Study of macroscopic fundamental diagram on Shanghai urban expressway network in China

The macroscopic fundamental diagram（ MFD） is studied to obtain the aggregate behavior of traffic in cities. This paper investigates the existence and the characteristics of different types of daily MFD for the Shanghai urban expressway network. The existence of MFD in the Shanghai urban expressway network is proved based on two weeks＇ data.Moreover, the hysteresis phenomena is present in most days and the network exhibits different hysteresis loops under different traffic situations. The relationship between the hysteresis phenomena and the inhomogeneity of traffic distribution is verified. The MFDs in the years of 2009 and 2012 are compared. The hysteresis loop still exists in 2012, which further verifies the existence of the hysteresis phenomenon. The direct relationship between the length of the hysteresis loop（ ΔO） and the congestion is proved based on sufficient data. The width of the hysteresis loop, i. e., the drop in network flow（ ΔQ） has no relationship with the congestion, and it varies from day to day under different traffic situations.

Perimeter traffic control strategy based on macroscopic fundamental diagrams

A perimeter traffic signal control strategy is proposed based on the macroscopic fundamental diagram theory(MFD)to solve the signal control problem in oversaturated states.First,the MFD of a specific regional network can be derived using VSS IM simulation software.Secondly,the maximum number of cumulative vehicles that the network can accommodate is determined based on the MFD.Then,through monitoring the influx flow,the number of vehicles existing in and exiting from the network,a perimeter traffic control model is proposed to optimize the signal timing of the boundary intersections.Finally,a virtual network simulation model is established and three different kinds o f traffic demand are loaded into the network.Simulation results show that a fer the strategy implementation,the number o f vehicles accumulating in the network can be kept near the optimal value,while the number o f both entering and exiting vehicles increases significantly and the road network can be maintained at a large capacity.Simultaneously,the queue length at the approach of the border intersections is reasonably controlled and vehicles entering and exiting the network can maintain a more efficient and stable speed.The network performance indices such as the average traffic delay and average number of stops can be improved to a certain degree,thus verifying the effectiveness and feasibility of the perimeter control strategy.

Vehicle emissions calculation for urban roads based on the Macroscopic Fundamental Diagram method and real-time traffic information

Vehicle emissions calculation methods mostly use ownership information or annual road monitoring data as the activity level to calculate air pollutant emissions,but it is hard to reflect either the emissions intensity under different conditions or the spatiotemporal characteristics in various sections based on such approaches.This paper presents a method based on the Macroscopic Fundamental Diagram and real-time traffic data to calculate vehicle emissions,which could reflect the operation conditions and emission characteristics of vehicles.Following the‘Technical Guide for the Compiling of Road Vehicle Air Pollutant Emissions Inventories’,the emissions of three roads with different lane numbers and road grades in Beijing were estimated and verified using this method.Compared with monitoring data,the average deviations of the traffic flow on the Fifth Expressway,Jingfu National Highway,and Jingzhou Provincial Highway were?25.5%,?26.5%,and?13.4%,respectively,and the average deviations of nitrogen oxides emissions were?27.7%,?12.9%,and?12%,respectively.This method showed good application potentials to construct the emissions inventory applied to the block-scale model and analyze the spatiotemporal distribution characteristics of motor vehicle emissions in urban areas.

Quantification of the Hysteresis of Macroscopic Fundamental Diagrams and Its Relationship with the Congestion Heterogeneity and Performance of a Multimodal Network

Recent studies have observed hysteresis loops in the macroscopic fundamental diagram (MFD). In particular, for the same network density, higher network flows occur during congestion onset than during congestion offset. To evaluate management strategies using the MFD, investigating the relationship between the size of these loops and network performance is needed. The existing literature has mainly discussed correlating loop width (difference in density) and height (capacity drop) with congestion heterogeneity, but has failed to prove a relationship between the capacity drop and traffic conditions. Moreover, quantification of the MFD loop in complex multimodal networks has not been investigated. The objective of this paper covers these aspects. We simulated the Sioux Falls network with different mode-share ratios (car and bus users) based on a multi-agent simulation, MATSim. We investigated the relationships between MFD loop size and congestion heterogeneity (standard deviation of density) and network performance (average passenger travel time), and found that both were directly correlated with loop width, while weakly correlated with loop height. Moreover, we divided the MFD loop into two parts according to congestion onset and offset periods and found that the heights of the two parts had opposite effects. Accordingly, we show why the relationship between capacity drop and congestion heterogeneity is not found in the literature. We also found that network performance inversely affected the height of part of the loop while the height of its other part increased with an increase in congestion heterogeneity. These results help to evaluate network performance in the presence of MFD hysteresis, leading to elaborated management decisions.

线上线下双模式教育的应用——以机械电子工程专业必修课《热力学与传热学基础》为例

后疫情时代,为适应教学要求,逐渐衍生出线上线下双模式的教学方式。本文以机械电子工程专业的必修课《热力学与传热学基础》为例介绍了线上线下双模式教育在教学中的应用。首先介绍了《热力学与传热学基础》这门专业必修课的主要授课内容、学习目标和考核标准,其次以理想气体基本热力学过程的p-v图和T-s图为例,说明了双模式教育的应用为教学带来的好处。

不同降雨量下基于宏观基本图的边界控制策略

在雨雪等不利天气条件下,城市交通拥堵加剧,北京、天津等大城市在降雨条件下经常发生多路段区域性交通拥堵。因此,根据天津市中心城区和市郊区域路网实际交通数据,以路网交通流宏观基本图模型为研究基础,对比不同降雨量以及不同路网的路网交通流时序和宏观基本图变化规律,分析不同降雨量对天津市中心城区和市郊区域路网交通状态的影响。基于不同降雨量下中心城区和市郊区域路网宏观交通流的变化规律,分别构建路网动态演化模型,并对模型的参数进行标定和有效性验证。针对降雨条件下路网发生的区域性拥堵问题,基于宏观基本图的边界控制分别设计了不同降雨量下中心城区和市郊区域路网控制策略,通过仿真实验分析验证了不同控制策略的效果,并给出了能够缓解中心城区和市郊区域路网拥堵的可行策略。结果表明:在小雨天气条件下,将从市郊区域向中心城区的交通流量的转移比例减小量控制在9%~50%范围之内时,中心城区与市郊区域交通状态更加均衡,路网调控效果更好;在大雨天气条件下,将从市郊区域向中心城区的交通流量的转移比例减小量控制在23%~50%范围之内时,中心城区与市郊区域交通状态更加均衡,路网调控效果更好。这表明该控制策略能够缓解中心城区和市郊区域路网的交通拥堵,保障路网交通系统的稳定运行。

面向行驶周期测算的细分速度区间的车辆行驶里程和时间算法

行驶周期是车辆尾气排放测算的核心参数之一,但传统建立行驶周期所采用的数据收集方法成本高,耗时长,样本代表性不足。为克服传统数据采集方法的缺陷,获取构建准确行驶周期所需的覆盖全路网的交通流参数,构建了面向行驶周期测算的细分速度区间的路网动态车辆行驶里程(V_(KT))和车辆行驶时间(V_(HT))算法。首先,基于多城市不同等级道路的多源交通流数据,构建宏观交通流基本图模型,利用路段速度数据推算全路网流量数据;其次,基于C++算法开发细分速度区间的路网动态V_(KT)和V_(HT)算法,研究道路等级和城市规模对V_(KT)和V_(HT)分布特征的影响;最后,构建动态V_(KT)和V_(HT)数据分析系统,对计算结果进行直观展示。结果表明,不同等级道路宏观交通流基本图模型能较好地模拟城市道路交通流,流量测算误差低于10%;细分速度区间V_(KT),V_(HT)分布会受道路等级和城市规模影响,其中道路等级影响更大;有必要针对不同城市规模和道路等级分别建立相应行驶周期;面向行驶周期开发的V_(KT),V_(HT)算法及数据分析系统,可对细分速度区间的动态V_(KT),V_(HT)进行快速计算,并对不同城市计算结果进行直观对比。研究成果提高了细分速度区间的动态车辆行驶里程测算精度,为构建准确刻画车辆行驶状况的行驶周期提供数据支撑。

基于宏观基本图的交通控制有效性与路网稳定性研究

区域城市路网在不同控制方法下呈现的网络交通流输出水平与维持能力不尽相同,要评价控制效果的优劣,需要分析区域控制方式下路网交通流与路网耦合时所体现出来的特征值。作为一种路网的基本属性,选择宏观基本图作为研究区域交通控制有效性与路网稳定性的评价载体,解析了相对于路段、交叉口等单点评价具有的优势。基于MFD的区域交通控制评价不受路网交通需求变化的影响。研究建立了路网最大加权流量、最佳累计车辆数、路网负载能力、交通流分布均衡度以及时序均衡度5评价指标,以对区域交通信号控制方法的有效性与路网运行的稳定性进行评价,解读路网高峰时段的交通流维持能力。以成都青羊区太升南路附近37个信号交叉口为路网,利用VISSIM二次开发技术,获得了该路网在定时控制、感应控制、自适应控制以及自适应协调控制下的路网宏观基本图形态。通过对比各控制方法的宏观基本图形态特征与评价指标,从多个维度比较了各控制方法的特征与适用性,分析了相应的控制有效性,解读了其从平峰至过饱和各阶段的交通流维持能力,进而可以了解特定控制方法下路网的运行效率。同时,根据相应控制方法的稳定性表现,指出了各自的局限性,有助于在实际应用中根据区域路网面临的环境,分析应采取的交通控制策略。

An identification model of urban critical links with macroscopic fundamental diagram theory

How to identify the critical links of the urban road network for actual traffic management and intelligent trans- portation control is an urgent problem, especially in the con- gestion environment. Most previous methods focus on traf- fic static characteristics for traffic planning and design. How- ever, actual traffic management and intelligent control need to identify relevant sections by dynamic traffic information for solving the problems of variable transportation system. Therefore, a city-wide traffic model that consists of three re- lational algorithms, is proposed to identify significant links of the road network by using macroscopic fundamental diagram （MFD） as traffic dynamic characteristics. Firstly, weighted- traffic flow and density extraction algorithm is provided with simulation modeling and regression analysis methods, based on MFD theory. Secondly, critical links identification algo- rithm is designed on the first algorithm, under specified prin- ciples. Finally, threshold algorithm is developed by cluster analysis. In addition, the algorithms are analyzed and applied in the simulation experiment of the road network of the cen- tral district in Hefei city, China. The results show that the model has good maneuverability and improves the shortcom- ings of the threshold judged by human. It provides an ap- proach to identify critical links for actual traffic management and intelligent control, and also gives a new method for eval- uating the planning and design effect of the urban road net- work.

Fundamental diagrams for pedestrian flows in a channel via an extended social force model

In this paper, an extended social force model was applied to investigate fundamental diagrams of pedestrian flows. In the presented model, both the static floor field and the view field were taken into account. Then each pedestrian can determine his/her desired walking directions according to both global and local information. The fundamental diagrams were obtained numerically under periodic boundary condition. It was found that the fundamental diagrams show good agreement with the measured data in the case of unidirectional flow, especially in the medium density range. However, the fundamental diagram for the case of bidirectional flow gave larger values than the measured data. Furthermore, the bidirectional flux is larger than the tmidirectional flux in a certain density range. It is indicated that the bidirectional flow may be more efficient than the unidirectional flow in some cases. The process of lane formation is quite quick in the model. Typical flow patterns in three scenarios were given to show some realistic applications.

Model on empirically calibrating stochastic traffic flow fundamental diagram

This paper addresses two shortcomings of the data-driven stochastic fundamental diagram for freeway traffic.The first shortcoming is related to the least-squares methods which have been widely used in establishing traffic flow fundamental diagrams.We argue that these methods are not suitable to generate the percentile-based stochastic fundamental diagrams,because the results generated by least-squares methods represent weighted sample mean,rather than percentile.The second shortcoming is widespread use of independent modeling methodology for a family of percentile-based fundamental diagrams.Existing methods are inadequate to coordinate the fundamental diagrams in the same family,and consequently,are not in alignment with the basic rules in probability theory and statistics.To address these issues,this paper proposes a holistic modeling framework based on the concept of mean absolute error minimization.The established model is convex,but non-differentiable.To efficiently implement the proposed methodology,we further reformulate this model as a linear programming problem which could be solved by the state-of-the-art solvers.Experimental results using real-world traffic flow data validate the proposed method.

船舶交通流速度离散特性研究

为更深刻地理解船舶交通流运行特性,以速度离散度研究为切入点,提出描述船舶交通流速度离散特性的两种指标,根据交通流基本图理论,对速度离散度在船舶交通流基本图上的分布及其与船舶交通流三参数之间的关系进行定量分析。结果表明:随着船舶流量、密度和速度的增加,速度离散度的分布趋于稳定,在低流量、高速度离散度区间内,速度离散度随着船舶流量的增加逐渐降低,在低速度离散度区间内,速度离散度随着船舶流量增加逐渐升高;基于改进的K-means-FCM算法,使用船舶交通流流量、密度、速度和速度离散度作为分类指标,对船舶交通流进行聚类分析,得到4个聚类中心与指标值的分布范围。基于试验结果,抽象总结船舶交通流运行状态图,将船舶交通流划分自由态、同质态、稳定态和异质态等4种状态。该研究对更好地认识船舶交通流特性起到一定的作用。

网联进口道考虑信号影响混合基本图研究

为研究含网联自动驾驶、自动驾驶和人工驾驶车辆的混合交通流在交叉口进口道受信号影响的混合基本图变化情况,构筑信号影响下的混合交通流跟驰模型,并进行混合基本图数值仿真。结果表明,同质交通流下CAV和AV能增加进口道最大通行能力,为12.47%和4.99%;信号信息能一定程度影响交叉口进口道通行能力;增加渗透率P能够提高混合交通流于交叉口进口道通行能力,当混合交通流为最佳密度时,渗透率为0.9、0.5,相较于0.1,进口道通行能力分别提高10.71%和6.52%。

基于改进密度峰值聚类的路网子区动态划分方法

在大型异构路网中,不同区域的交通运行特征存在显著差异,因此需要针对各个区域的具体特征制定相应的交通管理和控制策略。合理划分路网以获得交通特征均质的子区,对于有效的交通管控和分析至关重要。首先提出了一种改进的密度峰值聚类方法(Enhanced Density Peak Clustering,En-DPC),用于路网子区的初始划分。该方法基于质量概率相似性并考虑路网连接性约束,提升了算法对异常数据的鲁棒性,避免子区内路段不连续的问题。接着,利用En-DPC方法对初始划分的子区进一步合并,形成大小适中的新子区。最后,通过边界调整提高子区边界的平滑度,获得最终的划分结果。该方法能够根据路网交通状态自动确定子区数量,确保划分的合理性。此外,考虑到路网拥堵状态的时空演变,在静态划分基础上设计了一种动态划分方法,根据车辆密度的变化动态调整边界,以提升其在实时交通管控中的适用性,并利用瑞士苏黎世的线圈检测器数据验证了所提出方法的有效性。结果表明,本文提出的方法能够有效地将大型异构路网划分成均质子区,且每个子区都可获得一个清晰的宏观基本图。与现有文献中的路网划分方法如归一分割和“蛇”方法相比,本文方法不仅在归一化总方差、平均NcutSilhouette和模块度等性能评价指标上表现更优,而且子区划分时间明显低于其他两种方法。

考虑干扰的交通子区模糊迭代学习边界控制

针对固定增益迭代学习控制器跟踪交通子区内期望累计车辆数n^(*)的速度较慢,迭代次数过多的问题,并考虑到实际交通子区内存在的干扰对其运行状态的影响,建立了含有干扰项的交通流模型并对其进行离散化。采用开闭环比例微分(proportion differentiation,PD)型模糊迭代学习控制器,根据研究子区实际路网内的累计车辆数与期望累计车辆数n^(*)的误差及其变化率对开闭环PD型迭代学习控制器的迭代学习增益和反馈增益进行调整,使交通子区内的实际累计车辆数可以较快速地跟踪期望累计车辆数n^(*),并对开闭环PD型模糊迭代学习控制器的收敛性进行严格证明。通过对轻度和中度干扰影响下的交通子区进行仿真,结果显示,对比固定增益迭代学习控制器,开闭环PD型模糊迭代学习控制器的误差收敛速度分别提高24%和31%,平均排队长度分别减少24.52%和16.55%,平均延误分别减少21.63%和19.23%,平均停车次数分别减少23.68%和10.77%。

基于三维宏观基本图的城市机非混行交通研究

为解决城市交通中机动车与非机动车混合行驶造成的交通拥堵,提出一种三维宏观基本图的理论分析方法,探究机动车与非机动车混行交通流的微观运行特征和宏观演化规律,揭示在非机动车干扰下交通拥堵的形成、传播和消散机理。先分别建立描述非机动车流的元胞自动机模型和机动车流的元胞传输模型,并借助非机动车越线次数函数将两个模型耦合;然后,在Simulink仿真平台上,运行该耦合模型以求得路网的三维宏观基本图,并以此分析非机动车驶入流量与路网平均流量、冲突次数三者间的相互关系;最后,提出提高路网硬隔离占比和加大非机动车道宽度的两种方法,借助三维宏观基本图来验证其缓解交通拥堵的效果。研究结果表明:三维宏观基本图的分析方法可量化机动车和非机动车冲突的影响,指导混行交通拥堵的治理,该结论为改善中国城市交通出行秩序和出行环境提供了参考。

基于交通基本图的城市道路积水识别研究——以深圳为例

城市道路积水会造成道路交通系统运转失灵,增加交通事故的风险,是城市发展亟需解决的问题。虽可以通过设计道路积水识别监测系统有效减少城市内涝带来的损失,但现有的积水识别方式都存在较大缺陷。因此,本文提出一种基于交通流基本图(FD)的积水识别方法,该方法通过识别FD上的数据点位置并计算积水概率,实现积水点的实时智能识别。与现有方法相比,它适用于整个城市路网,且简单易实现,数据易获取。相关研究案例表明,该方法可以有效对暴雨期间路段积水进行实时识别,识别结果具有较高的可靠性。

基于PID控制的交通边界控制方法

随着城市化进程的不断加速,交通拥堵已成为城市面临的主要挑战之一。为了解决城市路网中心区域因车辆累积而导致的拥堵问题,本文利用宏观基本图的特有属性,提出了基于PID控制的交通边界控制方法。首先,建立被控区域车辆动态平衡方程,确定被控区域与外部区域车辆转移关系;其次,引入PID控制器作为路网边界控制器,通过调节进入被控区域的车辆转移比率,控制进入被控区域内部的车辆数;最后,利用VISSIM仿真软件的仿真结果表明:路网内部的车辆总数能够稳定在最佳累积值附近。

混有协同自适应巡航控制车辆的异质交通流稳定性解析与基本图模型

针对传统车辆和协同自适应巡航控制(cooperative adaptive cruise control,CACC)车辆构成的异质交通流,研究其稳定性与基本图模型.应用实车测试验证的CACC模型和智能驾驶员模型(intelligent driver model)分别作为CACC车辆和传统车辆的跟驰模型,建立异质流稳定性解析框架,研究不同平衡态速度、不同CACC车辆比例时的异质流稳定性.推导异质流基本图模型,并进行数值仿真实验.研究结果表明,在传统车辆稳定的速度范围,异质流处于稳定状态.在传统车辆不稳定的速度范围,CACC车辆比例增加以及平衡态速度远离9.6—18.6 m/s速度范围,均能够改善异质流的不稳定性.通行能力随着CACC车辆比例的增加而提高.此外,CACC模型的期望车间时距越大,异质流稳定域越大,但通行能力降低.因此,恒定车间时距CACC控制策略下的期望车间时距取值应权衡异质流稳定域和通行能力两个方面的影响.

北京市快速环路宏观基本图特征研究

对应于整个路网的宏观基本图反映了路网宏观变量间一种可再现的稳定的函数关系,该关系仅取决于路网属性和控制策略本身,而不依赖于具体交通需求.而作为世界上最大的城市之一,北京市的道路交通有着独一无二的特征.利用现有的高密度检测器数据,绘制了北京市快速环路的宏观基本图,在宏观上分析了北京市的交通特征;发现了北京市快速环路宏观基本图中不同于现有理论的特征,即在网络占有率方差与网络流量的对应关系中,二者亦可表现出相同的趋势,如较低占有率方差与较低网络流量间的对应关系.通过分析各环路交通拥堵的演化过程,认为北京市由中心向外扩散的放射形拥堵过程是形成这种特殊关系的内在因素.