On macroscopic freeway merge behavior: Estimation of merge ratios using asymmetric lane flow distribution

Data from several freeway merges reveal that, contrary to some previous findings, merge ratio can vary within a site with respect to the merge outflow and that the existing merge ratio estimates based on lane counts are not able to predict this within-site variation. Furthermore, the merge ratios estimated based on two well-known merging principles, “fair-share” and “zipper,” are found to be inaccurate for merges where merging streams compete directly due to a lane drop. In light of these findings, we estimate merge ratios using lane flow distribution (LFD) to better predict between and within site variations of merge ratio. In addition, we propose a merging principle specific for merges with a single lane-drop. The model was developed to better represent observed non-uniform redistribution of merging flow not captured by the current merge ratio estimation methods and merging principles. Empirical observations show that the proposed methods are able to improve merge ratio estimates, reproduce within-site variations of merge ratio, and represent more accurately non-uniform redistribution of merging flow dependent on the merge geometry.     

Integrated feedback ramp metering and mainstream traffic flow control on motorways using variable speed limits

Ramp metering (RM) is the most direct and efficient tool for the motorway traffic flow management. However, because of the usually short length of the on-ramps, RM is typically deactivated to avoid interference of the created ramp queue with adjacent street traffic. By the integration of local RM with mainstream traffic flow control (MTFC) enabled via variable speed limits (VSL), control operation upstream of active bottlenecks could be continued even if the on-ramp is full or if the RM lower bound has been reached. Such integration is proposed via the extension of an existing local cascade feedback controller for MTFC-VSL by use of a split-range-like scheme that allows different control periods for RM and MTFC-VSL. The new integrated controller remains simple yet efficient and suitable for field implementation. The controller is evaluated in simulation for a real motorway infrastructure (a ring-road) fed with real (measured) demands and compared to stand-alone RM or MTFC-VSL, both with feedback and optimal control results. The controller’s performance is shown to meet the specifications and to approach the optimal control results for the investigated scenario.     

Extended bottlenecks, the fundamental relationship, and capacity drop on freeways

This paper presents evidence that the commonly used point bottleneck model is too simplistic for freeway bottlenecks, the actual mechanism appears to occur over an extended distance. We find evidence of subtle flow limiting and speed reducing phenomena more than a mile downstream of a lane drop bottleneck. These phenomena impact the fundamental relationship, FD. Close to the lane drop the free flow regime appears to come from a “parabolic” FD, but further downstream the relationship straightens to a “triangular” FD and throughput increases. We develop a theory to explain the underlying mechanisms. These insights should help resolve the decades long debate about the shape of the FD. The phenomena also provide a mechanism that may contribute to the empirically observed capacity drop often seen at bottlenecks. Although we study a lane drop, this work should be transferable to other bottlenecks where the capacity restriction persists for an extended distance, e.g., a corridor with a fixed number of lanes and an on-ramp bottleneck.     

Applications of wavelet transform for analysis of freeway traffic: Bottlenecks, transient traffic, and traffic oscillations

This paper demonstrates the capabilities of wavelet transform (WT) for analyzing important features related to bottleneck activations and traffic oscillations in congested traffic in a systematic manner. In particular, the analysis of loop detector data from a freeway shows that the use of wavelet-based energy can effectively identify the location of an active bottleneck, the arrival time of the resulting queue at each upstream sensor location, and the start and end of a transition during the onset of a queue. Vehicle trajectories were also analyzed using WT and our analysis shows that the wavelet-based energies of individual vehicles can effectively detect the origins of deceleration waves and shed light on possible triggers (e.g., lane-changing). The spatiotemporal propagations of oscillations identified by tracing wavelet-based energy peaks from vehicle to vehicle enable analysis of oscillation amplitude, duration and intensity.     

On the macroscopic stability of freeway traffic

A simple model of traffic flow is used to analyze the spatio-temporal distribution of flow and density on closed-loop homogeneous freeways with many ramps, which produce inflows and allow outflows. As we would expect, if the on-ramp demand is space-independent then this distribution tends toward uniformity in space if the freeway is either: (i) uncongested; or (ii) congested with queues on its on-ramps and enough inflow to cause the average freeway density to increase with time. In all other cases, however, including any recovery phase of a rush hour where the freeway’s average density declines, the distribution of flow and density quickly becomes uneven. This happens even under conditions of perfect symmetry, where the percentage of vehicles exiting at every off ramp is the same. The flow-density deviations from the average are shown to grow exponentially in time and propagate backwards in space with a fixed wave speed. A consequence of this type of instability is that, during recovery, gaps of uncongested traffic will quickly appear in the unevenly congested stream, reducing average flow. This extends the duration of recovery and invariably creates clockwise hysteresis loops on scatter-plots of average system flow vs. density during any rush hour that oversaturates the freeway. All these effects are quantified with formulas and verified with simulations. Some have been observed in real networks. In a more practical vein, it is also shown that the negative effects of instability diminish (i.e., freeway flows increase) if (a) some drivers choose to exit the freeway prematurely when it is too congested and/or (b) freeway access is regulated in a certain traffic-responsive way. These two findings could be used to improve the algorithms behind VMS displays for driver guidance (finding a), and on-ramp metering rates (finding b).     

Platoon-based traffic flow model for estimating breakdown probability at single-lane expressway bottlenecks

This study investigates the mechanism of traffic breakdown and establishes a traffic flow model that precisely simulates the stochastic and dynamic processes of traffic flow at a bottleneck. The proposed model contains two models of stochastic processes associated with traffic flow dynamics: a model of platoon formation behind a bottleneck and a model of speed transitions within a platoon. After these proposed models are validated, they are applied to a simple one-way, one-lane expressway section containing a bottleneck, and the stochastic nature of traffic breakdown is demonstrated through theoretical exercises.     

城市占道施工区交通流分配研究

文章阐述了占道施工区交通流分配的概念及相关措施,采用改进的LOGIT多路径概率模型作为占道施工区路网交通流分配模型,确定路网的有效出行路径及合理的交通分配量,并以广州市天河区黄埔大道占道施工为例,运用该模型对其周围路网交通流分配量进行分析,所得结果较实际地反映了交通状况,证明该模型算法是科学合理的。     

高原地区公路运行速度模型研究

文章基于对青藏公路车辆运行速度、车辆组成的调查,采用层次分析法对不同车辆组成、不同路段速度进行分析,建立了不同车辆组成下的运行速度模型,并结合交通事故数据,提出了确保交通安全的常年冻土区公路运行速度值。该运行速度模型的应用研究,为减少道路交通事故提供了一种新思路。