Queue Warning Research Articles

Traffic safety performance evaluation in a connected vehicle environment with queue warning and speed harmonization applications

With the increased adoption of connected vehicle (CV) technologies, safety information is becoming increasingly available to drivers. This study investigates three main questions (1) Do CV-based traffic management applications improve safety on roadways with existing infrastructure-based traffic management systems? (2) Can combining two CV technologies have a greater impact on safety than a single CV technology? and (3) Do geometric and traffic composition factors impact the efficiency of CV technologies? We applied a rarely-used CV dataset and conducted a comprehensive simulation analysis of varying conditions and CV penetration rates that studies have not considered. Two CV applications (queue warning and speed harmonization) implemented in the Intelligent Network Flow Optimization experiment in Seattle, WA were evaluated. Results showed that driver safety performance, based on speed metrics (standard deviation and percentage of extreme values) improved under the CV driving conditions. Combining conventional variable speed limit systems with queue warnings also improved safety for CV drivers. Furthermore, the implementation of a single CV application (queue warning) showed positive changes in the aforementioned speed metrics, congestion mitigation, and reduced conflicts. With the two CV applications combined, no significant differences were observed. Additional tests investigated the impacts of lane changes and roadway attributes on safety in the CV environment.

Assessment of Queue Warning Application on Signalized Intersections for Connected Freight Vehicles

Connected vehicle (CV) systems are at the core of intelligent transportation systems (ITS) for their capability to support a variety of ITS applications and to unite vehicles and infrastructure elements into a well-integrated transportation system. CV refers to vehicles that exchange information with each other and with the infrastructure. The queue warning application (Q-WARN) uses CV technologies to allow vehicles within the queue to broadcast their queued status information automatically to upstream vehicles and to infrastructure. Queue warnings are sent to oncoming vehicles to prevent rear-end or other secondary collisions. This paper focuses on Q-WARN applications for freight vehicles at signalized intersections adjacent to I-80 in Wyoming, which are characterized by heavy truck traffic. The algorithms use the latitude/longitude coordinates of freight CVs and intersections to form a communication link and to share information. Tests were performed in VISSIM microsimulation with Econolite ASC/3 software-in-the-loop controller emulator for different CV market penetration rates. Three locations in Wyoming were used as test-bed cases. The developed Q-WARN algorithms are successful in reducing vehicle delays by an average of 2% to 5%. Time to collision (TTC) significantly increased with an increase in CV rates, by two to five times. The abundance of information obtained from CVs can be used further to enhance signal control algorithms. The developed algorithms can easily be implemented in the field, since they use existing CV communication protocols and signal control logic.

Development of Decision Support System for Integrated Active Traffic Management Systems Considering Travel Time Reliability

A new decision support system (DSS) using travel time reliability was developed for integrated active traffic management (IATM) including freeways and arterials. The DSS consists of recommendation and evaluation of response plans. The DSS also includes three representative traffic management strategies: variable speed limits, queue warning, and ramp metering. The recommendation of response plans for recurring traffic congestion was generated from the logics of the three strategies. The evaluation of response plans was conducted by travel time reliability through the prediction of traffic conditions with response plans. The near-future prediction of traffic conditions with control strategies was conducted through METANET for freeways and arterials. The developed DSS was evaluated under three types of traffic congestion: extreme, heavy, and moderate. According to the evaluation results, the developed DSS recommended an IATM strategy with the highest synergistic relationships in real time and contributed to enhancing the effectiveness of the IATM strategies. It was confirmed that arterials should have the allowable residual capacity for the improvement of traffic flow of the entire corridor network. Furthermore, the DSS demonstrated a more balanced traffic condition between freeways and arterials.

Cross-Vendor and Cross-State Analysis of GPS Probe Data Latency

Crowd sourced GPS probe data have become a major source of real-time traffic information applications. In addition to traditional traveler advisory systems such as dynamic message signs (DMS) and 511 systems, probe data are being used for automatic incident detection, integrated corridor management (ICM), end of queue warning systems, and mobility-related smartphone applications. Several private sector vendors offer minute by minute network-wide travel time and speed probe data. The quality of such data in terms of deviation of the reported travel time and speeds from ground-truth has been extensively studied in recent years, and as a result concerns over the accuracy of probe data have mostly faded away. However, the latency of probe data—defined as the lag between the time at which disturbance in traffic speed is reported in the outsourced data feed, and the time at which the traffic is perturbed—has become a subject of interest. The extent of latency of probe data for real-time applications is critical, so it is important to have a good understanding of the amount of latency and its influencing factors. This paper uses high-quality independent Bluetooth/Wi-Fi re-identification data collected on multiple freeway segments in three different states, to measure the latency of the vehicle probe data provided by three major vendors. The statistical distribution of the latency and its sensitivity to speed slowdown and recovery periods are discussed.

Simulation of hard shoulder running combined with queue warning during traffic accident with CTM model

Hard shoulder running (HSR) and queue warning are two active traffic management (ATM) strategies and are commonly used to alleviate highway traffic congestion. This study proposes an optimisation model for HSR operation in coordination with queue warning service during non-recurring traffic accident condition using an updated cell transmission model (CTM). The CTM in this study is updated in two aspects: first, the capacity reduction caused by traffic accident is considered in the CTM; second, a purposive desired lane-changing rate is updated in the CTM considering a driver's movement from one lane to another depending on the lane blocking after receiving queue warning. The HSR operation combined with queue warning is optimised to minimise the total time delay during traffic accident. The model is tested on the A12 highway in the Netherlands under two typical accident scenarios and presents a certain application value.

Safety Impacts of Queue Warning in a Connected Vehicle Environment

Queue warning systems (QWSs) have been implemented to increase traffic safety by informing drivers about queued traffic ahead so that they can react in a timely manner to the queue. Existing QWSs rely on fixed traffic sensors to detect the back of a queue. It is expected that if the transmitted messages from connected vehicles (CVs) are used for this purpose, detection can be faster and more accurate. In addition, with CVs, delivery of the messages can be done with onboard units instead of dynamic message signs and provide more flexibility on how far upstream of the queue the messages are delivered. This study investigates the accuracy and benefits of the QWS on the basis of CV data. The study evaluated the safety benefits of the QWS under different market penetrations of CVs in future years. Surrogate safety measures were estimated with simulation modeling combined with the surrogate safety assessment model tool. Results from this study indicate that a relatively low market penetration—about 3% to 6%—for the congested freeway examined in this study was sufficient for an accurate and reliable estimation of the queue length. Even at 3% market penetration, the CV-based estimation of back-of-queue identification was significantly more accurate than that based on detector measurements. The results also found that CV data allowed faster detection of the bottleneck and queue formation. Further, the QWS improved the safety conditions of the network by reducing the number of rear-end conflicts. Safety effects become significant when the compliance percentage with the queue warning messages is more than 15%.

Work Zone Impact Assessment Methods and Applications

Two work zone impact assessment methods were developed and their applicability was illustrated with data from recent construction projects on the I-35 Central Texas corridor. The first method was developed for postevent analyses of the impacts of freeway construction activities, incidents, and special events. The impacts were evaluated in terms of travel times and delay. An interval estimate for maximum queue length was also provided. Travel times and speeds were obtained by Bluetooth address matching. The method has also been used for determining work zone mobility performance measures, verifying the suitability of queue warning systems, and providing feedback for future deployment decisions. Based on more than 3 years’ experience with the analysis of impacts of road construction and maintenance projects as well as some major incidents on the I-35 corridor, the Bluetooth-based postevent closure analysis tool has proved to be very cost-effective. The second method was developed for determining the best closure schedule and start time for planned work zone lane closures. The best closure start time is the one that is expected to create the shortest queue lengths and has the least negative impact on travelers. The required input includes historical traffic volumes at a point upstream of the planned lane closure and estimated work zone capacity. The work zone capacity can vary over the duration of the work zone. The method runs input–output analysis in a dual-loop framework to analyze the impact of all available closure schedule scenarios and selects the best closure start time with the shortest expected queues.

Evaluation of Operational Effects of I-66 Active Traffic Management System

In September 2015, the Virginia Department of Transportation instituted an active traffic management system on I-66 in Northern Virginia. I-66 is a major commuter route into Washington, D.C., that experiences significant recurring and nonrecurring congestion. The active traffic management system sought to manage existing capacity dynamically and more effectively with hard shoulder running, advisory variable speed limits, lane use control signs, and queue warning systems. An initial before-and-after analysis of the system’s operational effectiveness was performed with probe-based travel time data from the provider, INRIX, and used records from the active traffic management’s traffic operations center. On weekdays, statistically significant improvements were often observed during off-peak periods, but conditions did not improve during peak periods. Weekends showed the greatest improvements, with travel times and travel time reliability measures improving by 10% to 14%. Segment-level analysis revealed that most of the benefits were attained because of the use of hard shoulder running outside of the peak periods, which created additional capacity on I-66. Benefits due to advisory variable speed limits were inconclusive because of limited data.

Harmonization with Variable Speed Limits on Motorways

This paper presents an investigation and a state-of-the-art description of traffic effects of harmonization with variable speed limits. Expansion of the E4 south of Stockholm has been carried out during the years 2009-2013. The expansion includes a rearranged lane configuration within the existing road section (25 m) and traffic management systems. Previous design with two lanes and hard shoulder has been replaced with three lanes without shoulder. In addition, the road has been equipped with variable speed limits (with red ring), queue warning system with recommended speed and emergency refuge areas (ERAs). The queue warning functionality was activated in 2011 and harmonization in 2013. Reduced speed variance is an important goal for the traffic management system and is considered to both reduce rear-end collisions and the risk of capacity breakdown. German experience shows that harmonization affects capacity down to about 80 kph. English experience is that the harmonization and monitoring can reduce accidents by 40% at normal speed of 70 mph (112 kph). Germany and England decrease the speed with approximately 20 kph at the degree of saturation of 0.7 to let the flow be harmonized before the traffic density has increased too much. A measure to describe disturbed and undisturbed traffic can be the relative change in the standard deviation of the average speed for 5-minutes periods updated each minute, called CVS. In case of accidents, CVS may be twice as high as normal. The standard deviation of all vehicles is calculated for each minute and aggregated into five-minute periods. The harmonization settings on E4 south of Stockholm has been iteratively designed and ultimately set to 325 vehicles/5 min, which has worked well. The variable speed limit is reduced to 80 kph and stabilizes the flow. The harmonization indicates a potential risk of queue for the road users, which becomes more prepared if the queue warning is activated further along the road due low speed and the risk of sudden braking. The main results can be summarized as follows: • The average speed during rush hour on weekdays has increased by 2.5 kph after the installation of the traffic control system, of which 25% is assumed to be attributable to the traffic management system • The harmonization has delayed the onset of collapse. During the periods in which harmonization has not been activated but there has been a breakdown, it is considered that harmonisation would have passed the half period and then collapse would occurred. • The accidents have been reduced by half, of which 25% is assumed to depend on traffic management (harmonization, queue warning and VMS) The queue warning has worked well, but could be improved by also exploiting the so-called CVS. The measure reveals the instabilities in the flow, sometimes tens of minutes before the AID-alarm due low speeds is enabled. In rush hour when traffic is dense (> 325 vehicles/5 min) and harmonization has already been activated the CVS measure gives further information, which could be exploited to improve the queue warning algorithm.

Sketch-Level Approach to Incorporate Active Traffic Management into the Regional Planning Process

Active traffic management (ATM) investments, such as variable speed limits (VSLs), queue warning systems, hard shoulder running, and dynamic ramp metering, offer mobility, safety, and environmental benefits. ATM investments differ from conventional capacity investments in a number of ways, such as lower capital costs, shorter planning horizons, reduced right-of-way requirements, and heightened need to consider longer-term operations. As a result, agencies sometimes have difficulty in determining how to incorporate ATM into the planning process. This paper develops a conceptual framework to enable consideration of ATM investments alongside traditional capacity investments in the regional transportation planning process. Although states and metropolitan planning organizations (MPOs) already consider operational initiatives to some degree, the proposed framework offers 10 practices to strengthen regional planning for ATM initiatives. Practices include (a) linking ATM to the MPO's congestion management plan, (b) using operations-related performance measures such as delay and reliability, (c) using sketch planning techniques to estimate the safety and environmental aspects of ATM when detailed simulation studies are infeasible, (d) encouraging the inclusion of operations-related goals in local and regional plans, and (e) relating ATM to statewide policies. The framework uses a mixture of two strategies: translating ATM effects into a planning-related narrative and modifying the planning process to be compatible with ATM. The rationale is not to promote ATM as more effective than other types of investments but rather to compare ATM objectively with such investments. The framework is demonstrated with a case study of VSLs on Interstate 66 in Virginia.

Development of Active Management Screening Tool

This paper summarizes an FHWA-directed project for the development of an active management screening tool (AMST) for agencies with congestion management process planning tools. The purpose of the AMST is to help those agencies better assess the potential of active management strategies for their region within the congestion management process. Furthermore, the AMST is structured to provide beneficial information and general guidance related to active management strategies in all areas and levels of transportation planning. The tool directly links the transportation planning process with operations by providing regional managers with information about operational strategies that they may find most beneficial to include in their plan for the regional transportation network. The AMST ascertains, at the appropriate screening level, major attributes about candidate corridors that help determine the suitability and appropriateness of an active management strategy. In successive steps the AMST ascertains the best strategy and its companion support that will enhance the facility's and program's responsiveness to the mobility, safety, and environmental needs of the corridor. Active management strategies included in the tool are high-occupancy vehicle lanes, high-occupancy toll lanes, express toll lanes, nontolled express lanes, exclusive or dedicated truck lanes, exclusive transitways, temporary shoulder use on freeways, speed harmonization, queue warning, dynamic rerouting and traveler information, ramp metering, dynamic merge control, and automated enforcement.