Effects of low speed limits on freeway traffic flow

Recent years have seen a renewed interest in Variable Speed Limit (VSL) strategies. New opportunities for VSL as a freeway metering mechanism or a homogenization scheme to reduce speed differences and lane changing maneuvers are being explored. This paper examines both the macroscopic and microscopic effects of different speed limits on a traffic stream, especially when adopting low speed limits. To that end, data from a VSL experiment carried out on a freeway in Spain are used. Data include vehicle counts, speeds and occupancy per lane, as well as lane changing rates for three days, each with a different fixed speed limit (80 km/h, 60 km/h, and 40 km/h). Results reveal some of the mechanisms through which VSL affects traffic performance, specifically the flow and speed distribution across lanes, as well as the ensuing lane changing maneuvers. It is confirmed that the lower the speed limit, the higher the occupancy to achieve a given flow. This result has been observed even for relatively high flows and low speed limits. For instance, a stable flow of 1942 veh/h/lane has been measured with the 40 km/h speed limit in force. The corresponding occupancy was 33%, doubling the typical occupancy for this flow in the absence of speed limits. This means that VSL strategies aiming to restrict the mainline flow on a freeway by using low speed limits will need to be applied carefully, avoiding conditions as the ones presented here, where speed limits have a reduced ability to limit flows. On the other hand, VSL strategies trying to get the most from the increased vehicle storage capacity of freeways under low speed limits might be rather promising. Additionally, results show that lower speed limits increase the speed differences across lanes for moderate demands. This, in turn, also increases the lane changing rate. This means that VSL strategies aiming to homogenize traffic and reduce lane changing activity might not be successful when adopting such low speed limits. In contrast, lower speed limits widen the range of flows under uniform lane flow distributions, so that, even for moderate to low demands, the under-utilization of any lane is avoided. These findings are useful for the development of better traffic models that are able to emulate these effects. Moreover, they are crucial for the implementation and assessment of VSL strategies and other traffic control algorithms.     

A trajectory smoothing method at signalized intersection based on individualized variable speed limits with location optimization

Traffic signals on urban highways force vehicles to stop frequently and thus causes excessive travel delay, extra fuel consumption and emissions, and increased safety hazards. To address these issues, this paper proposes a trajectory smoothing method based on Individual Variable Speed Limits with Location Optimization (IVSL-LC) in coordination with pre-fixed traffic signals. This method dynamically imposes speed limits on some identified Target Controlled Vehicles (TCVs) with Vehicle to Infrastructures (V2I) communication ability at two IVSL points along an approaching lane. According to real-time traffic demand and signal timing information, the trajectories of each approaching vehicle are made to run smoothly without any full stop. Essentially, only TCVs’ trajectories need to be controlled and the other vehicles just follow TCVs with Gipps’ car-following model. The Dividing RECTangles (DIRECT) algorithm is used to optimize the locations of the IVSLs. Numerical simulation is conducted to compare the benchmark case without vehicle control, the individual advisory speed limits (IASL) and the proposed IVSL-LC. The result shows that compared with the benchmark, the IVSL-LC method can greatly increase traffic efficiency and reduce fuel consumption. Compared with IASL, IVSL-LC has better performance across all traffic demand levels, and the improvements are the most under high traffic demand. Finally, the results of compliance analysis show that the effect of IVSL-LC improves as the compliance rate increases.     

Emergency steering control of autonomous vehicle for collision avoidance and stabilisation

ABSTRACT

Collision avoidance and stabilisation are two of the most crucial concerns when an autonomous vehicle finds itself in emergency situations, which usually occur in a short time horizon and require large actuator inputs, together with highly nonlinear tyre cornering response. In order to avoid collision while stabilising autonomous vehicle under dynamic driving situations at handling limits, this paper proposes a novel emergency steering control strategy based on hierarchical control architecture consisting of decision-making layer and motion control layer. In decision-making layer, a dynamic threat assessment model continuously evaluates the risk associated with collision and destabilisation, and a path planner based on kinematics and dynamics of vehicle system determines a collision-free path when it suddenly encounters emergency scenarios. In motion control layer, a lateral motion controller considering nonlinearity of tyre cornering response and unknown external disturbance is designed using tyre lateral force estimation-based backstepping sliding-mode control to track a collision-free path, and to ensure the robustness and stability of the closed-loop system. Both simulation and experiment results show that the proposed control scheme can effectively perform an emergency collision avoidance manoeuvre while maintaining the stability of autonomous vehicle in different running conditions.     

细粒土界限含水率液、塑限联合测定自动化方案的分析与探讨

为解决现行细粒土界限含水率液、塑限联合测定法效率低下和试验流程难以自动化的难题,通过对现行液、塑限联合测定法试验原理的扩展应用进行讨论,得出无论是对烘干土还是天然土试样进行液、塑限联合测定,都可通过监测土膏调制过程中水的增量求得各级土膏含水率,而无需对各级土膏采用烘干法逐级测试含水率的结论,并给出了试样干、湿2 种状态下求解各级土膏含水率的计算方法;以此为基础,结合昆明轨道交通4 号线岩土勘察获取的大量细粒土液、塑限联合测定试验操作的计时观察结果,对现行试验方法系统功能进行优化分析,进而得出界限含水率试验的优化流程。该流程人工操作时长可减少75%,完成单组试样界限含水率测试所需时长􀰐t 可由16. 5 h 缩减为6. 5 h。根据优化试验流程和试验方法特征,提出液、塑限闭式联测法这一自动化实现方案,并对其试验应用条件进行初步讨论。     

美国对车辆速度的管理

1987年美国国会准许各州把乡村州际公路某些路段的限速从89 km/h提高到105 km/h,1995年国家公路系统取消了最大限速89 km/h的规定,并把制定限速的权力下放给各个州,1996年32个州对不同的公路都提高了限速标准.通过对比限速前后交通流的变化,研究速度和交通事故的关系,美国提出了管理车辆速度的原因和方法,分析了限速对交通安全的影响,提出了管理车速应该考虑的因素,以及执行限速管理的要点.通过介绍美国管理车辆速度的背景、经验和效果,特别是限速作为速度管理的一个重要手段,为我国的车速管理提供了借鉴方法.     

汽车交流发电机中性点二极管作用原理探究

从理论上分析了星形连接交流发电机中性点二极管的作用原理，中性点二极管通过将电动势绝对值较低的发电机定子绕组及时“短路”，减少发电机的内部损失、增加发电机输出。确定了中性点二极管发挥作用的转速条件和负载条件，即发电机转速高于3．3倍空载转速、负载电阻小于一相定子绕组的阻抗。简要分析了影响中性点二极管作用的因素，包括负载电阻、发电机转速、磁极对数、定子绕组电感等。     

郑西高铁CTCS-3级列控系统临时限速设置常见问题分析

详细剖析CTCS-3级列控系统临时限速设置中常见的问题,为快捷分析和处理限速设置故障提供了新的思路和方法。     

机车车辆区间故障数据置信限近似估计方法

针对机车车辆产品可靠性数据特点,在分析传统区间故障数据置信限估计方法的应用范围及其不足的基础上,依据秩统计理论,提出一种机车车辆区间故障数据置信限近似估计方法并据此开发计算软件,对机车继电器、机车柴油机水泵等相关产品可靠性数据进行计算分析,并与传统估计方法计算结果进行比较。结果表明:本文方法在小样本数据情形下,可获得满意的估计结果,克服了传统方法的大样本苛求;在相同置信水平下,本文方法给出的置信限估计结果更偏安全,适用于对可靠性要求较高的机车车辆产品。     

如何有效的治理公路货运双超难题

公路货运车辆的超载、超限问题由来已久,久抓未绝,严重扭曲了正常的市场规则。治理双超是为了维护社会主义市场经济秩序,建立健康、规范、公平、有序的道路运输市场的需要。治理车辆双超需要通过经济、法律、行政等手段的综合运用,各要素充分整合协调,才能有效的解决这一难题。就公路货运双超问题进行分析,并就如何治理提出建议。     

基于燃料可燃界限的隧道火灾回燃研究

地铁隧道属于大长细比的通风受限空间,隧道火灾在一定条件下会出现回燃．地铁隧道1：8缩尺比例的火灾模型实验表明,决定回燃是否产生的关键参数是燃料挥发份的质量分数,单隧道自然通风条件下均临界值为8．78％;利用燃料的可燃界限图所得到的质量分数的临界值为8．66％,与实验值接近．自然通风条件下的单隧道回燃和腔体回燃相似,但产生回燃的燃料挥发份质量分数的,临界值不同．在确定隧道火灾回燃之前,借助于燃料的可燃界限图初步判断其挥发份质量分数临界值的范嘲,对现场真实火灾回燃的分析和控制有重要的参考价值．