Improving the Effectiveness of Zipper Merge Lane Control in Freeway Work Zones

Savolainen, Peter T.; Gates, Timothy J.; Brown, Henry; Edara, Praveen K.; Sun, Carlos; Gupta, Gagan; Mohammadpour, Matin; Qing, Zhu; Chang, Dae Yeol · 2024 · ROSA P / Smart Work Zone Deployment Initiative

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Summary

This study investigates methods to improve the effectiveness of zipper (late) merge lane control strategies in freeway work zones, addressing the variability in driver compliance and operational efficiency. While zipper merges are known to increase capacity by encouraging drivers to use both lanes until the merge point, driver familiarity and adherence vary significantly. The research, sponsored by the Smart Work Zone Deployment Initiative and the Federal Highway Administration, aimed to assess factors influencing lane merge controls, evaluate driver perceptions, and determine optimal signage and messaging strategies to enhance safety and mobility. The researchers employed a multi-method approach comprising a literature review, a nationwide survey of state Departments of Transportation (DOTs), a road user survey across nine states, and field evaluations in Michigan and Missouri. The DOT survey, which received responses from 45 agencies, analyzed current practices regarding static versus dynamic lane merges. The road user survey assessed driver preferences, familiarity, and comfort with early versus zipper merges, as well as their reactions to various signage configurations, including Portable Changeable Message Signs (PCMS). Field studies were conducted at four freeway work zones to measure lane utilization and driver behavior under different signage conditions, using video cameras in Michigan and radar sensors in Missouri to collect nonintrusive traffic data. Key findings indicate that while 93% of DOTs use static lane merge controls, only 40% utilize dynamic merges, which are more common in urban areas. Driver surveys revealed that although drivers typically prefer early merging, compliance with zipper merges increased significantly when supplemented by PCMS. Drivers preferred signs with textual messages, either alone or combined with graphics, over graphical-only signs. Perceptions of the zipper merge’s safety and efficiency were mixed, with familiarity and comfort being strong determinants of positive views. Field evaluations demonstrated that zipper merges improved capacity utilization, particularly at higher traffic volumes. Specifically, installing an upstream PCMS displaying "USE BOTH LANES/DURING BACKUPS" increased lane utilization by 2.3% to 5.1% depending on distance from the taper. A second PCMS near the taper had only a marginal additional effect. Lane utilization also correlated positively with traffic volume and density in the open lane, while heavy vehicle presence in the closed lane reduced utilization. The study concludes that outreach campaigns are necessary to improve driver familiarity and comfort with zipper merges. It recommends that if only one PCMS is available, it should be positioned approximately one mile upstream of the taper to encourage lane usage during backups. If a second sign is available, it should be placed within 1,000 feet of the lane closure to instruct drivers to take turns. These findings provide actionable guidance for transportation agencies to optimize work zone operations, enhance driver compliance, and maximize the efficiency of lane closure strategies through targeted signage and public education.

Key finding

Compliance with zipper merge strategies and lane utilization significantly increase when portable changeable message signs are used as supplementary devices, particularly when positioned approximately one mile upstream of the taper.

Methodology

mixed_methods

Provenance

The full processing record for this entry. Every stage of this paper's journey through the pipeline is logged — what ran, with which tool and model, how many attempts it took, and when it last completed. Discovered via bulk_ingest_rosap on 2026-05-23 (6 acquisition events logged).

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

Topics

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• work zones
• perceptual countermeasures