Ioannis Papamichail

Effects of Variable Speed Limits on Motorway Traffic Flow

Variable speed limits (VSLs) displayed on roadside variable message signs (VMSs) have emerged as a widespread traffic control measure on motorways in many countries, leading to substantial traffic safety benefits; however, there is no clear evidence of improved traffic flow efficiency in operational VSL systems. The available information on VSL impact on aggregate traffic flow behavior is summarized, and the issue is investigated in more detail with real traffic data from a European motorway. It is found that VSLs decrease the slope of the flow–occupancy diagram at undercritical conditions, shift the critical occupancy to higher values, and enable higher flows at the same occupancy values in over-critical conditions. Implications of the derived findings for more efficient VSL control strategies are discussed.

Optimal Motorway Traffic Flow Control Involving Variable Speed Limits and Ramp Metering

The impact of variable speed limits (VSL) on aggregate traffic flow behaviour on motorways is shown to bear similarities to the impact of ramp metering, in particular, when addressing potentially active bottlenecks. A quantitative model of the VSL impact is proposed that allows for VSL to be incorporated in a macroscopic second-order traffic flow model as an additional control component. The integrated motorway network traffic control problem involving ramp metering and VSL control measures is formulated as a constrained discrete-time optimal control problem and is solved efficiently even for large-scale networks by a suitable feasible direction algorithm. An illustrative example of a hypothetical motorway stretch is investigated under different control scenarios, and it is shown that traffic flow efficiency can be substantially improved when VSL control measures are used, particularly in integration with coordinated ramp metering.

Supercritical fluid extraction of celery seed oil

The supercritical fluid extraction of oil from milled celery seeds, using CO2 as a solvent, is presented in this study. The effect of the process parameters — pressure and temperature of extraction, particle size of celery seeds and flow rate of CO2 — on the extraction rate was examined in a series of experiments. The results indicated a significant increase in extraction rate with increase of pressure or decrease of the particle size of celery seed. A similar effect was observed with the increase of the solvent flow rate and decrease of temperature. The experimental data were described by an empirical model and two mass balance models: (1) a simplified model, which takes into account only the time dependence of the extract concentration in the two phases and is coupled with various equilibrium relationships, and (2) an extended Lacks model. The correlation results were satisfactory especially for the mass balance models, which account for the double regime — solubility and diffusion controlled regimes — of the extraction.

A Rigorous Global Optimization Algorithm for Problems with Ordinary Differential Equations

The optimization of systems which are described by ordinary differential equations (ODEs) is often complicated by the presence of nonconvexities. A deterministic spatial branch and bound global optimization algorithm is presented in this paper for systems with ODEs in the constraints. Upper bounds for the global optimum are produced using the sequential approach for the solution of the dynamic optimization problem. The required convex relaxation of the algebraic functions is carried out using well-known global optimization techniques. A convex relaxation of the time dependent information is obtained using the concept of differential inequalities in order to construct bounds on the space of solutions of parameter dependent ODEs as well as on their second-order sensitivities. This information is then incorporated in the convex lower bounding NLP problem. The global optimization algorithm is illustrated by applying it to four case studies. These include parameter estimation problems and simple optimal control problems. The application of different underestimation schemes and branching strategies is discussed.

Local Feedback-Based Mainstream Traffic Flow Control on Motorways Using Variable Speed Limits

Recent research has proposed mainstream traffic flow control (MTFC), enabled via variable speed limits (VSLs), as a novel motorway traffic management tool and has demonstrated its efficiency based on sophisticated optimal control methods that may face difficulties in practical field implementations. A simple local MTFC feedback controller is designed in this paper, taking into account a number of practical requirements and restrictions. The MTFC controller relies only on readily available real-time measurements (no online model usage and no demand predictions are needed) and is therefore robust and suitable for field implementations. The controller is evaluated in simulation and compared with optimal control results. Despite its simplicity, the new controllers performance is shown to approach the optimal control results while considering several practical and safety restrictions for a number of investigated scenarios.

Local feedback-based mainstream traffic flow control on freeways using variable speed limits

Recent research has proposed Mainstream Traffic Flow Control (MTFC), enabled via variable speed limits (VSL), as a novel freeway traffic management tool and has demonstrated its efficiency based on sophisticated optimal control methods that may face difficulties in practical field implementations. A simple local MTFC feedback controller is designed in this paper, taking into account a number of practical requirements and restrictions. The MTFC controller relies only on readily available real-time measurements and is robust and suitable for field implementations. The controller is evaluated in simulation and compared to the optimal control results. The controllers performance is shown to approach the optimal control results, whilst considering several practical and safety restrictions, for a number of investigated scenarios.

Global optimization of dynamic systems

Many chemical engineering systems are described by differential equations. Their optimization is often complicated by the presence of nonconvexities. A deterministic spatial branch and bound global optimization algorithm is presented for problems with a set of first-order differential equations in the constraints. The global minimum is approached from above and below by generating converging sequences of upper and lower bounds. Local solutions, obtained using the sequential approach for the solution of the dynamic optimization problem, provide upper bounds. Lower bounds are produced from the solution of a convex relaxation of the original problem. Algebraic functions are relaxed using well-known convex underestimation techniques. The convex relaxation of the dynamic information is achieved using a new convex relaxation procedure. Parameter independent as well as parameter dependent bounds on the dynamic system are utilized. The global optimization algorithm is illustrated by applying it to case studies relevant to chemical engineering, where affine functions of the optimization variables are used as a relaxation of the dynamic system.

Heuristic Ramp-Metering Coordination Strategy Implemented at Monash Freeway, Australia

A new traffic-responsive feedback control strategy, heuristic ramp-metering coordination (HERO) that coordinates local ramp-metering actions in freeway networks is presented. The proposed coordination scheme is simple and reactive [i.e., based on readily available real-time measurements without the need for real-time model calculations or external disturbance (e.g., demand) prediction]. HERO employs an extended version of the feedback regulator ALINEA at the local level. The new strategy overcomes the problem of uncertain freeway capacity by targeting the critical occupancy for maximum throughput. HERO outperforms uncoordinated local ramp metering and approaches the efficiency of sophisticated optimal control schemes. HERO has been implemented by VicRoads, the road authority for the state of Victoria, Australia, at six consecutive inbound on-ramps on the Monash Freeway in Melbourne, Australia. This pilot project is part of the Monash–CityLink–West Gate (MCW) upgrade. The obtained results show a significant increase of traffic throughput and a reduction of travel times compared with the previously used ramp-metering system. To maximize performance across the entire 75-km route of the MCW upgrade, HERO is currently under implementation at 63 on-ramps.

Overview of Traffic Signal Operation Policies for Ramp Metering

A ramp metering control algorithm includes typically two distinct components: the (most important) control strategy, which makes real-time decisions on the ramp exit flow to be implemented, and the translation of this decision into specific traffic light settings according to the applied metering policy. This study focuses on the second component, providing an overview of practiced metering policies (one car per green, n cars per green, full cycle, and discrete release rates) along with a discussion of their advantages and shortcomings. Further related issues, such as traffic signals, multiple metering lanes, bypass lanes, check-in and check-out detectors, integration of both control components, and on–off switching logic, are also addressed.

Integrated Ramp Metering and Variable Speed Limit Control of Motorway Traffic Flow

Abstract The impact of variable speed limits (VSL) on the aggregate traffic flow behaviour is reflected in the quantitative model proposed in this paper. VSL are incorporated in a general second-order traffic flow model as an additional control component. The integrated motorway network traffic control problem is formulated as a constrained discrete-time optimal control problem which is solved very efficiently even for large-scale networks by a suitable feasible-direction algorithm. An illustrative example is presented under different control scenarios and it is shown that traffic flow efficiency can be substantially improved when VSL control measures are used, particularly in integration with coordinated ramp metering.