Lane-drop Bottleneck Research Articles

Dynamic capacity drop propagation in incident-affected networks: Traffic state modeling with SIS-CTM

Mitigating the impact of traffic incidents on incident-affected networks is a critical requirement for traffic incident management. To this end, previous studies have focused on the capacity drop (CD) phenomenon at lane-drop bottlenecks. However, investigations of CD and propagation of CD in a network-level traffic flow modeling context have been limited, especially considering their dynamic interactions and temporal characteristics. This study presents a network-level traffic flow model for an incident-affected network to describe the time-varying characteristics of traffic flow on links. The cell transmission model (CTM) is adopted as the fundamental dynamic model. However, previous research on CTM and its stochastic characteristics has focused on unraveling the inherent stochasticity of the fundamental diagram. In this study, a new approach to investigate the stochastic nature of CD and congestion is used; the susceptible-infectious-susceptible (SIS) model is introduced and the SIS-CTM is proposed. This provides a probabilistic description of the link state. The proposed model can differentiate states with potential congestion and describe the unstable transitions as links switch between congested and uncongested conditions. It also can describe the influence of CD, congestion spread, and the propagation of CD with congestion spread. Numerical simulations in the Nguyen–Dupuis and Ziliaskopoulos networks case studies demonstrate the superiority of SIS-CTM. SIS-CTM can reflect the stochasticity of congestion and CD in time and space, while CTM cannot. Further, CTM underestimates the impact of congestion and CD, which is demonstrated as SIS-CTM spends 9.26% more time dissipating traffic demand in the network.

Congestion Mitigation Strategy at Lane Drop Bottleneck with Low Connected and Automated Vehicle (CAV) Penetration

Congestion Mitigation Strategy at Lane Drop Bottleneck with Low Connected and Automated Vehicle (CAV) Penetration

Differential variable speed limit control strategy consider lane assignment at the freeway lane drop bottleneck

Reduction in the number of lanes (lane drop) is typical on freeways due to road design, incidents, or road maintenance. As traffic demand increases, the bottleneck becomes activated because of there are many of vehicle lane-changing behaviors at the bottleneck. At the same time, the traffic density increases, resulting in a capacity drop phenomenon at the bottleneck. The degree of the capacity drop is also closely related to the traffic density at the bottleneck. Variable Speed Limit (VSL) is a widely used Active Traffic Management (ATM) method. In traditional research, most VSL control strategies consider all lanes uniformly and set uniform dynamic speed limits. However, traffic varies by lane. This study proposed a differential variable speed limit (DVSL) control strategy for each lane considering lane assignment. The DVSL control strategy sets the dynamic speed limit for each lane to attract a certain number of vehicles to change lanes before the bottleneck and reduce the impact of traffic capacity drop. This strategy is simulated based on extending the cell transmission model (CTM) to multi-lane. The multi-level iterative optimization algorithm obtains each lane's dynamic speed limit values. The experimental results show that the proposed DVSL control strategy can alleviate traffic congestion and improve efficiency. Total travel time (TTT) improved by 7.48 % compared to the no-control measures.

Formation control for connected and automated vehicles on multi‐lane roads: Relative motion planning and conflict resolution

Existing research has revealed that multi-vehicle coordinated decision making and control can achieve an improvement in both traffic efficiency and driving safety. In the multi-lane scenarios, a typical coordination method is multi-vehicle formation control. The existing formation control methods predefine the formation switching process and have not considered or explained the collision-free behaviour of vehicles in detail. In this paper, a multi-lane formation control strategy for connected and automated vehicles (CAVs) is proposed. The planning framework is bi-level, which can switch the structure of multi-vehicle formation in different scenarios smoothly and effectively. In the upper-level, the relative coordinate system is built to plan the collision-free relative paths for vehicles. In the lower-level, multi-stage trajectory planning and tracking are modelled as an optimal control problem with path constraints. The case study verifies the function of the formation control method in three lane-number-changing scenarios. Large-scale simulations in the lane-drop bottleneck scenario are conducted under different input traffic volumes, and the numerical results indicate that the proposed formation control method reduces congestion and improves both traffic efficiency and fuel economy at high traffic volumes.

A Cooperative Space Distribution Method for Autonomous Vehicles at A Lane-Drop Bottleneck on Multi-Lane Freeways

With the help of inter-vehicle communication (IVC), autonomous vehicles (AVs) can drive cooperatively, thus significantly improve road safety, traffic efficiency, and environmental sustainability. While substantial research has been conducted to investigate the efficiency of AVs in transportation systems, few attempts have been carried out to explore the lane-changing operations of AVs in multi-lane freeways, especially the dynamics of AVs at lane-drop bottlenecks. To this end, this paper aims to develop a cooperative space distribution method (CSDM) not only to increase the lane-drop bottleneck’s throughput but also to equally distribute the AVs in the dropped lane to other lanes by efficiently coordinating the dynamics of AVs upstream of the bottleneck in a multi-lane freeway. More specifically, we propose a novel framework where the freeway (region of interest) is divided into three segments: i) platoon segment, ii) acceleration segment, and iii) merging segment. In the first segment, AVs travel together in the platoon to guarantee their safety; they will then speed up to attain the maximum velocity and reach the determined position in the second segment. Finally, these AVs change lanes in the last segment and pass through the bottleneck with maximum velocity and minimum gap (i.e., gap distance). Simulation results are tested to demonstrate the performance of the proposed method, where we show that the proposed framework can significantly decrease the average travel time of all vehicles.

Improving Traffic Flow Efficiency at Motorway Lane Drops by Influencing Lateral Flows

Lane drops are a common bottleneck source on motorway networks. Congestion sets in upstream of a lane drop as a result of the lane changing activity of merging vehicles. This causes the queue discharge rate at the bottleneck to decrease and drop below the capacity, leading to capacity drop and further congestion. The objective of this study is to minimize the total travel time of the system by controlling lateral flows upstream of the lane drop. This is equivalent to maximizing the exit flows at the bottleneck. An optimization problem is formulated for a 3–2 lane drop section with high inflow. The problem is solved for different test cases where the direction of lateral flows being controlled is varied. An incentive based macroscopic model representing the natural lane changing scenario is used as a benchmark for comparison. The results showed that by influencing the lateral flows upstream of the bottleneck, the queue discharge rate increased by more than 4.5%. The total travel time of the system was consequently found to be reduced. The improvements in performance were primarily a result of the distribution of lane changing activity over space and the balancing of flow among the lanes which lead to the decrease in the severity of congestion. The findings reveal a potentially effective way to reduce the severity of congestion upstream of lane drop bottlenecks during high demand which could be implemented using roadside and in-car advisory systems.

Optimal location problem for variable speed limit application areas

Some studies consider variable speed limit (VSL) control as a viable option to prevent traffic breakdown at bottlenecks by limiting the mainline flow with reduced speed limits. However, few studies consider the location of the application area as a design variable of the problem. This paper explains why the location of a VSL control area is crucial to prevent the capacity drop phenomenon at lane drop bottlenecks. We first define two types of stationary states, congested and uncongested, inside a lane drop bottleneck assuming the Lighthill-Whitham Richards model with bounded acceleration. In particular, the characteristics of these stationary states and their admissible conditions are discussed thoroughly. If the speed limit imposed is low enough, the location of the VSL application area is irrelevant to ensure an uncongested stationary state inside the bottleneck. However, for a given range of speed limits, the location of the VSL application area should be designed carefully to allow for uncongested stationary states and prevent the occurrence of the capacity drop. We formulate an optimization problem and show that, contrary to the general belief, the larger the speed limit, the farther the VSL application area should be from the bottleneck. Finally, the results are extended to other types of bottlenecks, such as sag or tunnel bottlenecks. To the best of our knowledge, this is the first study to analytically identify, formulate, and solve the optimal location problem for variable speed limit application areas. It makes fundamental contributions to both traffic flow theory (by analyzing the stationary states for VSL-controlled bottlenecks) and traffic control (by determining the optimal location of a VSL application area). Moreover, the results presented are of practical relevance because they can help to establish some guidelines for practitioners to implement VSL control strategies.

Integrated Variable Speed Limits and Lane-Changing Control for Freeway Lane-Drop Bottlenecks

There are generally two kinds of traffic control strategies to relieve traffic congestion in lane-drop bottlenecks: variable speed limits (VSL) control and lane-changing (LC) control. However, VSL has limited or even no effect due to many mandatory LC maneuvers near bottlenecks, while LC fails to reduce traffic congestion when traffic demand is high. Although a few control methods combine VSL and LC, they do not consider the interaction between VSL and LC, which rules out many potentially good alternatives. We instead propose an integrated VSL and LC control method under a connected and automated vehicle (CAV) environment, which can consider the interaction and simultaneously find the values of LC numbers and speed limits to maximize traffic efficiency. Our control is in the framework of the model predictive control (MPC), which consists of prediction, optimization, and implementation. We adopt an improved multi-class cell transmission model (CTM) for traffic state prediction, then use the genetic algorithm (GA) for optimization which optimizes traffic network performance, and implement our control method in the SUMO platform. Simulation results demonstrate that our control method greatly improves the capacity of the road and is robust to different traffic demands and scenarios. Our control outperforms no control and VSL-only control in travel time and exhaust emissions, which reduces total travel time by 23.86% to 44.62% and exhaust emissions by 10.29% to 48.19%.

Optimisation of lane-changing advisory at the motorway lane drop bottleneck

Reduction in the number of lanes (lane drop) is common on motorways due to road design, incidents, or road maintenance, and it can be an active bottleneck if the traffic demand is high. If congestion occurs, the lane drop capacity will decrease 10–20%. In order to avoid capacity drop, this study analysed the reason and proposed lane-changing advisory control on the merge lane to distribute lane-changing using Cooperative Intelligent Transport Systems (C-ITS) technology. Further, this study used hyper-heuristic optimisation to obtain the lane-changing advisory proportion of each segment upstream of lane drop. Conditions of different traffic demands were analysed using the microscopic traffic simulation software AIMSUN and its Application Program Interface (API) function. Results indicated that the proposed lane-changing advisory strategy could reduce traffic congestion and obviously improve traffic efficiency. This study also analysed the effects with different proportions of connected vehicles and found that if the connected vehicle ratio is less than 20%, the lane-changing advisory has little impact on the lane drop performance, and that if the penetration rate is more than 50%, the gain in performance is marginal.

Integrated Traffic Control for Freeways using Variable Speed Limits and Lane Change Control Actions

The wide deployment of vehicle automation and communication systems (VACS) in the next decade is expected to influence traffic performance on freeways. Apart from safety and comfort, one of the goals is the alleviation of traffic congestion which is a major and challenging problem for modern societies. The paper investigates the combined use of two feedback control strategies utilizing VACS at different penetration rates, aiming to maximize throughput at bottleneck locations. The first control strategy employs mainstream traffic flow control using appropriate variable speed limits as an actuator. The second control strategy delivers appropriate lane-changing actions to selected connected vehicles using a feedback-feedforward control law. Investigations of the proposed integrated scheme have been conducted using a microscopic simulation model for a hypothetical freeway featuring a lane-drop bottleneck. The results demonstrate significant improvements even for low penetration rates of connected vehicles.

Optimal Variable Speed Limit Control at a Lane Drop Bottleneck: Genetic Algorithm Approach

AbstractThis paper develops a genetic algorithm (GA) approach to solving the variable speed limit (VSL) control problem at a lane drop bottleneck. A multiobjective nonlinear integer model is formul...

A first-order behavioral model of capacity drop

Understanding traffic dynamics at lane-drop bottlenecks, especially the mechanism of capacity drop, is critical for developing and evaluating centralized and decentralized control strategies in a freeway system. In this study, we propose a first-order behavioral model of capacity drop at a continuous lane-drop bottleneck. By extending the theoretical framework for analytically solving the generalized Riemann problem in Jin (2017), we introduce vehicles’ bounded acceleration on the LWR (Lighthill-Whitham-Richards) stationary states inside the lane-drop zone as an additional constraint for the optimization formulation of the entropy condition. We demonstrate that the optimization problem is uniquely solved with well-defined instantaneous continuous standing waves, comprised of the LWR stationary states inside the lane-drop and the bounded acceleration stationary states in the downstream acceleration zones. In the solutions to the generalized Riemann problem, the boundary flux as well as the stationary states and kinematic waves on both upstream and downstream links are the same as those in the phenomenological model of capacity drop proposed in Jin et al. (2015). This is a behavioral model of capacity drop, since both the dropped capacity and the capacity drop ratio are endogenous and can be calculated from the factors related to the fundamental diagram, road geometry, bounded acceleration, and lane changes. We present the Cell Transmission Model for the behavioral and phenomenological models and verify the theoretical results with numerical examples. We calibrate and validate the model with observed stationary speeds at a lane-drop bottleneck. Combined with Jin (2017), this study provides a proof of the conjecture by Hall and Agyemang-Duah (1991) that capacity drop results from the acceleration process when “drivers accelerate away from the (upstream) queue”.

Kinematic wave models of lane-drop bottlenecks

Lane-drop bottlenecks lead to both capacity reduction and capacity drop, but there are no systematic studies on traffic dynamics, especially the stationary traffic patterns when the upstream link is congested, at such critical bottlenecks within the framework of kinematic wave theory. In this paper, we study three kinematic wave models for both discontinuous and continuous lane-drop bottlenecks, where the number of lanes drops either suddenly or gradually. In the first two models, we apply the multilane LWR model (Lighthill and Whitham, 1955; Richards, 1956) for a discontinuous and continuous lane-drop bottleneck, respectively; in the third model, we assume that the standing wave is instantaneous inside a continuous lane-drop bottleneck. For the first model, we apply the kinematic wave theory developed in Jin et al. (2009) to solve the Riemann problem. In particular, we present an optimization formulation for the entropy condition, solve the stationary states and kinematic waves on both upstream and downstream links, and define instantaneous discontinuous standing waves connecting the upstream and downstream links’ stationary states. For the second model, we define a generalized Riemann problem and show that the asymptotic stationary states and kinematic waves on both links are the same as those in the first model, but the asymptotic standing wave is continuous, comprising of stationary states inside the whole lane-drop zone. Assuming standing waves to be instantaneous as in the first model but continuous as in the second model, we present a new model of a continuous lane-drop bottleneck and a new theoretical framework to solve the generalized Riemann problem. Finally we develop three corresponding Cell Transmission Models and verify theoretical results, especially the presence and structure of continuous standing waves inside a continuous lane-drop zone, with numerical examples. We find that vehicles’ acceleration rates across the standing waves can be infinite in the first model and unrealistically large in the other two and that there is no capacity drop in all of the models. The theoretical framework of the third model forms a basis for a follow-up study, (Jin, 2017), in which we introduce bounded acceleration as an additional constraint on the instantaneous standing waves and present a behavioral model of capacity drop.

Feedback control speed harmonization algorithm: Methodology and preliminary testing

The capacity drop phenomenon, which reduces the maximum bottleneck discharge rate following the onset of congestion, is a critical restriction in transportation networks that produces additional traffic congestion. Consequently, preventing or reducing the occurrence of the capacity drop not only mitigates traffic congestion, but can also produce environmental and traffic safety benefits. In addressing this problem, the paper develops a novel bang-bang feedback control speed harmonization (SH) or Variable Speed Limit (VSL) algorithm, that attempts to prevent or delay the breakdown of a bottleneck and thus reduce traffic congestion. The novelty of the system lies in the fact that it is both proactive and reactive in responding to the dynamic stochastic nature of traffic. The system is proactive because it uses a calibrated fundamental diagram to initially identify the optimum throughput to maintain within the SH zone. Furthermore, the system is reactive (dynamic) because it monitors the traffic stream directly upstream of the bottleneck to adjustment the metering rate to capture the dynamic and stochastic nature of traffic. The steady-state traffic states in the vicinity of a lane-drop bottleneck before and after applying the SH algorithm is analyzed to demonstrate the effectiveness of the algorithm in alleviating the capacity drop. We demonstrate theoretically that the SH algorithm is effective in enhancing the bottleneck discharge rate. A microscopic simulation of the network using the INTEGRATION software further demonstrates the benefits of the algorithm in increasing the bottleneck discharge rate, decreasing vehicle delay, and reducing vehicle fuel consumption and CO2 emission levels. Specifically, compared with the base case without the SH algorithm, the advisory speed limit increases the bottleneck discharge rate by approximately 7%, reduces the overall system delay by approximately 20%, and reduces the system-wide fuel consumption and CO2 emission levels by 5%.

Capacity drop: a comparison between stop-and-go wave and standing queue at lane-drop bottleneck

ABSTRACTIn freeways, the capacity drop means that the maximum traffic flow is higher than congestion discharge rates there. Various capacity drop magnitudes have been empirically observed before. But the mechanism behind this wide capacity drop range is not yet found. This contribution fills in the gap by relating the congestion discharge rates to different congestions in empirical observations. Two days’ data show that the outflows of stop-and-go waves are always lower than those of standing queues. Different discharge rates, ranging from 5220 to 6040 veh/h at the same site, always accompany different congestion states. Moreover, the different observations show that a higher discharge rate means a higher density in the free-flow branch in the fundamental diagram. This contribution shows that discharging rates probably could be controlled by transforming the congestion states. For instance, transforming a stop-and-go wave into a standing queue at a bottleneck might increase the bottleneck throughput.

Vehicular Traffic Flow Model with Driver Aggressiveness Component in a Multilane Road

In this study, we outline the Kerner’s 3-phase traffic flow theory, which states that traffic flow occurs in three phases and these are free flow, synchronized flow and wide moving jam phases. A macroscopic traffic flow model that is factoring driver aggressiveness is developed and its features discussed. By construction of the solution to the Riemann problem, the model is written in conservative form and solved numerically. Using the Godunov numerical method we go ahead to simulate traffic flow on a multilane road with a lane-drop bottleneck. The model is compared with the Aw-Rascle model and features of the model are shown to reproduce the features of a three phase traffic flow which the Aw-Rascle model cannot reproduce. It is also shown that the model respects aspects of traffic by responding to frontal stimuli only and it does not produce negative travel.

Simulating Lane-changing Dynamics towards Lane-flow Equilibrium Based on Multi-lane First Order Traffic Flow Model

It is well known that under the condition of high traffic volume lane-flow distribution becomes unbalanced; more traffic tends to use a median lane rather than a middle and outer lane, which causes the deterioration of traffic capacity at bottleneck sections. As intensive development of ITS, active and dynamic lane management has been practically implemented. By employing the technology of ITS, balancing lane-flow distribution is one of the feasible solutions to increase the throughput of bottleneck flow. However, the mechanism of the unbalanced usage among lanes is still unclear and traffic flow models enabling online and network-wide evaluation of dynamic and strategic lane management have not been proposed due to the lack of the method to computing lane-based traffic flow including lane-changing dynamics. This paper developed the multi-lane first order traffic flow model, which depicts the dynamics of lane-changing. It is assumed that each vehicle changes the lane to improve its utility or decrease its disutility, and also that the equilibrium of lane flow distribution is achieved as the condition of stochastic user equilibrium (SUE), where all drivers believe that they cannot improve their utility by changing the lanes. Thus, in the model, lane-changes are represented as the dynamics towards lane-flow equilibrium. As a result of simulating multilane traffic on freeway without any merging and diverging, it is revealed that the proposed model can represent the equilibrium curve of lane-flow distribution, and depict the propagation of traffic congestion at a lane-drop bottleneck section.

Integrated Variable Speed Limits Control and Ramp Metering for Bottleneck Regions on Freeway

To enhance the efficiency of the existing freeway system and therefore to mitigate traffic congestion and related problems on the freeway mainline lane-drop bottleneck region, the advanced strategy for bottleneck control is essential. This paper proposes a method that integrates variable speed limits and ramp metering for freeway bottleneck region control to relieve the chaos in bottleneck region. To this end, based on the analyses of spatial-temporal patterns of traffic flow, a macroscopic traffic flow model is extended to describe the traffic flow operating characteristic by considering the impacts of variable speed limits in mainstream bottleneck region. In addition, to achieve the goal of balancing the priority of the vehicles on mainline and on-ramp, increasing capacity, and reducing travel delay on bottleneck region, an improved control model, as well as an advanced control strategy that integrates variable speed limits and ramp metering, is developed. The proposed method is tested in simulation for a real freeway infrastructure feed and calibrates real traffic variables. The results demonstrate that the proposed method can substantially improve the traffic flow efficiency of mainline and on-ramp and enhance the quality of traffic flow at the investigated freeway mainline bottleneck.

Control of a lane-drop bottleneck through variable speed limits

The discharging flow-rate of a lane-drop bottleneck can drop when its upstream is congested, and such capacity drop leads to additional traffic congestion as well as safety threats. Even though many studies have demonstrated that variable speed limits (VSL) can effectively delay and even avoid the occurrence of capacity drop, there lacks a simple approach for analyzing the performance of a VSL control system. In this study, we formulate the VSL control problem for the traffic system in a zone upstream to a lane-drop bottleneck based on two traffic flow models: the Lighthill–Whitham–Richards (LWR) model, which is an infinite-dimensional partial differential equation, and the link queue model, which is an ordinary differential equation and approximates the LWR model. In both models, the discharging flow-rate is determined by a recently developed model of capacity drop, and the upstream in-flux is regulated by the speed limit in the VSL zone. We first analytically study the properties of the control system with the link queue model. For an open-loop control system with a constant speed limit, we prove that a constant speed limit can introduce an uncongested equilibrium state, in addition to a congested one with capacity drop, but the congested equilibrium state is always exponentially stable. Then we apply a feedback proportional-integral (PI) controller to form a closed-loop control system, in which the congested equilibrium state and, therefore, capacity drop can be removed by both I- and PI-controllers. Both analytical and numerical results show that, with appropriately chosen controller parameters, the closed-loop control system is stable, effect, and robust. Finally, we show that the VSL strategies based on I- and PI-controllers are also stable, effective, and robust for the LWR model. Since the properties of the control system are transferable between the two models, we establish a dual approach for studying the control problems of nonlinear traffic flow systems. We also confirm that the VSL strategy is effective only if capacity drop occurs. The obtained method and insights can be useful for future studies on other traffic control methods and implementations of VSL strategies in lane-drop bottlenecks, work zones, and incident areas.

Investigation of the early-onset breakdown phenomenon at urban expressway bottlenecks in Shanghai

Based on the analyses of recurring bottlenecks in Shanghai's expressways using spatial–temporal diagrams, the method of transformed cumulative curves was used to investigate the traffic flow characteristics revealed by loop detector data at two typical isolated bottlenecks (a lane-drop and an on-ramp). The pre-queue flow (PQF) and queue discharge flow (QDF) of the two bottlenecks were analysed. The QDFs were an average of 18% and 27% higher than the PQF in the lane-drop and on-ramp bottlenecks, respectively. These findings differ from the results of bottlenecks in other countries (e.g. the M4 motorway near London, UK; I-494 in Minneapolis, MN, USA; the Queen Elizabeth Way (QEW) and the Gardiner Expressway in metropolitan Toronto, Canada) and indicate that lane-drop and on-ramp bottlenecks in Shanghai experience early onsets of traffic breakdown. The potential causes of the early onset of breakdown were discussed from the perspectives of driving behaviour, the temporal features of the flow rate and the features of merging.