Driver Expectations When Navigating Complex Interchanges

Chapter 1. Introduction

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 includes providing initial recommendations for navigation signage to aid complex interchange design.

Background

The focus of this report 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 the following:

• High mainline traffic volumes.
• Dense spacing of ramps resulting in extensive use of auxiliary lanes.
• System (freeway to freeway) interchanges with service (freeway to roadway) interchanges closely adjacent to and/or nested within the functional limits of the system movements.
• Multiple lane changes needed for following a desired route.
• Rapid sequencing of driver decision points coupled with high driver attention demands.
• An egress-type movement to the left or right for continuing on the designated route.
• Maneuvers that may be contrary to driver expectations or directional intuition.

Complex interchanges are often associated with uncommon vehicle maneuvers, such as lane splits, lane drops, and left exits, which can cause confusion among 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. One problem with interchanges is that drivers are confronted with time-sensitive task demands (i.e., forced-paced tasks). More specifically, drivers at unfamiliar interchanges must read the available signage, observe pavement markings, and determine a path through the interchange before they reach the gore point. As an additional source of stress, driver errors at interchanges are often more difficult to correct since drivers transfer to a grade-separated freeway, highway, or roadway which provide limited access points for their return to the original roadway. These factors emphasize the need for clear navigation signage to guide drivers and minimize errors.

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 specific objectives of this project were as follows:

• Develop a method for determining driver expectations at interchanges and use that method to determine how these expectations affect driver behavior at interchanges that vary in level of complexity.
• Develop recommendations for navigation signage to aid complex interchange design.

Project Overview

This project included multiple tasks to study driver expectations at complex interchanges. The task outcomes are summarized below with a brief description as follows:

• Task 2-Literature review: A review of prior work on driver navigation problems and driver expectations at interchanges and intersections.
• Task 3-Focus group protocol: A protocol for acquiring information from drivers about their expectations at interchanges and about the problems they encounter when navigating complex interchanges.
• Task 4-Gather feedback from drivers: A series of focus groups used to collect qualitative information about driver expectations from drivers in accordance with the task 3 protocol.
• Task 5-Task analysis: A task analysis of different interchange navigation scenarios used to provide information that, together with other task information, can be used to determine driver expectations at interchanges.
• Task 6-Data collection plan: A plan for collecting empirical data on driver performance given various complex interchange sign and marking alternatives.
• Task 7-Collect experimental data about driver expectations at interchanges: An experimental study to collect data on driver performance given various complex interchange signage alternatives in accordance with the task 6 plan.

Note that some of the activities in this project were done in collaboration with TTI, which conducted related research on improving signing and markings at complex interchanges as part of task 6.

Figure 1 shows the relationships between the project tasks (blue) and the project objectives/outcomes (orange). Tasks 2, 3, and 6 were mainly used for planning other tasks and gaining background information about the issues of interest. Tasks 4, 5, and 7 were data collection tasks.

Figure 1. Illustration. Overview of study tasks and information flow between tasks.

Each of the links in figure 1 represents a contribution that one task made to the following task. These relationships are further discussed by task in the following sections.

Task 2-Literature Review

The project began with a literature review. Through this review, a list of geometries and expectation elements that can be challenging to drivers at interchanges was created. These geometries and expectation elements were used to search for candidate scenarios in the task 3 focus group protocol. Task 2 was also used to contribute the methodology to the task 6 data collection plan. In addition, task 2 contributed directly to the study outcomes by providing a summary of existing design principles and guidance for designing roadway elements that conform to driver expectations.

Task 3-Focus Group Protocol

The scenarios developed in the task 3 focus group protocol incorporated the geometries and interchange elements associated with driver expectations identified in the literature review. These were used in addition to the complexity factors identified in the TTI project.⁽⁶⁾ The task 3 outputs included a moderator guide, dynamic scenario videos, and a response booklet for the task 4 focus groups.

Task 4-Gather Feedback from Drivers

The focus group activities provided inputs for several tasks and contributed directly to the overall project objectives of identifying driver expectations at interchanges. Specifically, the key themes related to driver navigation strategies from the focus groups led to the formulation of the driver navigation model in the task 5 task analysis. Drivers in the focus groups provided details about driver strategies for using the information that they read, filling in information that they needed, and creating new expectations. Additionally, focus group comments validated the driver activities and provided the specific scenarios and key maneuvers used in the task analysis.

Some of the driver challenges or interesting expectation trends identified in the focus groups were incorporated into the data collection plans developed in task 6. These plans also utilized some of the guide signs identified as being problematic in certain driving scenarios discussed in the focus groups.

Task 5-Task Analysis

The task analysis primarily contributed to the study objectives by providing information about how various interchange elements affect driver tasks and performance and by identifying the key driving challenges during specific interchange navigation scenarios. The results of this analysis are included in a separate report.⁽⁴⁾

Task 6-Data Collection Plan

The task 6 data collection plan was developed using a methodology identified during the literature review, and it used several of the signs that gave drivers problems during the focus group driving scenarios. Candidate topics were selected in conjunction with TTI as part of their driving simulator scenario development activities done in parallel with this project.⁽⁶⁾

Task 7-Collect Experimental Data about Driver Expectations at Interchanges

The task 7 data collection primarily contributed to the study objectives by measuring driver behavior in relation to specific sign elements and determining very specific relationships between navigation sign elements and driver decisions.

Overview of Report

This report provides a description of the tasks, activities, and results of the literature review, focus groups, and data collection activities conducted in this project. The body of this report contains the following three chapters, one for each of these tasks:

• Chapter 2: Task 2-Literature Review.
• Chapter 3: Task 4-Focus Groups.
• Chapter 4: Task 7-Collect Experimental Data about Driver Expectations at Interchanges.

The technical chapters are followed by an overall "Conclusions and Recommendations" chapter and report appendices.