Speed Limit Control Research Articles

Procedure for Determining the Deployment Locations of Variable Speed Limit Signs to Reduce Crash Risks at Freeway Recurrent Bottlenecks

The effectiveness of the variable speed limit (VSL) control is affected by the deployment locations of VSL signs. In this paper, a procedure was proposed to help determine the deployment location of VSL signs to reduce collision risks at freeway recurrent bottlenecks. The procedure is started from determining the hazardous recurrent bottleneck section by constructing the profile of the collision risks. The length of the hazardous section was determined according to the pre-selected threshold of crash risks. Various scenarios were considered for different lengths of VSL controlled sections with various densities of VSL signs. A modified cell transmission model (CTM) was used for modeling the traffic flow at the freeway bottlenecks under the VSL control. The VSL control factors were optimized by using the genetic algorithm. The safety effects of VSL were greatly affected by the placement of VSL signs. In general, the scenario with a longer VSL controlled section and more speed limit signs was more effective in reducing rear-end collision risks. The cost-benefit analyses showed that the placement of VSL signs in scenario C2 with the controlled section length of 11.7 mi and average VSL density of 1.0 mi had the best benefit/cost effect. Using 12 VSL signs, the collision risks were reduced by 69.16% and the total travel time was increased slightly by 7.73%. The procedure can help determine the optimal deployment of VSL signs on freeways, considering both the safety benefits and the cost.

Research on a Sliding Mode Vector Control System Based on Collaborative Optimization of an Axial Flux Permanent Magnet Synchronous Motor for an Electric Vehicle

In this paper, a sliding mode vector control system based on collaborative optimization of an axial flux permanent magnet synchronous motor (AFPMSM) for an electric vehicle is proposed. In order to increase the high efficiency range of electric vehicles and improve the cruising range, a collaborative optimization control strategy is firstly proposed. Due to the use of a dual stator-single rotor AFPMSM, the multi-motor efficiency optimization map and torque cooperative control are used to realize the working mode conversion of single stator and double stator, and the torque ripple caused by the working mode conversion is improved by fuzzy control. In order to improve the torque tracking capability, speed limiting characteristics, and operating characteristics, a speed limit and current vector control strategy based on a sliding mode controller is proposed and studied. The dynamic performance of electric vehicles is improved by a sliding mode vector control. Finally, a drive control system was developed for the proposed control strategy, and the complete vehicle test was carried out. The collaborative optimization control experiment and torque tracking and speed limiting experiments verify the correctness and effectiveness of the proposed control strategy. The acceleration performance and endurance experiments show that the proposed control strategy can effectively improve the cruising range and the acceleration performance of electric vehicles.

Stability analysis and variable speed limit control of a traffic flow model

The cell transmission traffic flow model (CTM) has attracted considerable interest in the field of transportation due to its simplicity as well as the ability to capture most of the macroscopic traffic flow characteristics. The stability properties of the CTM under different demand and capacity constraints are not always obvious. In addition, the impact of microscopic phenomena such as forced lane changes at bottlenecks leading to capacity drop is not captured by the CTM. In this paper, we start with a single section and modify the CTM to account for capacity drop. We analyze the stability properties of the CTM under all possible demand and capacity constraints as well as all possible initial density conditions. The analysis is used to motivate the design of variable speed limit (VSL) control to overcome capacity drop and achieve the maximum possible flow under all feasible traffic situations. The results are extended to multiple sections, where the stability properties of the open-loop system are analyzed and a VSL control scheme is designed and shown to achieve the objective of maximizing the traffic flow under different demand and capacity constraints. Unlike the open loop system where an infinite number of equilibrium points exist under certain demand levels, the proposed nonlinear VSL scheme guarantees exponential convergence to a unique equilibrium point that corresponds to maximum possible flow and speed under all possible demand levels and capacity constraints.

Safety Analysis of Interchange Functional Areas

Limited access facilities, such as freeways and expressways, are generally designed to the highest standards among public roads. Consequently, these facilities demonstrate crash, injury, and fatality rates that are significantly lower than other road facility types. However, these rates are generally elevated in the immediate vicinity of interchanges because of increases in traffic conflicts precipitated by weaving, merging, and diverging traffic. Given the extensive costs involved in interchange construction, it is important to discern the expected operational and safety impacts of various design alternatives. To this end, the objective of this study was to analyze safety performance within the functional areas of interchanges. The study involves the integration of traffic crash, volume, and roadway geometric data from 2010 to 2014 in the state of Iowa. Separate analyses were conducted for the freeway mainline and ramp connections. A series of safety performance functions (SPFs) were estimated for both the mainline and ramps. Random effects negative binomial models were estimated, which account for correlation in crash counts at the same location over time. The results show the frequency of crashes to vary based on traffic volume, interchange configuration, speed limit, and traffic control at the ramp terminal. The random effects models are shown to significantly outperform pooled models, which suggest there are several important location-specific factors that are not included in the analysis dataset. The SPFs from this study are also compared with several reference models from the extant research literature.

Enhancing Freeway Safety through Intervening in Traffic Flow Dynamics Based on Variable Speed Limit Control

New technologies and traffic data sources provide great potential to extend advanced strategies in freeway safety research. The High Definition Monitoring System (HDMS) data contribute comprehensive and precise individual vehicle information. This paper proposes an innovative Variable Speed Limit (VSL) based approach to manage crash risks by intervening in traffic flow dynamics on freeways using HDMS data. We first conducted an empirical analysis on real-time crash risk estimation using a binary logistic regression model. Then, intensive microscopic simulations based on AIMSUN were carried out to explore the effects of various intervention strategies with respect to a 3-lane freeway stretch in China. Different speed limits with distinct compliance rates under specified traffic conditions have been simulated. By taking into account the trade-off between safety benefits and delay in travel time, the speed limit strategies were optimized under various traffic conditions and the model with gradient feedback produces more satisfactory performance in controlling real-time crash risks. Last, the results were integrated into lane management strategies. This research can provide new ideas and methods to reveal the freeway crash risk evolution and active traffic management.

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...

Improving Traffic State Prediction Model for Variable Speed Limit Control by Introducing Stochastic Supply and Demand

Variable speed limit (VSL) is becoming recognized as an effective way to improve traffic throughput and road safety. In particular, methods based on traffic state prediction exhibit promising potential to prevent future traffic congestion and collisions. However, field observations indicate that the traffic state prediction model results in nonnegligible error that impacts the next step decision making of VSL. Thus, this paper investigates how to eliminate this prediction error within a VSL environment. In this study, the traffic state prediction model is a second-order traffic flow model named METANET, while the VSL control is model predictive control (MPC) based, and the VSL decision is discrete optimized choice. A simplified version of the switching mode stochastic cell transmission model (SCTM) is integrated with the METANET model to eliminate the prediction error. The performance of the proposed method is assessed using field data from a VSL pilot test in Edmonton, Canada, and is compared with the prediction results of the baseline METANET model during the road test. The results show that during the most congested period the proposed SCTM-METANET model significantly improves the prediction accuracy of regular METANET model.

Decision support model for variable speed limit control in recurrent congestion

ABSTRACTThough previous studies have shown the potential of variable speed limits (VSL) and VSL paired with ramp metering (VSLRM) control to improve safety and mobility measures at freeway bottlenecks, little guidance has been developed to assist decision makers in the planning phase of a congestion mitigation project. The objective of this study was to develop and demonstrate control decision guidance to facilitate accurate, robust, and efficient planning level guidance for these control strategies. This study simulated over 1,000 traffic environments under VSL, VSLRM, and no control. The outputs were used to evaluate the mobility and safety changes of VSL and VSLRM relative to the no control scenarios. These metrics were used to develop a multinomial logistic regression model for control decision guidance. The model was then demonstrated on real-world candidate sites. The results suggest that the model has the potential to assist in the planning of VSL and VSLRM control.

Variable speed limit control scheme for vehicle in the vicinity of signalized intersection

In this paper, a variable speed limit control approach for improving the efficiency of signalized intersection is proposed from the cyber-physical view. In the proposed control scheme, vehicle-to-vehicle communication is being utilized fully such that the preceding vehicle’s optimal information is exploited for optimizing the motion of the current vehicle. To prepare the control scheme for dynamical optimization, nonlinear model predictive control technique is used to optimally control vehicle speed limit and a new traffic model is constructed by considering vehicle dynamics. Moreover, this paper evaluates the influence of communication distance on the traffic. The results show that extending communication distance in the vicinity of signalized intersection can improve vehicles’ average fuel economy.

A Study on the Merits for Coordinated Use of Ramp Metering and Variable Speed Limit Traffic Control Systems

Aim:The aim of this research was to investigate the merits for further improvements of traffic operation on freeways and expressways through coordinated use of Ramp Metering and Variable Speed limit (VSL) control systems.Methods:In this research, the widely used ALINEA Ramp Metering strategy was coordinated with a rule-based VSL strategy so that the total flow entered from the upstream freeway and entry ramp is maintained below the merge downstream capacity. The developed algorithm was then examined on a freeway network comprising two merge and one diverge sections, using VISSIM microscopic simulation model. The performance of the simulated network was examined under three scenarios namely, No-control, Ramp Metering only and Ramp Metering plus VSL controls. The network performance under each scenario was then assessed and compared using three measures of performance namely, average travel time, overall delay and freeway throughput. The ANOVA test was used to analyze and compare the impacts of specified scenarios.Results:The results indicated that the best performance is achieved under coordinated Ramp Metering plus VSL scenario as it produced a significantly better performance in comparison with the other two scenarios.Conclusion:The results can be attributed to the synergistic effects of coordinated and integrated use of these control systems on the freeway network and therefore, coordination of such systems is recommended.

Safety Impact of Weaving Distance on Freeway Facilities with Managed Lanes using Both Microscopic Traffic and Driving Simulations

With the challenges of increasing traffic congestion, the concept of managed lanes (MLs) has been gaining popularity recently as a means to effectively improve traffic mobility. MLs are usually designed to be left-lane concurrent with an at-grade access/exit. Such a design forms weaving segments since it requires vehicles to change multiple general purpose lanes (GPLs) to enter or exit the ML. The weaving segments could have a negative impact on traffic safety in the GPLs. This study provides a comprehensive investigation of the safety impact of different lengths for each lane change maneuver on GPL weaving segments close to the ingress and egress of MLs through two simulation approaches: VISSIM microsimulation and driving simulator. The two simulation studies are developed based on traffic data collected from freeway I-95 in Miami, Florida. The results from the two simulation studies support each other. Based on the two simulation studies, it is recommended that 1,000 feet be used as the optimal length for per lane change at the GPLs weaving segments with MLs. The safety impact of traffic volume, variable speed limit control strategies, and drivers’ gender and age characteristics are also explored. This study can provide valuable insight for evaluating the traffic performance of freeway weaving segments with the presence of concurrent GPLs and MLs in a highway safety context. It also provides guidelines for future conversion of freeways to include MLs.

Tabu Search Strategies for Variable Speed Limit Control at a Lane Drop Bottleneck

AbstractA tabu search–based algorithm is developed in this study and applied for optimizing a variable speed limit (VSL) control system. A macroscopic traffic flow model is used to predict the traf...

Predicting Traffic Dynamics with Driver Response Model for Proactive Variable Speed Limit Control Algorithm

Variable Speed Limit (VSL) control contributes to potential crash risk reduction by suggesting a suitable dynamic speed limit to achieve more stable and uniform traffic flow. In recent studies, researchers adopted macroscopic traffic flow models and perform prediction-based optimal VSL control. The response of drivers to the advised VSL is one of the most critical parameters in VSL-controlled speed dynamics modeling, which significantly affects the accuracy of traffic state prediction as well as the control reliability and performance. Nevertheless, the variations of driver responses were not explicitly modeled. Thus, in this research, the authors proposed a dynamic driver response model to formulate how the drivers respond to the advised VSL during various traffic conditions. The model was established and calibrated using field data to quantitatively analyze the dynamics of drivers’ desired speed regarding the advised VSL and current traffic state variables. A proactive VSL control algorithm incorporating the established driver response model was designed and implemented in field-data-based simulation study. The design proactive control algorithm modifies VSL in real-time according to the traffic state prediction results, aiming to reduce potential crash risks over the experiment site. By taking into account the real-time driver response variations, the VSL-controlled traffic state dynamics was more accurately predicted. The experimental results illustrated that the proposed control algorithm effectively reduces the crash probabilities in the traffic network.

Integrated control of highway traffic flow

Several efforts have been made during recent years on the control of traffic flow using ramp metering, variable speed limit and lane change control. Most of these techniques reported significant traffic mobility improvements in macroscopic simulations, which cannot be confirmed in some microscopic simulation cases. We demonstrate that the lack of travel time improvement is due to the forced lane changes at the bottleneck which introduce a capacity drop. In this paper, a lane change controller is proposed which provides lane change recommendations to avoid the capacity drop. Combined with the lane change control, a feedback linearisation variable speed limit controller which maintains the bottleneck flow at the maximum level and coordinates with ramp metering controllers is proposed. The integrated VSL, RM and LC controller can improve system performance, manage the ramp queues and maintain the fairness between mainline and on-ramp flows. Microscopic simulations show consistent improvements under different scenarios.

Reinforcement Learning-Based Variable Speed Limit Control Strategy to Reduce Traffic Congestion at Freeway Recurrent Bottlenecks

The primary objective of this paper was to incorporate the reinforcement learning technique in variable speed limit (VSL) control strategies to reduce system travel time at freeway bottlenecks. A Q-learning (QL)-based VSL control strategy was proposed. The controller included two components: a QL-based offline agent and an online VSL controller. The VSL controller was trained to learn the optimal speed limits for various traffic states to achieve a long-term goal of system optimization. The control effects of the VSL were evaluated using a modified cell transmission model for a freeway recurrent bottleneck. A new parameter was introduced in the cell transmission model to account for the overspeed of drivers in unsaturated traffic conditions. Two scenarios that considered both stable and fluctuating traffic demands were evaluated. The effects of the proposed strategy were compared with those of the feedback-based VSL strategy. The results showed that the proposed QL-based VSL strategy outperformed the feedback-based VSL strategy. More specifically, the proposed VSL control strategy reduced the system travel time by 49.34% in the stable demand scenario and 21.84% in the fluctuating demand scenario.

Integrated Cooperative Adaptive Cruise and Variable Speed Limit Controls for Reducing Rear-End Collision Risks Near Freeway Bottlenecks Based on Micro-Simulations

Freeway bottlenecks lead to traffic congestion and speed reduction, resulting in increased risks of rear-end collision. This paper aimed to develop a control strategy of an integrated system of cooperative adaptive cruise control (CACC) and variable speed limit (VSL) to reduce rear-end collision risks near freeway bottlenecks. A microscopic simulation testbed was first constructed, in which the realistic PATH CACC models and surrogate safety measures of the time exposed time-to-collision (TET) and time integrated time-to-collision (TIT) were used. A feedback control algorithm was then developed for the proposed vehicle to infrastructure system of CACC and VSL. The simulation results showed that the proposed integration system with 100% CACC penetration rate can reduce the rear-end collision risks effectively, with the TIT and TET declined by 98%. The average travel time was also decreased by 33%, compared with the manual vehicles without any control. Moreover, the safety improvements of the proposed integrated system are quite stable at the various bottlenecks with different magnitudes of speed reductions. The sensitivity analyses suggested that the penetration rate of CACC has significant impact on safety performance. The VSL control plays an important role in reducing rear-end collision risks when the penetration rate of CACC is low. The combination of CACC and VSL controls mitigates the negative effects of the mixed traffic flow of the manual and CACC vehicles.

Variable speed limit design based on mode dependent Cell Transmission Model

In this paper a mode dependent variable speed limit (VSL) control strategy is developed for motorway networks. The suggested rolling horizon and coordinated algorithm uses switching mode Cell Transmission Model (CTM) and purports to maximize network throughput.In this line, first, a VSL signal scheduled piecewise affine switching mode CTM is derived based on the polyhedral description of Godunov fluxes. Second, a two-stage, coordinated, rolling horizon VSL sequence generation procedure is proposed. The set of possible VSL signs is selected by applying input constraints in order to eliminate spatial and temporal VSL oscillations. Then, the set of modes is further reduced according to the stable and adjacent reachable modes of the switching mode CTM. Over the remaining set of input signals, network capacity is maximized with the help of solving a mixed integer optimization problem under the form of reference density tracking objective. The method is implemented in simulation environment to demonstrate its computational efficiency and viability to attenuate shockwaves.

Variable Speed Limit Control for Delay and Crash Reductions at Freeway Work Zone Area

AbstractImproving operational efficiency and safety on freeway segments has long been recognized as a priority in traffic communities. This study presents a comprehensive variable speed limit (VSL)...

Stochastic stability of Markov jump hyperbolic systems with application to traffic flow control

In this paper, we investigate the stochastic stability of linear hyperbolic conservation laws governed by a finite-state Markov chain. Both system matrices and boundary conditions are subject to the Markov switching. The existence and uniqueness of weak solutions are developed for the stochastic hyperbolic initial–boundary value problem. By means of Lyapunov techniques some sufficient conditions are obtained by seeking the balance between the boundary condition and the transition probability of the Markov process. Particularly, boundary feedback control of a stochastic traffic flow model is developed for the freeway transportation system by integrating the on-ramp metering with the speed limit control.

Effects of rainy weather on traffic accidents of a freeway using cellular automata model**Project supported by the National Natural Science Foundation of China (Grant No. 50478088) and the Natural Science Foundation of Hebei Province, China (Grant No. E2015202266)

The aim of this work is to investigate the influence of rainy weather on traffic accidents of a freeway. The micro-scale driving behaviors in rainy weather and possible vehicle rear-end and sideslip accidents are analyzed. An improved CA model of two lanes one-way freeway is presented, where some vehicle accidents will occur when the necessary conditions are simultaneously satisfied. The characteristics of traffic flow under different rainfall intensities are discussed and the accident probabilities are analyzed via the simulation experiments by using variable speed limit (VSL) and incoming flow control. The results indicate that the measures are effective especially during heavy rainstorms or short-time heavy rainfall. According to different rainfall intensities, an appropriate strategy should be adopted in order to reduce the probability of vehicle accidents and enhance traffic flux as well.