Nathan H. Gartner

A multi-band approach to arterial traffic signal optimization

Progression schemes are widely used for traffic signal control in arterial streets. Under such a scheme a continuous green band of uniform width is provided in each direction along the artery at the desired speed of travel. A basic limitation of existing bandwidth-based programs is that they do not consider the actual traffic volumes and flow capacities on each link in their optimization criterion. Consequently they cannot guarantee the most suitable progression scheme for different traffic flow patterns. In this paper we present a new optimization approach for arterial progression that incorporates a systematic traffic-dependent criterion. The method generates a variable bandwidth progression in which each directional road section can obtain an individually weighted bandwidth (hence, the term multi-band). Mixed-integer linear programming is used for the optimization. Simulation results indicate that this method can produce considerable gains in performance when compared with traditional progression methods. It also lends itself to a natural extension for the optimization of grid networks.

Implementation of the OPAC adaptive control strategy in a traffic signal network

The Real-time Traffic Adaptive Control System (RT-TRACS) represents a new, state-of-the-art system in advanced traffic signal control. It has been developed cooperatively by a team of U.S. academic, private and public researchers under the guidance of the Federal Highway Administration (FHWA). The system provides a framework to run multiple traffic control algorithms, existing ones as well as new adaptive algorithms. The OPAC (Optimized Policies for Adaptive Control) control strategy, which provides a dual capability of distributed individual intersection control as well as coordinated control of intersections in a network, is the first adaptive algorithm implemented within the RT-TRACS framework. OPAC was the first comprehensive strategy to be developed in the U.S. for real-time traffic-adaptive control of signal systems. This paper presents the operational features of the OPAC algorithm and describes the implementation and-field testing of OPAC within the RT-TRACS system.

Area Traffic Control and Network Equilibrium

Area traffic control systems play an important role in determining the equilibrium between demand and supply in an urban highway network. The paper describes some of the methods that have been developed for the operation of these systems and derives the level-of-service that would result from a given set of flows. Currently used techniques attempt to optimize network performance assuming a fixed pattern of demands. It is shown, via an example, that control measures can be used to affect the demand pattern in such a way that total network performance is improved. A model for achieving this objective is discussed.

Arterial-based control of traffic flow in urban grid networks

Urban networks are typically composed of a grid of arterial streets. Optimal control of the traffic signals in the grid system is essential for the effective operation of the network. In this paper, we present mathematical programming models for the development of optimal arterial-based progression schemes. Such schemes are widely used for traffic signal control in arterial streets. Under such a scheme, a continuous green band is provided in each direction along the artery at the desired speed of travel to facilitate the movement of through traffic along the arterial. Traditional schemes consist of uniform-width progressions. New approaches generate variable bandwidth progressions in which each directional road section is allocated an individually weighted weighted that can be adapted to the prevailing traffic flows on that link. Mixed-integer linear programming is used for the optimization. Simulation results indicate that this method can produce considerable gains in performance when compared with traditional progression methods. By introducing efficient computational techniques, this method also lends itself to a natural extension for incorporation in a dynamic traffic management system.

MULTIBAND-96: A PROGRAM FOR VARIABLE-BANDWIDTH PROGRESSION OPTIMIZATION OF MULTIARTERIAL TRAFFIC NETWORKS

Progression schemes are widely used for traffic signal control in urban arterial streets. Commonly available programs such as the MAXBAND or PASSER programs use the traditional approach, which consists of a uniform bandwidth design for each arterial. The multiband criterion, on the other hand, has the ability to adapt the progressions to the specific characteristics of each link in the network and thus obtain improved performance. The development and application of the multiband signal optimization scheme in multiarterial grid networks are described. The MULTIBAND-96 model optimizes all the signal control variables, including phase lengths, offsets, cycle time, and phase sequences, and generates variable bandwidth progressions on each arterial in the network. It uses the MINOS mathematical programming package for the optimization and offers considerable advantages compared with existing models. Simulation results using TRAF-NETSIM are given.

Pilot study of computer-based urban traffic management

The objectives of urban traffic management are to make more efficient use of existing transportation resources and provide for the movement of people in an efficient manner through the development of low-cost short-range management strategies. Implementation of such strategies can provide for reduced travel costs, savings of energy, reduced air pollution, and improved safety and convenience for the users of the facilities. It can also help to reduce necessary capital expenditure for new facilities to accommodate urban travel demands in the longer range. This paper presents a modelling framework for multimodal urban traffic management, explicitly modelling the divergent objectives of traffic managers and trip-makers. Facilities analyzed are arterial streets and freeways with their associated access and egress links. Modes considered are private automobiles, carpools, trucks, and buses. Decision parameters may include signal controls, ramp metering rates, priority lane assignments, variable-message signs, etc. Effects of modal performance on mode choice are also included in the analysis. Computational results are reported of a pilot application of the framework using heuristic solution techniques.

Integration of Dynamic Traffic Assignment with Real-Time Traffic Adaptive Control System

Intelligent transportation systems (ITS) are being designed to provide real-time control and route guidance to motorists to optimize traffic network performance. Current research and development efforts consist of a dynamic traffic assignment capability that can predict future traffic conditions and a real-time traffic adaptive control system (RT-TRACS) for generation of signal control strategies. Although these models are intimately connected, so far they have developed independently of one another. A framework is presented here for integrating the two models into a combined system with a practical approach for realizing it. First the static case involving the interaction between travelers (demand) and transportation facilities (supply) under recurrent conditions is discussed. This model is applicable in the design and planning of transportation systems management actions. The framework is then extended to the quasi-dynamic and the dynamic cases, which involve incorporation of advanced ITS technologies in the form of advanced traffic management systems and advanced traveler information systems. An innovative application of this framework to advanced traffic-adaptive signal control is presented using the hierarchic structure of RT-TRACS.

Combined model for signal control and route choice in urban traffic networks

Traffic signals have a significant effect on the choice of routes by motorists in urban areas. They are of primary importance in the development of advanced traffic management strategies that involve dynamic rerouting of traffic flows through signal-controlled street networks. A combined network model that simultaneously accounts for both the route choices made by motorists and the desired signal controls to match these choices is presented. Given origin-destination travel demand information, the model generates signal controls to optimize network performance and calculates the resulting traffic volumes in the network. This optimization model inherently reflects the mutual consistency between traffic flows and signal controls. The model is applicable to both fixed-time and demand-responsive signals. Computational procedures and sample network solutions are presented.

DYNAMIC CONTROL AND TRAFFIC PERFORMANCE IN A FREEWAY CORRIDOR: A SIMULATION STUDY

This paper describes simulation studies that were conducted to assess the performance of a freeway corridor control system. The system combines an advanced traffic management system with a motorist information system that provides route guidance to individual drivers. It has a hierarchical structure: The corridor level control acts in a supervisory capacity dynamically allocating traffic among alternative corridor facilities, including freeways, frontage roads, and signalized arterials. The local level control then selects control parameters for the individual facilities based on the predicted usage at the corridor level. A user specified performance function is optimized in the process. Both recurrent and nonrecurrent congestion scenarios were simulated using the SCOT model as a test bed. It is shown that, in most cases, significant benefits in performance can be obtained when the system operates as designed.

Analysis of Traffic Flow Characteristics on Signalized Arterials

The characteristics of traffic flows on signalized arterials are examined within a cellular automaton microsimulation model. The model is used to analyze arterial throughput and travel times for given densities, coordination schemes, and signal spacings. A fundamental three-dimensional relationship is established between flow, density, and offsets for signalized arterials. In particular, it is shown that arterial throughput is dependent on offsets and that the constituent single-intersection limiting capacity, as determined by the saturation flow and the green splits, can be realized only under optimal coordination conditions for a limited range of densities on the arterial. This finding is a manifestation of the important role that signal coordination and, in fact, intelligent transportation systems in general play in the operation of urban street networks.