Impacts Assessment of Dynamic Speed Harmonization with Queue Warning: Task 3, Impacts Assessment Report

Dowling, R.; Skabardonis, A.; Barrios, J.; Jia, A.; Nevers, B. · 2015 · ROSA P / United States. Department of Transportation. Federal Highway Administration. Office of Operations

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Summary

This report assesses the impacts of a prototype system combining Dynamic Speed Harmonization (SPD-HARM) and Queue Warning (Q-WARN), components of the Intelligent Network Flow Optimization (INFLO) bundle. The research aims to evaluate how these connected vehicle applications affect traffic safety, efficiency, and reliability under various market penetration levels and operational conditions. The study was motivated by the need to understand the trade-offs between reducing speed shockwaves for safety and potential reductions in average freeway speeds. The assessment employed a dual-method approach: extensive microsimulation analysis and a small-scale field demonstration. The simulation utilized a VISSIM model of the US 101 freeway corridor in San Mateo, California, calibrated with historical traffic, weather, and incident data. The experimental design tested various response rates—combining market penetration, communication loss, latency, and driver compliance—to address key questions regarding deployment timing, communication methods, and required penetration levels. Additionally, a small-scale demonstration was conducted on a 23-mile stretch of I-5 in Seattle, Washington, involving 21 connected vehicles equipped with both Dedicated Short-Range Communications (DSRC) and cellular networks to validate system functionality and driver feedback in real-world conditions. Simulation results indicated that the prototype significantly reduces the magnitude of speed drops (shockwaves) between vehicles, even at a 10% market penetration level, thereby lowering the probability of collisions where free-flowing traffic meets queue backs. However, this safety benefit comes with a trade-off: the system increases the geographic impact of bottlenecks by expanding the upstream distance affected by congestion. Consequently, average freeway speeds decreased by up to 20%, with the most significant impacts observed at a 50% connected vehicle level. The prototype also increased lane-changing frequency, likely due to speed differentials between informed connected vehicles and uninformed non-connected vehicles. Benefits increased rapidly within the first 20% of fleet response rate, after which gains diminished. The small-scale demonstration confirmed system stability, showing no evidence of data loss or algorithm disruption during switches between DSRC and cellular networks. The findings suggest that SPD-HARM and Q-WARN offer substantial safety improvements by mitigating shockwaves, though they may reduce throughput and average speeds in congested scenarios. The study concludes that while the system is effective at low penetration levels, widespread deployment requires careful consideration of the trade-off between safety and travel time reliability. The results provide a basis for future deployment strategies, highlighting the importance of communication robustness and the diminishing returns of benefits beyond initial market adoption thresholds.

Key finding

The prototype significantly reduces speed drop magnitudes at 10 percent market penetration but increases upstream congestion impact and average speed reductions of up to 20 percent.

Methodology

simulator

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• traffic density
• speed choice