Driver Merging and Lane Utilization Behavior under Zipper Merge Lane Control

Work zone lane closures require drivers to determine when and where to merge from a lane that is about to close to an adjacent open lane. The “zipper merge” strategy encourages drivers to stay in their lanes until reaching a defined merge area, where drivers alternate merging, analogous to a zipper. The zipper merge has been shown to provide better operational performance than the standard “early merge” scenario, reducing congestion and speed differentials between lanes. However, there is considerable variability in driver familiarity and behavior when encountering a zipper merge. To that end, this study evaluated the impacts of using supplemental portable changeable message signs (PCMS) to encourage drivers to follow the zipper merge strategy. Driver behavior was assessed through a series of field studies that compared vehicle lane utilization and merging behavior. The utilization rate of the soon-to-close lane was evaluated under three scenarios, which included (1) standard static zipper merge signage; (2) static signage with a PCMS displaying “USE BOTH LANES DURING BACKUP” upstream of the merge area; and (3) standard signage with two PCMS, with the second (downstream) sign displaying “MERGE HERE/TAKE TURNS” close to the start of the taper. A regression model was estimated to further investigate lane utilization while accounting for the effects of traffic volume, density, and the presence of heavy vehicles in the open and soon-to-close lanes. Installing an upstream PCMS improved lane utilization compared to static signs alone; however, the additional PCMS closer to the taper showed negligible impacts on lane utilization.