A multi-zone differential variable speed limit strategy for on-ramp bottlenecks

Effects of low speed limits on freeway traffic flow

Mainline freeway traffic flow control is one of the primary methods of traffic management, which can present the best network situation. In this paper, we integrate variable speed limit (VSL) strategy into the cell transmission model (CTM). Then the implementation of the integrated model on freeway traffic network is discussed. A novel optimal model of controlling freeway traffic flow is proposed for minimizing the total travelling time in the network. A solution algorithm is designed by using a simulation method. Considering the main purpose of the speed limit strategy is to control the mainstream flow, we compare the case where the VSL is used with the one without VSL. A simulation is implemented to show that the control strategy is efficient in describing system’s dynamic performance and the dynamic speed limit strategy significantly alleviates congestion.

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

In recent years, variable speed limit (VSL) strategies have proven to be an efficient control measure to mitigate traffic congestion at freeway bottlenecks. In general, the VSL change time is a constant value. However, there are certain limitations in traffic situations handled by a constant cycle VSL. Therefore, in this paper, we develop a dynamic cycle strategy of VSL based on predictive control. We first analyze the applicable situation of the dynamic control cycle and establish a probability model to determine the range of cycle selection. Then, the cell transmission model predicts the parameters of macroscopic traffic flow. Finally, an optimization algorithm is designed that is suitable for this strategy, which optimizes the cycle and speed limit options. An objective optimization function is formulated to minimize the total travel time. A sensitivity analysis is applied to compare different control strategies under a variety of road bottleneck structures by both the numerical analysis and simulation experiments. The simulation results show that the strategies and algorithms proposed in this paper can effectively reduce traffic congestion duration and enhance the service level of a freeway network.

Despite of the fact that the traffic control zone for maintenance work (work zone) has been recognized as one of major priorities to guarantee the traffic safety, only one conventional posted speed limit (PSL) strategy is applied into the organization and management. This article presents the strategy of the variable speed limit (VSL) on highway work zones that brings about gradual deceleration and low speed variance. To evaluate the safety of the proposed VSL strategy, this study uses the microscopic simulation software VISSM to estimate the traffic flow and adopt transversal and longitudinal coefficients of safety (MSDE and cv) to compare the different speed limit strategies. The results of simulation and analysis confirm that VSL yield a substantial decrease the traffic turbulence caused by speed limit and increase the traffic safety throughout work zones.

This paper develops a novel anticipatory Variable Speed Limit (VSL) control strategy that incorporates driver behavior based on trajectory data from probe vehicles. In particular, the research examines how the speed limit controls can be coordinated and optimized to reduce lane changing and overall braking, thus to achieve greater traffic throughput, safety, and sustainability. Driver behavior, such as acceleration/deceleration and lane changing, as reflected in individual trajectory data from GPS enabled probe vehicles are crucial for early detection of shockwave formation and proactive selection of speed limits; and consequently delay, or even eliminate breakdown formation. The core of this approach is the incorporation of a lane changing model which provides a more robust integrated speed limit selection and shockwave detection framework than the one we have developed in an earlier paper. The control principle is formulated in a generic fashion that finds the optimal speed limit control variables for either separate or simultaneous reduction of travel time, crash rates, and fuel consumption over a prediction horizon. The findings from the paper suggested that the VSL strategy was able to result in fewer lane changing rate (LCR) compared to the No-VSL case. Also, the developed algorithm was able to produce higher frequency of lower acceleration and deceleration rates than the No-VSL case, which indicated a smoother acceleration and deceleration pattern that corresponded to lower emission and fuel consumption. The performance of the algorithm was also examined under different probe penetration rates and congestion levels to identify the % of probe needed to achieve simultaneous mobility, safety, and environmental objectives.

Determining safety aspects of Differential Speed Limit on Indian Expressway

In the conventional human driving environment, due to poor traffic information transmission and the inability to actively control vehicles, variable speed limits (VSLs) and ramp metering (RM) strategies often fail to achieve the desired results when dealing with the merging zone of an expressway under high traffic demand. To improve traffic efficiency, an integrated control method for traffic flow in the merging area of expressways is proposed by combining connected and autonomous vehicle (CAV) active lane change (LC) technology with traditional VSL and entrance RM strategies. This method uses an upgraded cell transmission model (CTM) to estimate the state of the mainline and ramp input traffic volume and then uses the genetic algorithm (GA) to solve for the optimal combination of the mainline speed limit, number of mainline vehicles changing lanes, and ramp regulation under maximized flow in the merging zone. This integrated control strategy incorporating VSL, LC, and RM (VSL-LC-RM) enables synergistic control of the mainline and ramp traffic entering the merging zone in time-space. Finally, simulation experiments are conducted with the mainline two-lane and on-ramp single-lane expressway merging zone scenarios. The results show that compared with the VSL strategy and the strategy incorporating only VSL and RM (VSL-RM), the proposed VSL-LC-RM strategy can enhance the regulation of traffic flow, improve the flow of merging bottlenecks, and make full use of the road space-time resources.

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.

This paper provides an overview of various variable speed limit (VSL) strategies developed over the last two decades. It first explains the theoretical background of VSL systems and how they can be used as dynamic traffic control devices to improve traffic conditions and increase safety. An overview of VSL control strategies, ranging from early rule-based approaches to the most advanced network-wide coordinated approach, is provided, followed by a discussion of their associated potential benefits and areas for improvement. The likely environmental benefits of VSL strategies are also discussed, along with some further related aspects of VSL application, such as driver compliance to VSL and the integration of a differential speed limit (DSL) with VSL. Finally, the paper presents a critical review, pointing out some key issues that were identified from the literature review and recommends some directions for future research that need to be addressed as recommendations for best practices to underpin further refinement, development, and application of VSL.

Fog is a weather condition that can significantly threaten expressway safety and efficiency by reducing natural visibility and the road friction coefficient. The variable speed limit (VSL) strategy is an effective traffic-management method for harmonising vehicle speed and reducing rear-end crash potential under foggy conditions. Two key inputs – driver sight distance and the road friction coefficient – determine the reliability of VSL calculation. However, a driver's sight distance in a running vehicle (i.e. the dynamic sight distance (DSD)) is significantly lower than that at rest. Additionally, a dust–water coupled film on the expressway surface will also affect the dynamic friction coefficient (DFC). Based on a stopping sight distance model, a new VSL strategy that considers the DSD, DFC, alignment and slope is proposed. The results obtained from simulations based on the CarSim software program and the Simulink programming environment showed that, in foggy conditions with a natural visibility of 200 m, the speeds calculated by the proposed VSL strategy in straight and curved road segments were 70 km/h and 67 km/h, respectively, 16% and 11% higher than China's legal speed limit. A braking experiment on Wentai expressway in Zhejiang province, China confirmed that these speeds are safe and could improve efficiency. The research results provide a theoretical basis and support for the management of expressways in foggy conditions.

Historically, researchers and practitioners have utilized spot speeds and microscopic simulation methodologies to evaluate the operational impact of differential or uniform speed limits for trucks and passenger vehicles. This paper presents a methodology that uses connected truck data to develop a statistical characterization of both passenger car and truck speeds. These techniques were applied to three adjacent states, Illinois, Indiana and Ohio. Illinois and Ohio have 70 mph speed limits for both trucks and cars. Indiana has a differential speed limit for heavy trucks (65 mph) and passenger cars (70 mph). The statistical distribution of truck speeds was then compared among Illinois, Indiana and Ohio. These speeds were derived from over 8 million connected truck records traveling along Interstate 70 in Illinois, Indiana and Ohio during a one-week period from May 8-14, 2022. Statistical test results over selected 20-mile sections in each state showed that median truck speeds in Indiana with its differential speed limit of 65 mph were only 1 - 2 mph lesser than the neighboring states of Illinois and Ohio who observe a uniform speed limit of 70 mph for all traffic.

Dynamic Control Cycle Speed Limit Strategy for Improving Traffic Operation at Freeway Bottlenecks

This study re-evaluates Egypt’s Differential Speed Limit (DSL) policy by proposing Lane-Based Speed Limits (LBSL), an innovative strategy where speed limits vary across highway lanes. To address a critical research gap, we developed a microsimulation framework using Simulation of Urban Mobility (SUMO) with parameters specifically justified for Egyptian driver behavior. While constrained by data availability, our model moves beyond generic defaults by integrating a key stand-still gap parameter (CC0 = 1.5 m) calibrated from previous research on Egyptian conditions, ensuring a more accurate representation of local driving patterns. We simulated three-lane highway scenarios under varying traffic volumes (500–1750 veh/h/lane), heavy vehicle percentages (2.5–20%), and speed limits (100, 110, 120 km/h). This compared LBSL configurations (speed reductions of 10, 15, and 20 km/h between lanes) against Uniform Speed Limit (USL) and current DSL strategies. Performance was assessed through average delay, travel speed, lane-changing frequency, and conflict ratios using Time-to-Collision thresholds (≤ 2.50, ≤ 1.50, and ≤ 0.50 s). The results indicate that Egypt’s existing DSL strategy offers no statistically significant safety advantages over USL, questioning its effectiveness. Conversely, LBSL significantly improved safety, reducing conflicts by up to 60%, though it introduced a trade-off with operational efficiency by increasing delays by 71–870%. The primary contribution is a contextually grounded analysis demonstrating LBSL as a safer strategic alternative to Egypt’s incumbent DSL policy. This provides Egyptian policymakers with evidence-based insights for potential traffic safety reforms and offers a methodological framework for adapting microsimulation to other regions with similar data constraints.

Efficient traffic safety management necessitates real‐time crash risk prediction using expressway characteristics. With the emergence of autonomous vehicles (AVs), the development and evaluation of variable speed limit (VSL) strategies, a key active traffic management technique, become crucial for enhancing safety and mobility in mixed traffic flows. This underscores the need for optimized VSL strategies to accommodate both conventional and AVs. This paper presents a study on the development of VSL control algorithms using deep reinforcement learning in a microscopic traffic simulation. As the rewards function, time‐to‐collision and speed were considered. To enhance traffic safety, VSL strategies were refined across various market penetration of connected AVs. Analysis revealed that safety and traffic density are improved by 53% and 59%, respectively, in market penetration rate (MPR) 50, marking significant safety improvements in congested and low MPR scenarios. These findings present the importance of developing and evaluating VSL strategies for mixed traffic flow, particularly in the context of increasing the prevalence of connected and AVs.