Highway Capacity Manual改訂の動向--テイラ-教授の講演より

This paper shows that a macroscopic fundamental diagram (MFD) relating average flow and average density must exist on any street with blocks of diverse widths and lengths, but no turns, even if all or some of the intersections are controlled by arbitrarily timed traffic signals. The timing patterns are assumed to be fixed in time. Exact analytical expressions in terms of a shortest path recipe are given, both, for the street’s capacity and its MFD. Approximate formulas that require little data are also given. For networks, the paper derives an upper bound for average flow conditional on average density, and then suggests conditions under which the bound should be tight; i.e., under which the bound is an approximate MFD. The MFD’s produced with this method for the central business districts of San Francisco (California) and Yokohama (Japan) are compared with those obtained experimentally in earlier publications.

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An analytical approximation for the macroscopic fundamental diagram of urban traffic

A field experiment in Yokohama (japan) revealed that a macroscopic fundamental diagram (MFD) linking space-mean flow, density and speed exists on a large urban area. It was observed that when the highly scattered plots of flow vs. density from individual fixed detectors were aggregated the scatter nearly disappeared and points grouped along a well defined curve. Despite these and other recent findings for the existence of well-defined MFDs for urban areas, these MFDs should not be universally expected. In this paper we investigate what are the properties that a network should satisfy, so that an MFD with low scatter exists. We show that the spatial distribution of vehicle density in the network is one of the key components that affect the scatter of an MFD and its shape. We also propose an analytical derivation of the spatial distribution of congestion that considers correlation between adjacent links. We investigate the scatter of an MFD in terms of errors in the probability density function of spatial link occupancy and errors of individual links' fundamental diagram (FD). Later, using real data from detectors for an urban arterial and a freeway network we validate the proposed derivations and we show that an MFD is not well defined in freeway networks as hysteresis effects are present. The datasets in this paper consist of flow and occupancy measures from 500 fixed sensors in the Yokohama downtown area in Japan and 600 loop detectors in the Twin Cities Metropolitan Area Freeway network in Minnesota, USA. (C) 2010 Elsevier Ltd. All rights reserved.

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Properties of a well-defined Macroscopic Fundamental Diagram for urban traffic

Recently, some authors have provided experimental evidence of the existence of an urban-scale macroscopic fundamental diagram (MFD). Their convincing results were obtained on the basis of 500 urban fixed detectors placed 100 m upstream of most major intersections in the city of Yokohama, Japan. Those authors assume that the network in which data are collected is homogeneous in regard to congestion occurrence. This paper is devoted to exploring the impact of heterogeneity on the existence of an MFD. All data available for a medium-size French city are used. The data set encompasses measurements on highways, urban center streets (congested during business hours), and residential area streets. Data were collected by loop detectors with a distance from a downstream signal that can vary from 1,000 to 10 m. Heterogeneity is examined here in various aspects: differences between the surface and highway network, impact of the distance between the loop detector and the traffic signal in the surface network, and dif...

Exploring the Impact of Homogeneity of Traffic Measurements on the Existence of Macroscopic Fundamental Diagrams

This report serves as a comprehensive guide to traffic signal timing and documents the tasks completed in association with its development. The focus of this document is on traffic signal control principles, practices, and procedures. It describes the relationship between traffic signal timing and transportation policy and addresses maintenance and operations of traffic signals. It represents a synthesis of traffic signal timing concepts and their application and focuses on the use of detection, related timing parameters, and resulting effects to users at the intersection. It discusses advanced topics briefly to raise awareness related to their use and application.

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Traffic signal timing manual.

Results of an exploratory study of networklevel relationships in an isolated network with a fixed number of vehicles circulating according to the microscopic rules embedded in the NETSIM traffic simulation model are presented. The primary concern was to assess the usefulness of such simulation-based approaches in the investigation of macroscopic network-level traffic relationships. Three specific objectives were addressed: (a) identification of network-level descriptors that are related in operationally useful and simple ways, (b) exploration of some aspects of the two-fluid theory of town traffic, and (c) examination of the traffic flow distribution over the network components. A number of simulations were conducted on the same network under two different sets of control schemes and traffic characteristics at widely varying vehicle concentrations. The results were analyzed with respect to the three objectives, yielding useful insights into network-level traffic phenomena and suggesting promising avenues for further research.

Investigation of network-level traffic flow relationships: some simulation results

Addressed in this paper are the development and comparative assessment of macroscopic network-level traffic flow models, which describe the behavior and interrelation between traffic variables defined at the network level. These variables include average speed, concentration, flow, the fraction of vehicles stopped in the network, and the two-fluid running time variables. Three alternative sets of interrelated models, each with a different starting postulate, are presented and tested in terms of their performance against a series of microscopic simulation runs corresponding to different concentration levels. In each model system, a different functional form is postulated for either the speed-concentration relation or the fraction of vehicles stopped versus concentration relation. The functional form for the other relation is then derived from the postulated model by invoking the two-fluid theory of town traffic. The models are calibrated and tested using the simulation results. The analysis indicates that the network-level traffic variables are interrelated in a manner similar to that captured by the traffic models established for individual road sections. In particular, a well-known linear speed-concentration model as well as a nonlinear alternative are found to be generally applicable at the network level.

Urban traffic network flow models

Presented in this paper is a systematic exploration, using microscopic simulation, of the sensitivity of network-level traffic flow descriptors and relationships, particularly those of the two-fluid theory of town traffic, to network features, traffic control, and traffic-interfering urban activity levels. Moving traffic interference, which is represented by stochastic short-term lane blockages of varying duration and frequency, is shown to be a key determinant of the traffic character of an urban street network and of the behavior described by the two-fluid theory and verified operationally. In addition, the sensitivity of the two-fluid model parameters to a change in traffic control strategy, in this case the coordination of signals to achieve progression, is demonstrated. Furthermore, keeping the same network configuration, the effect of network topology on traffic flow is examined by changing the identical length of the links.

Analysis of traffic network flow relations and two-fluid model parameter sensitivity.

The basic postulate of the two-fluid theory of town traffic relates the average speed of moving vehicles to the fraction of moving vehicles in a street network; both representing averages over all vehicles in the network. Data collection to provide estimates of the model parameters to-date has consisted of sampling the network with a test car, replicating (as closely as possible) the trip histories of randomly selected vehicles in the network. Because the two-fluid model is nonlinear, it cannot be simultaneously applied at the individual vehicle level and the network level, as is shown in this article. However, due to the practical difficulties in collecting data for all the vehicles in the network, several potential sampling strategies are identified and are evaluated with computer simulation. The simulation experiments suggest that aggregating the trip histories of 10 to 20 test vehicles over 10 to 15 minutes yields parameter estimates very close to the true value.

Sampling strategies for two-fluid model parameter estimation in urban networks

The relation between street network geometric and control features and the network quality of traffic service is investigated. The goal is to quantify macroscopically the degree of improvements made when an urban street network undergoes modifications in its control or geometric features. The quality of traffic service in several city networks is assessed through field calibration of the two-fluid model. The model parameters are then correlated to 10 geometric and control features for each city network. It is found that under low traffic concentrations in the network, the average speed limit and the degree of signal progression are the most influential features. On the other hand, for high-concentration conditions, the fraction of one-way streets, the average number of lanes, and the fraction of signals actuated most affect the service quality. The density of signalized intersections affects peak and off-peak traffic. The signal density is beneficial during peak periods but detrimental during off-peak periods.

James C. Williams

Papers

Investigation of network-level traffic flow relationships: some simulation results

Urban traffic network flow models

Analysis of traffic network flow relations and two-fluid model parameter sensitivity.

Sampling strategies for two-fluid model parameter estimation in urban networks

Influence of urban network features on quality of traffic service