Zipper UP – Merge Design Helps Reduce Crashes

by: Tracy Zafian, Research Fellow

zipper
Texas DOT blog

A recent study from the University of Massachusetts (UMass) Amherst evaluated the impact of various merge signs and road geometries on driver perception and behavior at merges.  Alyssa Ryan, a UMass Amherst Transportation Engineering graduate student working with Dr. Michael Knodler presented findings of the “Alternative Merge Design Downstream of Intersections,” focused on alternative merges, also known as zipper merges.  The study was discussed at the Road Safety and Simulation Conference in the Netherlands in October; additional results were presented at the annual meeting of the Transportation Research Board in Washington, D.C. in January. Discussing the importance of this research, Dr. Knodler said, “If we are able to change driver behavior to better promote alternate merging, we have the potential to improve both roadway safety and efficiency.”

The zipper merges occur when two lanes of traffic equally merge into one, rather than a standard merge from the right lane to the left, or left to right. Ideally, for zipper merges, similar levels of traffic occupy the left and right lanes approaching the merge, and then the vehicles merge into the single lane alternating from the left lane and then the right lane, vehicle by vehicle, as in the two sides of a zipper coming together. A demonstration video of a zipper merge by the Alberta Motor Association can be seen here. The UMass Amherst researchers hypothesized that “by changing signage from the traditional “Lanes End Sign” to experimental signs that promote alternative merging, drivers will be more likely to merge evenly downstream” of the lane drop location…. Even merging will present a greater balance in lane utilization, which will lead to reduced congestion and potentially improved safety.”

Traffic merges can create challenging situations for drivers and safety. The Highway Safety and Information System (HSIS), a multi-state database of a crash, traffic volume, and roadway inventory data, includes information on the driving maneuvers that result in the most truck and car crashes. In an examination of HSIS crash data for twenty-six driving maneuvers, improper merges were listed as the second most dangerous, behind inattentive driving.

The current standard traffic sign used to let drivers know that they are approaching a merge is the W4-2 sign, also known as the “Lane Ends” sign, defined in the Manual on Uniform Traffic Control Devices (MUTCD). This is the first sign on the left in the chart below. With this sign, there is no indication of how the vehicles should merge into one lane, and it also appears that the right-lane traffic will yield to traffic on the left, which is a different merging approach that is used with the zipper merge.

The UMass study had two main components. The first involved a survey of drivers regarding different potential merge signs. Drivers were shown pictures of different signs and different road-perspective views of merging road lanes. The survey asked them questions such as “In the lane drop pictured, a merge is required. With the given sign below, which lane do you prefer to approach the lane drop in?” Drivers were also shown road pictures and asked: “Knowing that you will be merging ahead, as shown, which sign would you prefer to promote EVEN MERGING?”  The different signs shown to the survey participants are in the figure here. Aside from the W4-2 sign, each of the other signs has been used in signage studies before, either at the Federal Highway Administration (FHWA)’s Human Factors Laboratory or for a study conducted in Connecticut.

zippergraph
Participant Sign Preference to Encourage Even Merging

 

The study found that with the traditional “Lanes Ends” sign, or with no sign before the merge, most survey participants indicated that they would prefer to stay in the left lane approaching the merge. With the other no-text sign options, between 30-40% of participants indicated that they had no preference between the left and right lane approaching the merge. With each of the text sign options, participants showed a preference for staying in the left lane approaching the merge though the percentages selecting the left lane or indicating no preference was almost equal for the “Alternative Merge” sign.

On the question of which sign(s) participants thought would best promote even merging, the “Lane Ends” sign was the least preferred. The “Alternative Merge” sign was the most preferred of the 8 sign options, for promoting even merging. At the same time, however, one in five (20%) of the survey respondents also rated it the least preferred.

The second part of the UMass study involved testing the most preferred signs for even merging on the UMass-Amherst advanced driving simulator. This allowed the research team to validate whether the stated preferences from the survey would match drivers’ lane choices in merge scenarios in a driving simulation environment. The preliminary results of the simulator study will be discussed more in April’s Innovative Outlook.

The findings of this research may be helpful on roadways with alternative merge configurations, either more permanent or in work zone areas where one lane is closed on a temporary basis.

For more information on this study, you can contact Alyssa Ryan by email at alyssaryan@umass.edu.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

w

Connecting to %s