Defined as a method to improve traffic flow, zipper merging offers several advantages, including reduced congestion and enhanced safety, but also faces challenges with driver education and cultural acceptance. Join traffic modeling expert Bine Terefe as he explores the overall concept, outlines its advantages and disadvantages, and presents additional considerations for implementation.
Zipper merging is a traffic management technique that allows drivers to use both lanes of a road right up to the point of lane closure where they take turns merging into the open lane. The name comes from the resemblance of the alternating merge to the “teeth” of a zipper. It is also known as late merge or dynamic late merge.
This form of merging is most effective in situations where there is a high volume of traffic and lane closure due to roadwork, accidents, or other incidents. It is especially beneficial when traffic volumes are close to roadway capacity. Zipper merging can also be applied to other scenarios where two lanes must merge into one, such as highway ramps, bridges, tunnels, or toll booths. However, it is not recommended for low-traffic situations, where drivers can merge early and smoothly without causing delays or conflicts.
Zipper merging has several benefits for both drivers and traffic flow, including:
The zipper merge technique is not a perfect solution for every traffic situation. Some possible drawbacks are:
The zipper merging technique was evaluated and compared to a non-zipper merger using detailed microsimulation models for various roadway scenarios. Microsimulation is a computer-based tool that simulates the behavior and interaction of individual vehicles and drivers within a traffic network. This method helps assess the capacity of zipper merging by modeling traffic conditions, lane closure configurations, and driver responses to the technique.
The microsimulation modeling was performed using PTV Vissim 2024 (SP05), an industry-leading traffic microsimulation software which includes features that enable precise modeling of zipper merging scenarios.
Various roadway types, as shown in Table 1, were modeled to evaluate the impact of zipper merging under different roadway conditions.
The microsimulation models for each scenario were run 10 times with different random speeds. Key performance indices (KPIs) were averaged to ensure robust evaluation results. The KPIs used to evaluate and compare merging scenarios included:
The evaluation of zipper merging covers travel segments of approximately 1 mile (5,283 ft) for urban low-speed and high-speed roadways and 1.65 miles (8,747 ft) for freeway merging segments. The microsimulation analyses results summarized in Table 2 consistently demonstrated reduced travel times for zipper merging scenarios compared to early (without-zipper) merging alternatives across all types of roadways. For low-speed urban roadway types, zipper merging reduces travel time through lane closure segments by 16%.
In high-speed urban roadway conditions, it achieves a 42% reduction compared to early (non-zipper) merging situations. For freeway merging segments, zipper merging reduces travel time by 54% compared to non-zipper scenarios, leading to significant increases in average operating speeds through lane closure sections across all roadway types. Figures 2, 3, and 4 depict speed heatmaps for low-speed urban, high-speed urban, and freeway merge segments, illustrating the progressive buildup of queues upstream of the lane closure location.
The evaluated scenarios demonstrate the highly effective role of zipper merging in minimizing upstream queue development at lane closures. In low-speed urban road sections, zipper merging reduced queue length by 22% compared to non-zipper merging scenarios. The impact of zipper merging is particularly pronounced in high-speed urban roadways and freeway merging segments. Results from Table 4 show complete elimination of queues when zipper merging is applied on high-speed urban road types. In freeway scenarios, zipper merging has reduced queues by 87% compared to early merging without zipper implementation.
Volume throughput quantifies the number of vehicles passing through a congested roadway section, reflecting its operational capacity under the proposed road closure. Microsimulation results indicate that zipper merging enhances traffic volume throughput in lane closure segments. Specifically, freeway segments saw an 8.5% increase in volume throughput, while high-speed urban arterials experienced a 2.7% rise. The impact on low-speed urban arterials was comparatively minimal. Figure 5 illustrates volume throughput comparisons between zipper and non-zipper merging scenarios across different roadway types
When tested in detailed microsimulations on various roadway types, zipper merging has delivered impactful results. It shows reduced travel time through the lane closure section by 16% for low-speed urban roads, 42% for high-speed urban arterials, and 54% for freeway merge segments, compared to the early (non-zipper) merging approach. It also shows reduced queue length by 22% for low-speed urban roads, 100% for a high-speed urban arterial, and 87% for a freeway merge segment, compared to the early (non-zipper) merging approach, further improving volume throughput on high-speed urban roads and arterials.
These results present a compelling case for optimizing traffic flow and safety during lane closures. When implemented correctly and with public awareness, zipper merging can significantly reduce congestion, improve travel times, and enhance safety for drivers and road workers. It is a technique that requires cooperation and understanding among drivers, and its success is dependent on proper education and signage while understanding the importance of considering specific road conditions and traffic patterns to ensure the best outcomes. Ultimately, this approach is a smart traffic management strategy that, when embraced, can lead to smoother and safer driving experiences.
This is not a one-size-fits-all approach. It requires careful planning, design, and evaluation to ensure its suitability and effectiveness for different road environments and traffic patterns. Here are some considerations:
Binebeb Terefe, PE*, PTOE**, is a senior traffic engineer and modeling specialist with extensive traffic and modeling expertise. He has a hands-on approach to delivering strategic travel demand and micro-simulation projects across the globe. With a comprehensive background in freeway and urban corridor optimization, his skills extend to travel demand modeling, traffic impact assessments, transportation master plan studies, and corridor traffic operations and analyses.
*Registered Professional Engineer in CO, TX