Researchers
Peter Savolainen
Timothy Gates
Henry Brown
About the research
The late merge, or “zipper” merge, has become a common strategy to increase work zone capacity by encouraging drivers to stay in their lanes until they reach a defined merge area, where they alternately merge. While the zipper merge has been shown to provide improved operational performance, there is considerable variability in driver familiarity and behavior when encountering the zipper merge. In addition to determining where and when merging should occur, agencies also determine whether lane merge control is static or dynamic. This study provides insights into the use and efficacy of various types of lane merge control strategies. The research included a literature review, a state department of transportation (DOT) survey, a road user survey, and field evaluations conducted at several freeway work zones in Michigan and Missouri. The state DOT survey found that 93% of agencies use static lane merge compared to only 40% that use dynamic lane merge, the latter of which is more widely used in urban freeways than rural freeways. Various factors are considered when deciding whether to use a dynamic lane merge, including annual average daily traffic, peak hour volumes, and duration of work. The road user surveys showed that drivers would typically merge closer to the taper under zipper merge lane control compared to early merge; however, compliance with this strategy increased significantly when a portable changeable messages sign (PCMS) was used as a supplementary device. Drivers also indicated that traffic signs with textual messages, either with or without supplementary graphical messages, were preferred over graphical-only signs. Driver merging behavior was also found to vary depending upon both the merging strategy (i.e., early versus late/zipper) and the vehicle’s location with respect to the start of the taper. Interestingly, there was significant variability in respondents’ perceptions of whether the zipper merge impacts traffic safety and operations. Familiarity and comfort with the zipper merge were strong determinants of driver behavior and perceptions. The results suggest that outreach campaigns may help to raise awareness of the zipper merge. The field evaluations showed that the zipper merge tended to result in better use of available capacity. However, at low volumes, drivers tend to merge earlier without any adverse impact on operations. For example, the M-53 study location (average volume of 940 vehicles/hour) had utilization rates of less than 15% on average but did not experience any substantive negative impacts on operations. The field evaluations also assessed the impact of varying the location of the PCMS within the work zone, including near the taper and one mile upstream of the taper. It is recommended from the field evaluations that if only a single PCMS is to be used, it should be positioned nearly one mile upstream of the taper displaying USE BOTH LANES/DURING BACKUPS. If an additional PCMS is available, it is recommended to be positioned within 1,000 ft upstream of the lane closure displaying MERGE HERE/TAKE TURNS.