Safety Effects of Horizontal Curve and Grade Combinations on Rural Two-Lane Highways

CHAPTER 1—INTRODUCTION

This chapter presents an introduction to the report, including key background information for the research, research objectives and scope, and the organization of this report.

BACKGROUND

The safety effects of horizontal curves and grades on highways have been quantified separately, but it is not currently known whether and how the safety performance of horizontal curves and grades interact. Furthermore, there are established safety effects for crest and sag vertical curves, and it is not known whether and how the safety performance of crest and sag vertical curves interacts with any horizontal curves that may be present. The research presented in this report was undertaken as part of the Federal Highway Administration (FHWA) Highway Safety Information System (HSIS) contract to investigate these issues and provide results in a form useful to highway designers and highway safety engineers.

RESEARCH OBJECTIVES AND SCOPE

The objective of the research was to quantify the safety effects of horizontal and vertical alignment combinations and to present the results in a form suitable for incorporation in the American Association of State Highway and Transportation Officials (AASHTO) Highway Safety Manual (HSM).⁽¹⁾

The scope of the work initially included horizontal and vertical alignment for the four facility types whose safety performance is addressed in the first edition of the HSM:⁽¹⁾

• Rural two-lane highways.
• Rural multilane undivided highways.
• Rural multilane divided highways.
• Urban and suburban arterials.

REPORT ORGANIZATION

The remainder of this report is organized as follows:

• Chapter 2 reviews the existing state of knowledge about the safety effects of horizontal curves and grades prior to the current research.
• Chapter 3 describes the HSIS database used for analysis.
• Chapter 4 discusses the statistical analysis approach and presents the results of the safety analysis.
• Chapter 5 details how crash modification factors (CMFs) are derived.
• Chapter 6 presents the conclusions and recommendations.