Angela Di Febbraro

One-Way Carsharing

Carsharing services allow users to benefit from the advantages of a private car without the costs of owning one. One-way systems provide users with a higher level of service than traditional carsharing systems in terms of flexibility because users do not need to return to the station of origin. Moreover, the added option to leave the vehicle at any free parking area, which is not necessarily a station, increases the flexibility offered by the one-way system. Introduction of such improvements to the carsharing system, however, leads to a vehicle relocation problem, which should be addressed carefully to avoid concentration of vehicles in certain areas. This paper reports on a study of this issue with the use of discrete event systems (DESs), which allowed an easy representation of the complex dynamics of the carsharing system. A user-based methodology was proposed on the basis of an optimal relocation policy in a rolling horizon framework. This methodology not only offers greater flexibility to users, it also maximizes operator benefits by reducing the number of required staff to relocate vehicles among the stations and determines the minimum number of vehicles needed to satisfy system demand. The DES model was applied to a case study to evaluate the proposed approach. The results showed a significant decrease in the rejection rate from the worst scenario (no relocation) to the best (relocation of all vehicles by their users). The paper concludes with suggestions for additional research and improvements to this study.

On using petri nets for representing and controlling signalized urban areas: New model and results

A microscopic model of signalized urban areas and its representation by means of stochastic-timed Petri nets are described in this paper, where a traffic-responsive control strategy, whose objective is the optimization of green duration of each stage, is also proposed. The Petri net representation here proposed is the new version of a previously proposed one, which was based on deterministic-timed Petri nets. The motivations of the need of proposing a new version and the differences between the two Petri net representations are clearly described. In addition, some results on the application of the proposed methodology to a single signalized intersection are included in the paper.

A Deterministic and Stochastic Petri Net Model for Traffic-Responsive Signaling Control in Urban Areas

The problem of reducing congestion within urban areas by means of a traffic-responsive control strategy is addressed in this paper. The model of an urban traffic network is microscopically represented by means of deterministic and stochastic Petri nets, which allow a compact representation of the dynamic traffic network. To properly model traffic congestion, intersections are divided into crossing sections, and roads have limited capacity. Each intersection includes a multiphase traffic signal, whose sequence of phases is given and represented by a timed Petri net. The control strategy proposed in this paper aims at minimizing queue lengths by optimizing the duration of each signal phase. This is accomplished by heuristically solving a stochastic optimization problem within a receding-horizon scheme, to take into account the actual traffic flow entering the network, thus making the proposed approach traffic-responsive. In this framework, the Petri nets play a key role, as the cost function to be minimized is a function of the marking, and the constraints include the marking state evolution. The proposed strategy is applicable to both undersaturated and oversaturated traffic conditions.

INTRANET, A NEW SIMULATION TOOL FOR INTERMODAL TRANSPORTATION SYSTEMS

Abstract The issues of modelling, simulation, and control of an intermodal urban transportation system are addressed in this paper. The transportation network is modelled as an oriented graph in which nodes represent single-mode stations or intermodal stations. For the system under consideration, a discrete event model integrating different transportation services is presented. Some disturbances are included to model the stochastic nature of the system. Based on such a model, a special-purpose urban traffic simulator has been designed, including two major modules. The first one, the Traffic Simulation Kernel, allows one to study the dynamic behaviour of the transportation system, analyzing its performance and applying control strategies for performance optimization. The second one, the Passenger Information Service, gives the system users, at any time, updated information about the different intermodal paths between any pair of origin/destination nodes. In this paper attention is focused on the functioning of the simulation module. Some experimental results relevant to a case study, showing the effectiveness of the proposed control strategies, are presented and discussed.

On adopting a Petri net-based switching modelling system to represent and control urban areas

Deterministic timed Petri nets are a suitable mod- elling formalism to represent signalized urban areas microscopi- cally, when pursuing traffic management and control purposes. In the proposed model, an urban transportation network is considered to be composed of signalized intersections and roads. So as to properly model any traffic condition, including congestion, any intersection equipped with a multi-phase traffic signal is divided into several crossing sections. The modular deterministic timed Petri net representing the overall traffic network allows to implement a switching modelling system. In fact, the representation of some parts of such a Petri net can become more or less detailed, whenever some event modifying the current traffic state occurs. I. INTRODUCTION Nowadays, it is widely recognised that the search for effec- tive solutions to pressing problems of traffic in metropolitan areas is a primary task to perform. In the past decade, urban traffic has known an enormous growth, making the need for new approaches to deal with it critical, in order to spare time, money, and lives. The application of information technologies to model, analyze, and control transportation systems, that is, Intelligent Transportation Systems (ITS), seems to offer promising solutions to the above mentioned problems. In the related framework, the focus is two-folded, on both intelligent infrastructures and intelligent vehicles. An intelligent infrastructure could fulfil transportation needs inside and around cities, whereas intelligent vehicles could complement it by making a real-time traffic information exchange feasible and improving safety. Since the sixties, the flows of vehicles in metropolitan areas have usually been regulated via the implementation of some kind of control on traffic lights. That usually consists in changing, according to the time-varying behaviour of the incoming traffic, the intersection stage specification, the relative green duration of each stage, the intersection

A model for performance evaluation and sensitivity analysis of seaport container terminals

Abstract In the last decades, the growth of the containerised freight demand has led to a fast development of seaport container terminals. As an evidence of the relevance and of the interest that such intermodal terminals have reached, consider the significant amount of existing scientific works facing the problem of optimising their performances at operative, tactic, and strategic levels. In this framework, the present paper provides a model to face the problem of finding the best typology, and number, of resources for a given seaport container terminal. Evidently, on one hand, such problem is subject to “static” constraints due, for instance, to budget, manpower and space limitations, and on the other hand, to “dynamic” constraints that rise when the sequence of operations in the terminal, and the relevant timing, are taken into account. The paper is organised as follows: after a brief bibliography review, the proposed PN and (max,+)-algebra models are introduced. Then, some simulation results and the relevant sensitivity analysis are discussed.

Urban traffic control in modular/switching deterministic-timed Petri nets

Abstract Deterministic-timed Petri nets (DTPNs) are adopted in this paper in order to microscopically model a signalized urban area. The proposed representation may be viewed, at a higher level, as composed of modules which interact by means of immediate (that is, non-timed) transitions. Such a representation allows two levels of traffic control: the high level switches between different internal structures of the modules, in order to modify the model of the traffic system according to the system state, whereas the low level optimizes the performances of the traffic system by solving a mathematical programming problem which minimizes the number of vehicles in the system.

Experimental Validation of a Hybrid Petri-Net Based Model of Urban Transportation Networks

Abstract In this paper, a class of Hybrid Petri Nets is proved to provide a valuable model of urban networks of signalized intersections. Such networks can be fruitfully considered to be hybrid systems, where the vehicle flow behaviour is represented by means of a time-driven model, and the traffic light dynamics is represented by an event-driven model. The proposed model has been validated using real traffic data about the Italian city of Turin. Some relevant experimental results are reported and discussed

TRAFFIC-RESPONSIVE SIGNALLING CONTROL THROUGH TIMED PETRI NETS

Abstract This paper addresses the optimization of the green light durations in a signalized urban area, by means of a traffic-responsive control strategy. The adopted model, mainly consisting of signalized intersections and roads, is microscopically represented by means of deterministic-timed Petri nets (DTPNs), which allow a compact representation of the traffic behaviour. The proposed optimization algorithm includes the solution to a mathematical programming problem constrained by the DTPN state equations.

Modelling and Performance Analysis of Urban Transportation Networks

A discrete event approach is proposed to evaluate the performance and develop control strategies for a urban intermodal transportation network. The discrete event model presented integrates public and private means of transport. Some disturbances are included to model the stochastic nature of the system. Based on such a model, a urban traffic simulator (INTRANET) has been designed on purpose. INTRANET includes two major modules. The first one, the Traffic Simulation Kernel, allows to study the dynamic behaviour of the transportation system, analyzing its performances and applying control strategies to optimize them. The second one, the Passenger Information Service, gives the system users at any time updated information about the different intermodal paths between any pair of origin/destination nodes. A case study relevant to the transportation network of an Italian city is dealt with. Related experimental results, showing the effectiveness of the proposed control strategy, are presented and discussed.