Zipper UP – Lane Merge Design – Part 2

by Tracy Zafian, Research Fellow

merge

The March Innovative Outlook (IO) discussed the first part of a University of Massachusetts (UMass)-Amherst study that evaluated different signage options to encourage more zipper merging when two lanes of traffic are merging into one. Here we discuss the second part of the study, involving testing the first phase results on a full-immersion driving simulator to analyze driver behaviors and decision-making in different scenarios where two lanes are merging into one.

The simulator evaluation was presented briefly at the Road Simulation and Safety Conference in the Netherlands in October 2017, and additional results were shown at the annual Transportation Research Board meeting in Washington, D.C. in January 2018.

Ideally, for zipper merges, similar levels of traffic occupy the left and right lanes approaching the merge. The vehicles from both lanes then take turns moving into the single lane, alternating from the left lane and then from the right lane, vehicle by vehicle, as in the two sides of a zipper coming together. For the first phase of the UMass study, drivers were surveyed about their perceptions and preferences of different road signs for a merge ahead and how they respond as drivers when shown different signs. One of the signs in the study was the W4-2 sign, also known as the “Lane Ends” sign, defined in the Manual on Uniform Traffic Control Devices (MUTCD); it’s the top sign in the figure. The other signs had been used in previous signage studies conducted by the Federal Highway Administration’s Human Factors Laboratory.

Based on the results of the earlier driver surveys, three different merge signs were used for the simulator part of the study: the standard W4-2 sign, a sign showing an alternative merge graphically, and a sign with “Alternate Merge” in words. There were 12 different scenarios tested on the simulator. In addition to varying the signage between the scenarios, two other variables were changed as well: which lane the driver’s vehicle started in at the beginning of the scenario (left lane or right lane) and the surrounding traffic conditions (vehicles in front of or adjacent to the driver’s vehicle). After the simulator drives, study participants were given a questionnaire regarding their perceptions of the different merge signs.

Not unexpectedly, since the standard merge sign (W4-2) is already in use, the standard merge sign had the strongest results in terms of driver recognition and comprehension. Drivers were found to be most likely to make lane changes upstream of the merge in the simulator scenarios with the standard merge sign. At the same time, however, the questionnaire results indicated that the standard merge sign was the least preferred sign to promote even merging from the left and right lanes. Another result, which differed from the results of the earlier driver surveys, was that the “Alternate Merge” sign with words was no longer among the most preferred signs for promoting even merging. Some participants in the simulator study felt that the “words were harder to process than pictures” and that the sign has confusing wording. The majority of participants preferred the graphic alternative merge sign for promoting even merging.

Two other interesting results were seen across the simulator scenarios: (1) participants were much more likely to switch lanes upstream of the merge intersection when they were following vehicles that had already merged than when they were adjacent to other vehicles, and (2) participants were more likely to switch lanes when their vehicle started in the right lane compared to the left lane. This second finding likely reflects the participants’ familiarity with merging into traffic from the right lane and is influenced too by the standard merge sign currently in use.

Overall, there were no significant changes in driver behavior upstream of the merge intersection.  Still, a graphic alternative merge sign could have promise for encouraging more even, zipper merging, once drivers become more familiar with them. Additional study, potentially including field experiments, is needed to evaluate further the potential of alternative merge signs to improve traffic flow and safety and reduce traffic congestion at merge locations.

For additional information on this simulator study, you can contact graduate student Francis Tainter at ftainter@umass.edu. The simulator study has been accepted for publication in the Transportation Research Record. 

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