Factors Influencing Operating Speeds and Safety on Rural and Suburban Roads

CHAPTER 1. INTRODUCTION

United States traffic safety statistics show that fatalities resulting from motor vehicle crashes decreased from 2005 through 2011, but increased slightly in 2012. Despite these encouraging trends, the National Highway Traffic Safety Administration’s (NHTSA) Fatality Analysis Reporting System reported approximately 5.6 million traffic crashes in 2012, including 33,561 fatalities and more than 2.3 million injuries.⁽¹⁾ Rural fatalities accounted for 54 percent (18,170) of all traffic fatalities in 2012, although 19 percent of the U.S. population lived in rural areas. Further, in 2012, the fatality rate per 100 million vehicle mi traveled was 2.4 times higher in rural areas than in urban areas. This disparity between rural and urban road fatal crash rates has not changed appreciably in more than a decade.⁽²⁾

NHTSA defines speeding-related crashes as “the driver behavior of exceeding the posted speed limit (PSL) or driving too fast for conditions,” which has been cited as a contributing factor in approximately 31 percent of traffic fatalities in the United States. ⁽³⁾ This same research shows that a significant percentage of crashes occurring at night and on rural roads are speeding related. Speeding-related factors were also prevalent among single-vehicle run-off-road crashes and crashes on horizontal curves. It should be noted that speeding-related crash statistics that are compiled and reported annually by NHTSA rely on databases consisting of electronically coded police accident reports. Therefore, an element of subjectivity (i.e., the reporting officer’s expert opinion of whether speed was a contributory cause of a crash) is present. The extent of reporting bias is unknown.

The relationship between operating speeds and safety is not entirely clear. There is no consensus regarding the association between speed and crash frequency; however, there is agreement that crash outcomes will be more severe as speed increases. Therefore, it is important to consider how roadway and roadside features influence driver speed choice. Roadway and roadside features provide cues to drivers that influence their speed selections. Supplemental measures, such as traffic control devices and speed management strategies, are sometimes used to influence driver speed selections to be more consistent with target operating speeds. Designing a roadway to influence drivers to travel at a particular “target speed,” while discouraging them from traveling at an excessive or inappropriate speed, may help to prevent speeding-related crashes or reduce their severity when they do occur. In such cases, free-flow operating speeds near design speeds and posted speeds are likely correlated with community goals and expectations. The Federal Highway Administration’s (FHWA) Speed Concepts: Informational Guide reviewed a number of speed management practices and technologies and describes speed-based technical processes that often lead to operating speeds higher than design speeds and PSLs. ⁽⁴⁾ The FHWA suite of speed management resources also includes Engineering Countermeasures for Reducing Speeds: A Desktop Reference of Potential Effectiveness and Speed Manb agement: A Manual for Local Rural Road Owners. ^(5,6)

This report builds on these previous efforts by further exploring technical speed-related design processes and their speed outcomes as well as highway and traffic engineering features that influence operating speeds and safety on rural and suburban roads.

SCOPE OF RESEARCH AND REPORT ORGANIZATION

The objective of this project was to develop a technical report that describes features and treatments that influence operating speeds on curve and tangent sections of rural and suburban roads. The research team gathered information for the report from a review and critical synthesis of published literature as well as new research conducted as part of this project.

The remainder of this report is organized into the following chapters:

• Chapter 2 provides background and the results of a synthesis of literature related to the characteristics of rural and suburban roads, speed-related outcomes of current design and operational practices, and traffic engineering treatments, as well as design and retrofit decisions and practices that influence operating speeds.
  
  
• Chapter 3 describes the process of defining the scope of the research conducted as part of this project.
  
  
• Chapter 4 describes an operational evaluation of the high-friction surface treatments (HFST) applied to horizontal curves on rural two-lane roads in West Virginia.
  
  
• Chapter 5 documents a speed evaluation of optical speed bar (OSB) treatments applied to rural and suburban road segments in Arizona, Alabama, and Massachusetts.
  
  
• Chapter 6 presents an observational, cross-sectional study on the safety effects of lane width and shoulder width combinations on rural two-lane, two-way road segments.
  
  
• Chapter 7 summarizes the entire research effort, provides general conclusions, and outlines general considerations for future studies.