Field Experiment of Variable Speed Advisory (VSA)

Introduction and Problem Statement

VSA Project Overview

Traffic flow through a freeway corridor is primarily determined by the limits on flow established at its bottlenecks. To maximize traffic flow, it is necessary to guide traffic into the bottleneck at a rate that reduces or mitigates congestion and shockwave development, upstream of this bottleneck.

The most effective flow maximization approach is based upon the current conditions of traffic and whether or not congestion has already been established. When traffic is not yet congested, speed harmonization initiated upstream of a bottleneck can effectively delay the start of traffic congestion.  Under congested conditions, the ideal goal is to maintain a stable flow in the corridor. This is accomplished by managing the traffic flow rate, upstream of the bottleneck, at or below the bottleneck capacity flow. If this can be achieved, traffic queues or shockwaves propagating upstream of the bottleneck will be reduced or avoided, which produces safety benefits. However, achieving this goal is rather challenging. Due to significant differences in driver  behavior, demands from the upstream mainline and on-ramps and off-ramps, the traffic itself tends to be inhomogeneous. Further complicating matters, a freeway corridor usually has multiple bottlenecks, which establishes additional constraints on the selected approach.

Variable Speed Advisory (VSA) is an Active Traffic Management strategy with the goal of affecting driver behavior on freeway mainlines. VSA strategies have the potential to manage and minimize congestion in corridors that have bottlenecks, by dynamically managing traffic speed. In this project VSA was temporarily deployed on a frequently congested highway in southern California, which included bottlenecks as well as an active construction project.

Research Team

The principal investigator for this project was Dr. Xiao-Yun Lu of California PATH. Other team members from PATH included John Spring, Cheng-Ju Wu, David Nelson, and Yuheng Kan. 

The PATH project team wants to acknowledge the leadership of the Caltrans Project Manager Mr. Hassan Aboukhadijeh, of Caltrans' DRISI (Division of Reserach), and the members of the project panel for their guidance and strong support during the execution of the project. In particular, the PATH team wants to acknowledge the strong technical advice and continuous facilitative support from Caltrans' D11 TMC Traffic engineers: Mr. Fred Yazdan (retired), Mr. Shahin Adibi, and Mr. Anush Badii are gratefully acknowledged. We also would like to acknowledge the efficient and timely help of the engineering staff at TrafficLogix in LaSalle, Quebec. In particular, they changed their product software to meet the project requirements.

Partners

The Caltrans/UC Berekeley PATH Variable Speed Advisory Project is a joint project sponsored by the Caltrans Division of Research, Innovation and System Information (DRISI) and Caltrans District  11 (San Diego). It is developed and managed by the University of California at Berkeley Institute for Transportation Studies (ITS), Partners for Advanced Transporation Technology (PATH). The project was funded by the California Department of Transportation (Caltrans) Division of Research Innovation and System Information (DRISI) under Contract Number 65A0587.  

ObjectiveVariable Message Sign

The objective of this project was to determine the potential for Variable Speed Advisories (VSA) to improve mobility in a congested corridor, as well as determine the potential for an improvement in safety as well. This project was conducted through a field test, on State Route 78 Eastbound (SR-78E) in Caltrans D11. This site was selected through judicious traffic data analysis, as described in the final report. The image above shows the system scope, road geometry, planned VSA sign locations and construction area. 

Status, Conclusions and Recommendations

Field testing a new technology in active traffic presented multiple challenges to the project team.

The first challenge was the development of a system that could be deployed on the side of highway which, through automation, would communicate speed advisories based on realtime traffic information.

The second challenge was the deployment of the VSA system in a manner that did not negatively impact traffic.

The third challenge was to engage the traveling public in a manner that resulted in voluntary participation in complying with variable speed advisories.

And the final challenge was to objectively evaluate the resulting data.

Thefinal reportdocuments the field test of Variable Speed Advisory (VSA).  After different stages of the system development, integration, and installation process were completed, a progressive test procedure was executed to mitigate any potential negative impacts on traffic operation.  This procedure included dry-runs (saving data for analysis without roadside display), error detection, system tuning, preliminary testing, and extensive tests for data collection for four weeks.  The results of the performance analysis, conducted with an independent Performance Measurement (PeMS) data set, illustrated an improvement in all three performance measures. During the AM (6-9AM) peak hours: Vehicle Miles Traveled (VMT) increased by 2.72%; Vehicle Hours Traveled (VHT) decreased by 6.28%, and the average speed over the road segment or Q=VMT/VHT increased by 8.71%.  In PM peak hours (2-7PM), two of the three performance measures improved: VMT did not have noticeable improvement; VHT decreased by 1.47% on average;  and Q increased by 2.80% on average. On SR-78E the PM peak hours experience higher traffic demand, as well as a greater percentage of non-commuters. These two factors may have had some influence in the lower PM performance improvements. As for driver compliance, it gradually improved as the test progressed and the increase in driver compliance was generally in line with an improvement in system performance.

Related Content

The California Variable Speed Advisory Project web site

Further Reading 

Field Experiment of Variable Speed Advisory (VSA) Final Report

Caltrans Fact Sheet: Variable Speed Advisory

X. Y. Lu, D. J. Chen, and S. E. Shladover, 2014 Preparations for Field Testing of Combined Variable Speed Advisory (VSA) and Coordinated Ramp Metering (CRM) for Freeway Traffic Control, Final Report, UCB-ITS-PRR-2014-1, Jan 2014, DOI: 10.13140/2.1.3612.5442

X. Y. Lu, C. J. Wu, J. Spring, and S. E. Shladover 2017, Field Test of Coordinated Ramp Metering, California PATH Research Report, UCB-ITS-PRR-2017-01