehicles, and modeling and application of autonomous modular vehicles are discussed and future research directions are highlighted
Impede autonomous vehicles merging at on-ramps?
No full textTRB 2018, 97th Annual Meeting Transportation Research Board, Washington, ETATS-UNIS, 07-/01/2018 - 11/01/2018This paper sheds some light on the macroscopic and microscopic characteristic of traffic flow on freeways in the merging areas in presence of Human-Driven Vehicles (HDV) and Connected Autonomous Vehicle (CAV) by thoroughly digging into the literature, exploiting real data and simulating simple scenarios. In particular, in the first step, an extensive literature review on merging behaviour for current and future traffic conditions (i.e. having HDV or HDV and/or CAV on the road) has been carried out. This is followed by an analysis of single vehicle data from a freeway in Lyon to illustrate current speed and headway distributions which are most likely to change with the introduction of CAV's. Finally, some microscopic simulation results are presented which show that traffic flow typically gains from the introduction of CAV's, but may also display negative consequences under certain circumstances. This simulation study focused mainly on the traffic characteristics at the merging area in presence of CAV. It remains for future work to map out precisely where the introduction of CAV's improves the traffic system, and where amendments have to be made to prevent negative side-effects. It should be noted that calibrating and validating the micro-simulation tools which simulate traffic scenarios in presence of CAV's will indeed be challenging in the future. This needs extra attention for research
Optimizing distribution of metered traffic flow in perimeter control: Queue and delay balancing approaches
No full textPerimeter traffic flow control based on the macroscopic or network fundamental diagram provides the opportunity of operating an urban traffic network at its capacity. Because perimeter control operates on the basis of restricting inflow via reduced green times at selected entry (gated) links, vehicles on those links may be subject to queuing and delay. The experienced delay or resulting queue lengths depend on the adopted policy for the distribution of the inflows and corresponding green times at the gated links. The chosen policy may have a significant impact on the traffic system under control. For example, managing queue lengths may reduce the interference with upstream traffic whereas the management of delays may improve users’ perception with respect to equity and fairness. In this paper, an approach has been proposed to distribute the gated flow based on the queue lengths or experienced delay at the gated signalized junctions. This is in contrast to standard practice that distributes inflows proportionally to the gated links’ saturation flows. Perimeter control is then evaluated in a microscopic simulator for a realistic traffic network and compared in three configurations against fixed-time: perimeter control without queue or delay management; perimeter control with relative queue balancing; and perimeter control with delay balancing. It has been found that managing the queues at the gated links not only improves the overall network performance but also reduces the possibility of queue propagation to the upstream junctions. This improves traffic flow outside the protected network by managing the queue propagation at the gated links and reducing the possibility of queue spill-back to upstream intersections. In addition, the results indicate that perimeter control with delay balancing has a similar performance as the case without queue or delay management being a suitable approach for flow distribution among the gated links. In the scenarios with perimeter control with either queue or delay balancing the gap between the ordered flow by the controller and the actual flow crossing the stop-line at the gated links reduced remarkably
Impede Autonomous Vehicles Merging at On-Ramps?
No full textThis paper gives an overview over the phenomena on freeways to be expected when connected and autonomous vehicles (CAV) will be introduced into the traffic system. It discusses the physics of the phenomenon, together with the modelling and simulation approaches that have been described so far. This is followed by an analysis of single vehicle data from a freeway in Lyon to illustrate current speed and headway distributions which are most likely to change with the introduction of CAV’s. Finally, some microscopic simulation results are presented which show that traffic flow typically gains from the introduction of CAV’s, but may also display negative consequences under certain circumstances. More specifically, the traffic characteristics at the merging area in presence of CAV as been studied. It remains for future work to map out precisely where the introduction of CAV’s improves things, and where amendments have to be made to prevent negative side-effects
Urban congestion gating control based on reduced operational network fundamental diagrams
No full textNEARCTI
A model predictive perimeter control with real-time partitions
Get PDFPrevious studies through simulation and empirical data have shown that a Network
Macroscopic Fundamental Diagram (NMFD) exists and can be used for designing network
optimal perimeter control strategies. These control strategies rely on well defined NMFDs, which
highly depend on the homogeneity of the traffic condition in the network. However, it is known
that traffic dynamics change drastically during the day in different zones in a large-scale network,
and different control strategies might lead to heterogeneous traffic distribution across the urban
network. One potential direction is re-partitioning the network to maintain the well defined
NMFDs. However, re-partitioning the network changes each sub network’s size, such that it
makes the well-defined NMFDs unpredictable. This paper provides a model predictive controlbased optimization approach for perimeter control using real-time partitioning to avoid this
problem and utilize re-partitioning techniques. Results show that the proposed method can be
used in a heterogeneous network to improve control performance by redistributing accumulations
via re-partitioning over time. Our results, which are compared to no control and the traditional
model predictive control, yield that the proposed method is superior to the others
Perimeter Traffic Control via Remote Feedback Gating
No full textAbstractRecent studies demonstrated the efficiency of feedback-based gating control in mitigating congestion in urban networks by exploiting the notion of network fundamental diagram (NFD). The employed feedback regulator targets an operating NFD point of maximum throughput to enhance the mobility in the urban road network during the peak period, under saturated traffic conditions. In previous studies, gating was applied directly at the border of the protected network (PN), i.e. the network part to be protected from over-saturation. In other words, to implement gating, the usual traffic light settings have been modified at (one or more) junctions at the boundary of the PN. In this work, the recently developed feedback-based gating concept is applied at junctions located further upstream of the PN. This induces a time-delay, which corresponds to the travel time needed for gated vehicles to approach the PN. The resulting extended feedback control problem can be tackled by use of a PI (Proportional-Integral) regulator, albeit with different gain values compared to the case without time-delay. The reported results show a stable behaviour and improved mobility of the overall network in terms of mean speed and travel time
Modeling driver's evasive behavior during safety-critical lane changes:Two-dimensional time-to-collision and deep reinforcement learning
Get PDFLane changes are complex driving behaviors and frequently involve
safety-critical situations. This study aims to develop a lane-change-related
evasive behavior model, which can facilitate the development of safety-aware
traffic simulations and predictive collision avoidance systems. Large-scale
connected vehicle data from the Safety Pilot Model Deployment (SPMD) program
were used for this study. A new surrogate safety measure, two-dimensional
time-to-collision (2D-TTC), was proposed to identify the safety-critical
situations during lane changes. The validity of 2D-TTC was confirmed by showing
a high correlation between the detected conflict risks and the archived
crashes. A deep deterministic policy gradient (DDPG) algorithm, which could
learn the sequential decision-making process over continuous action spaces, was
used to model the evasive behaviors in the identified safety-critical
situations. The results showed the superiority of the proposed model in
replicating both the longitudinal and lateral evasive behaviors
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