Enhancing Safety and Operations at Complex Interchanges With Improved Signing, Markings, and Integrated Geometry

Chapter 1. Introduction

The urban freeway interchange can present a very demanding environment for drivers. Drivers approaching an interchange must undertake the navigation task while workloads from the guidance and control tasks are particularly high. In addition to selecting a lane appropriate to the desired route, which typically requires lane changes, drivers experience workload demands related to conflicting traffic from lane changes associated with entering and exiting maneuvers. The combined workload of guidance and control related to collision avoidance and navigation is easily exacerbated by limited available time to make a maneuver (due to speed or short distances between critical points) and factors associated with roadway geometric design, roadway cross section, and traffic volumes and density.

Freeway interchanges with exit-only lanes, multilane exits, and so-called option lanes may be readily understood to the degree that interactions between these features do not create a situation in which road users experience a workload that results in task saturation and primacy order rejection. Primacy order rejection is defined as placing a task of lower primacy but higher complexity (such as the navigation task) ahead of a task of higher primacy but lower complexity (such as the control task). For example, when the navigation task supplants the guidance task, abrupt and erratic maneuvers may occur. Often, task saturation and primacy order changes can result in late lane changes and erratic movements near the gore that may result in crashes.

The Federal Highway Administration’s (FHWA’s) Office of Safety Research and Development is undertaking a sequence of studies intended to address safety and operational issues related to complexity within interchange areas. The purpose of this study, which is part of this sequence of studies, was to develop recommendations for signing, delineation, and geometric design that will reduce workloads at critical points approaching interchanges that exhibit a high degree of complexity. These recommendations are based on human factors (HFs) analyses and practice evaluations conducted within a framework of logical application consistency.

Using This Report

This report is divided into nine chapters. This first chapter and chapter 2 provide background information and a contemporary literature and policies summary, respectively. Chapter 3 describes the development and categorization of attributes typical of complex interchanges. Chapter 4 describes the site evaluation and selection process, basis for the field study, and input for the practices evaluation. Chapter 5 presents the practices evaluation, including information on a new method of evaluating traffic control devices (TCDs) alongside this project’s examination of contemporary practices throughout the United States and Canada.

Chapters 6 and 7 describe the simulator study and field study, respectively, detailing the study methodologies, data collection, and analyses. Chapter 8 presents the research results, which in combination with the practices evaluation in chapter 5, forms the foundation of the recommendations in chapter 9.

Approach and Outcomes

Project activities are organized into seven tasks. The tasks’ purpose is to achieve the outcomes listed in table 1. For each project objective, an X has been placed in the column corresponding to the task in which work was performed to meet the objective.

X indicates work was done to meet this objective.

—This task was not performed to meet the corresponding project objective.

As illustrated in figure 1, the project team developed a work plan for completing these objectives that included three investigation activities: a discussion framework development activity and two experimental activities. The three investigation activities occurred under tasks 2 and 3 and included the literature and standards review (part 2), the working group discussions (included in part 3), and the practices evaluation (included in part 3). The practices evaluation is an additional activity not explicitly identified as a task, but it is valuable because this evaluation helped the project team identify trends in practice and differences between agencies and within agencies, a key to understanding how existing documentation and policy influence interchange design and operations.

These three investigation activities were the predecessors to the discussion framework development activity, which included a thorough evaluation of complex interchange characteristics. This activity culminated in the identification and selection of more than 200 attributes related to interchange complexity. From these attributes, a list of research topics comprising related attributes with interactions was developed, and these research topics formed the basis for the simulator study, treatment development, and field study. The development of the attributes—the framework for the entire project’s experimental activities and recommendations formulation—was undertaken as part of task 3 and is described in chapter 3.

Source: FHWA.

Figure 1. Graphic. Project process for developing and organizing information.

The ultimate result of this process is the preparation of recommended practices and a list of pertinent considerations. The recommendations are based on research results and the application of consistency principles outlined in the practices evaluation.

Basis of Recommendations

The recommendations made in chapter 9 address technical practice issues and functional policy issues. Each recommendation is made on the basis of one or more of the tasks in this report. A description of the basis for recommendations (included in table 2) also includes indexing symbols. The purpose of these symbols is to help the user of this report readily identify the supporting work for each recommendation.

The use of the consistency principle, explained in chapter 5, helps address the shortcomings of heuristic analyses by providing a means of evaluating TCD installations using a logic model. The consistency principle, when properly applied, can identify TCD uses for which a revision to the use will improve consistency and road-user expectancy. In many cases, it will also form the basis of the suggested revision to practice. In other cases, when further evaluation is necessary to determine what type of revision to practice would be suitable, such a need for evaluation is identified.

Table 2. Basis of recommendations with indexing symbols.

Indexing Symbol	Description
	Literature and policy review (chapter 2): Input from the literature review includes agency policy manuals and memorandums; standard plans and details; and published, peer-reviewed guidance from technical support organizations.
	Practitioner input and insights (chapter 3): Practitioner input was obtained from the working group and the technical evaluation panel. This input includes preferences based on heuristics, background on policy, and practice descriptions.
	Practices evaluation with consistency principle (chapter 5): The consistency principle provides for the assessment of TCD implementations by using a logic model that considers the interaction between geometric design and TCDs.
	Simulator study (chapter 6): The simulator study was used to evaluate various signing treatments approaching interchange divergences that exhibited complexity.
	Field study (chapter 7): The field study was used to evaluate real-world driver response to signing and pavement markings in complex interchange environments in several States.

Each recommendation made in chapter 9 is classified according to six treatments. These six treatments address corresponding topics used in the development of the simulator study and field study.

Preparation for the Final Report

In previous tasks, the project team completed a literature review, prepared a comprehensive list of attributes contributing to complexity within motorway interchanges, and identified sites where potential field analysis could occur. For each of these primary work products, a report was submitted, and the completed reports are listed in table 3.

Prior to preparing this final project report, the project team submitted a preliminary research results report to FHWA that discussed the research results from literature examination, practices evaluation, field studies and data analysis, and driving simulator study and analysis. It also presented the initial draft recommendations for practice concerning guide signing, regulatory signing, pavement markings and delineation, geometric design, and agency policy and practice associated with the selected topics identified in the attribute identification process.

As the project team developed the list of attributes contributing to complexity and examined the resulting topics over the course of the practices evaluations, a definition of complexity emerged, particularly related to driver needs within complex interchanges:

Complexity occurs when the choice of more than one movement is available from a lane or group of lanes where the decision or departure points occur successively in close proximity.

This working definition addresses the key issue of interchange complexity: drivers must make decisions about lane choice and point of departure in quick succession. This definition of complexity is independent of the overall design of the interchange, the number of overall ramps, and other factors that may challenge drivers (e.g., narrow lanes, insufficient or improper use of TCDs, or inadequate roadway and traveled way delineation).