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Publication Number: FHWA-HRT-13-048
Date: October 2013

 

Driver Expectations When Navigating Complex Interchanges

EXECUTIVE SUMMARY

Background

The focus of this project is on driver signing and marking expectations at complex interchanges. An interchange is defined as "a system of interconnecting roadways providing for traffic movement between two or more highways that do not intersect at grade."(pg. 15)(1) Some factors that contribute to interchange complexity include high mainline traffic volumes, dense spacing of ramps resulting in extensive use of auxiliary lanes, multiple lane changes needed for following a desired route, and rapid sequencing of driver decision points coupled with high driver attention demands, among other factors. Complex interchanges are often associated with uncommon vehicle maneuvers such as lane splits, lane drops, and left exits, which may be confusing to drivers.

Interchange navigation is a common driver task when traveling on freeways and highways, and it presents a range of challenges that are different from those associated with driving on continuous roads. Although there has been previous research performed on signage in general, research specifically on interchange signage has been limited. There is little consensus on a single best way to design signs for interchanges. In general, the data present an incomplete picture of guidance relevant to sign design, which suggests that a better understanding of driver expectations and actions at interchanges is required. The tasks in this project were designed to begin addressing these information needs.

Objectives

The purpose of this project was to develop a method for determining driver expectations at interchanges and to use that method to determine how these expectations affect driver behavior at interchanges that vary in level of complexity. This also included providing initial recommendations for navigation signage to aid complex interchange design.

Project Overview

This project included multiple tasks to study driver expectations at complex interchanges. The primary data collection tasks included the following:

Some activities in this project were done in collaboration with the Texas Transportation Institute (TTI), which conducted research on improving signing and markings at complex interchanges as part of a separate project. The key tasks from the current report are described in the following subsections.

Task 2-Literature Review

The objective of task 2 was to conduct a literature review of prior work on driver navigation problems and driver expectations at interchanges and intersections and identify the key issues. Task 2 focused on identifying relevant methods for investigating driver expectations for future project tasks (tasks 4 and 7). It was also used to gain an understanding of background literature on driver expectations.

Task 4-Gather Feedback from Drivers

The overall objective of task 4 was to collect qualitative information about driver expectations. Twelve focus groups in three metropolitan areas (Seattle, WA; Columbus, OH; and Washington, DC) were used to obtain driver opinions, thoughts, and beliefs about their expectations in complex interchange scenarios. In particular, the specific objectives of the focus groups were to obtain information about the sources of expectation-related problems that drivers encounter and the kinds of remedies that drivers suggest for these problems.

These objectives were addressed by having participants view dynamic scenarios comprised of video footage of a vehicle navigating a complex interchange and asking participants questions and engaging them in discussions at various points during the video drive. Each scenario began with participants being given an overview of the driving objectives for the scenario and any other relevant context (e.g., direction, etc.). The same video scenario was shown to participants three times. The first time was used to collect individual responses from participants without a group discussion. In particular, participants were shown the scenario video, which was paused at key points in the drive (referred to as "critical points"). These critical points corresponded to times or locations in which drivers needed to make key decisions, where they may have had difficulty, or where their expectations were of particular interest (e.g., after viewing a guide sign). During this pause, participants were asked questions, and they wrote down their answers individually. This process was repeated until all critical points within a scenario had been covered. In subsequent presentations of the dynamic scenario, the moderator engaged drivers in more detailed discussions of the various issues associated with each critical point.

Task 7-Collect Experimental Data about Driver Expectations at Interchanges

The objective of task 7 was to collect empirical data on driver performance given various complex interchange signage and marking alternatives. The basic approach involved using group data collection sessions to obtain information about driver lane preference responses and participants' interpretations of guide signs. A slideshow presentation showed static photographs of freeway backgrounds with overlaid fabricated interchange signs (similar to the method used by Chrysler et al.).(2) The participants were then asked specific questions about elements of the signs, lane selection, and lane permissions in relation to what was shown on the slide. A total of 183 licensed male and female drivers participated in the group data collection sessions in this study (84 males and 99 females). The ages of participants ranged from 18 to 76 years old (mean = 44.0 years and standard deviation (SD) = 15.2 years).

Findings and Conclusions

The conclusions are organized around the two primary objectives of this project, which involve answering the following questions: (1) how do expectations affect driver behavior and (2) what are the related recommendations for sign design?

How Do Expectations Affect Driver Behavior?

The activities conducted in this project provided information about several aspects of this question. These aspects are discussed in the following sections.

Definition of Driver Expectations

Based on the literature review, it was clear that the concept of driver expectations has not been thoroughly defined with specific regard to interchanges and common driver maneuvers at interchanges. However, this concept has received consideration with regard to broader driving tasks, and there are several existing definitions of driver expectations. The majority of definitions identified typically include some variation of the definition provided by Lunenfeld and Alexander that states that expectancy is "a driver's readiness to respond to situations, events, and information in predictable and successful ways."(pg. 153)(3)

Key Driver Expectations Regarding the Navigation of Complex Interchanges

The primary conclusions related to driver expectations come from the focus group discussions, which are presented in chapter 3 of this report. Some of the following conclusions are specific to individual scenarios covered in the focus groups, while others are relevant to more than one scenario:

Key Challenges to Drivers

The task analysis indicated that drivers face several key challenges in interchange driving.(4) These challenges include the following:

What are the Key Recommendations for Sign Design?

The following subsections summarize the key conclusions and findings regarding recommendations for sign design that were identified in this project.

Key Design Principles and Guidance from Existing Research

One outcome of the task 2 literature review was an initial set of key design principles. The "Results" section in chapter 2 of this report provides additional details about specific steps that can be taken to address each design principle. The initial set of key design principles includes the following:

  1. Provide adequate forward sight distance.
  2. Provide transition cues.
  3. Minimize attention-dividing conditions.
  4. Provide navigation information to address all of the driver information needs.
  5. Maintain compatibility between the interchange and the visual cues.
  6. Design to accommodate the drivers' expectations and abilities.
  7. Warn drivers of situations that may violate their expectations.
  8. Allow drivers to recover after making an error.
  9. Design for simplicity.
  10. Design for consistency and predictability.

The task 7 empirical activities yielded several findings that support the development of specific recommendations for guide sign design. The spatial organization and layout of guide sign information have an important influence on driver interpretation of signs and their expectations of upcoming interchange geometry. The most consistent finding from task 7 is that perceptual factors related to the organization of information on a sign influence how drivers interpret the sign. The focus group discussions also provided similar findings. These perceptual factors can involve the layout of informational elements, how sign elements are grouped, and/or the position of the sign on the sign bridge. The data suggest that some drivers formed strong expectations based on the arrangement of sign information in certain situations. Also, there is evidence from this same topic that drivers confused left-exit panels with exit only panels when those panels were located on the right side of a sign bridge. This suggests that the expectations that drivers had related to sign position biased their understanding of the sign. Under time-constrained driving conditions, these expectations can influence their reading of signs. Other specific findings related to the influence of perceptual factors are discussed in the chapter 4.

Understanding the perceptual factors that influence guide sign interpretation is important because they represent attributes that can be exploited to make signs more useful to drivers. However, if signs are designed without proper consideration of these factors, it could also lead to unnecessarily complicated or confusing guide signs. This is especially important for complex interchanges because signs typically communicate more information, the interchanges are more likely to be unique and unfamiliar to drivers, and the interchanges are uncommon and are more likely to involve atypical geometric elements.

Up arrows yield consistently better results than down arrows in terms of accurate driver understanding of permissible interchange movements and efficient option-lane usage. In topics 1, 2, and 4 (as part of the seven topics in task 7), up arrows led to more efficient option lane usage or better comprehension of permissible movements when compared to down arrows. Up arrows differ from down arrows in multiple ways. They often have a clearer visual alignment with a single roadway lane. The movement information can be inherent in the arrow (e.g., curved to the right for a right exit, straight for the through destination). Additionally, since the curvature of the arrow can vary according to destination direction, up arrows can be grouped with other up arrows pointing in the same direction (e.g., to indicate that both an option lane and an exit only lane lead to the same destination). Overall, these findings suggest that drivers require additional or specific information about option lanes and the movements they represent and that up arrows can be used more effectively to provide this option lane information.

An aspect of sign comprehension that was examined in this study was how drivers visually group destination information with other sign elements and particular roadway lanes. A key sign element used for communicating which information goes together is the type of separator used to divide destination labels on the sign. Three separators were investigated in this study: vertical lines, hyphens, and multiline separations. In addition to these, data suggest that the presence of an exit only panel-and more specifically, the space it occupied along the bottom of the sign-may also have acted as a visual separator. The results for the various separators include the following:

It is clear that separation cues influence how drivers associate destination information with specific lanes. Although these elements were not tested exhaustively or necessarily in isolation, these findings demonstrate the importance of the choice of separator, as each led to different expectations for the upcoming roadway, particularly when used in conjunction with an exit only panel. Additionally, the exit only panel itself deserves consideration as a separator since it appeared to influence how drivers grouped destination information with lanes.

Key Recommendations

Based on the work conducted in this project, the following recommendations were made for follow-up activities, which can be used to gain an understanding of driver expectations at complex interchanges:

 

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