Self-Enforcing Roadways: A Guidance Report

CHAPTER 1. INTRODUCTION

Reducing the number of fatal and injury crashes in the United States is a high priority among Federal, State, and local transportation agencies. The number of fatal crashes has declined from 36,254 (40,716 fatalities) in 1994 to 29,989 (32,675 fatalities) in 2014. (National Highway Traffic Safety Administration (NHTSA) 2015) Based on 2013 crash statistics, approximately 53 percent of the fatal crashes occurred in rural areas. The fatal crash rate in rural areas is 1.88 per 100 million vehicle-miles traveled, which is more than 2.5 times the urban fatal crash rate of 0.73. This indicates that traffic safety improvement programs with strategies for rural areas may be especially successful in reducing crash frequency and severity.

Of particular interest among the total traffic-related fatalities in the United States are those attributed to speeding, which comprised 29 percent (9,613) of the total fatalities. Speeding-related crashes are those defined as driving too fast for conditions or exceeding the posted speed limit. (NHTSA 2015a) Among the rural traffic fatalities, 30 percent (5,346 of 17,696) were codified as speeding-related. In 2014, the percentage of speeding-related crashes that occurred on roadways with a speed limit of 25 mph (40.3 km/h) or less was 7 percent; the percentage of speeding-related crashes on roadways with a speed limit higher than 25 mph (40.3 km/h), but less than or equal to 40 mph (64.4 km/h), was 28 percent; the percentage of speeding-related crashes that occurred on roadways with a speed limit between 45 (72.5 km/h) and 80 mph (128.8 km/h) was 65 percent. (NHTSA 2015) Collectively, these data suggest there is value in effective speed-management programs to reduce speeding-related crashes on moderate- and high-speed, two-lane rural highways.

Speeding is a complex issue that involves the interaction of many factors, including public attitudes, road-user behavior, vehicle performance, roadway design characteristics, posted speed limits, enforcement strategies, and judicial decisions. To be successful in mitigating speeding-related crashes, engineering, enforcement, and education must be integrated and coordinated.

From an engineering perspective, one design concept that has been developed to successfully guide appropriate road-user behavior is known as “self-enforcing roadways.” A self-enforcing road, also called a “self-explaining roadway,” is a roadway that is planned and designed to encourage drivers to select operating speeds consistent with the posted speed limit. Road designers use geometric elements to accomplish their goal of encouraging a target operating speed. Properly designed self-enforcing roadways can be effective in producing speed compliance and may contribute to less severe crash outcomes. This report provides guidance on how to produce self-enforcing roadways; the concepts can be applied to both planned and existing roadways.

Two-lane rural highways with posted speed limits of 35 mph (56.4 km/h) or greater are the focus of this report. A significant body of published literature related to operating speeds and safety is available for high-speed roads, which are defined as roadways with design speeds that are 50 mph (80.5 km/h) or greater. A report by Liu and Chen (2009) found that speeding-related crashes are more often coded as exceeding the posted speed limit on roadways with posted speed limits of 50 mph (80.5 km/h) or less, which indicates that the guidance included in this report should cover moderate-speed roadways with posted speed limits in the range of 35 to 50 mph (56.4 to 80.5 km/h). For lower-speed roadways (posted speed limits less than 35 mph (56.4 km/h)), information from the Institute of Transportation Engineers (ITE) Traffic Calming ePrimer can be used to provide speed-management solutions. (ITE 2017)

The remainder of this report is organized as follows:

• Chapter 2 examines the relationship between speed and safety, describing the interaction of crash frequency and severity with speed.
  
  
• Chapter 3 discusses the relationship between speed and geometric design, including horizontal and vertical alignments and cross-section elements. It contains the objective information used to inform many of the concepts described in chapter 5.
  
  
• Chapter 4 describes six self-enforcing road concepts and the processes needed to implement the concepts when designing or assessing two-lane rural highways.
  
  
• Chapter 5 presents two case study examples that apply several of the self-enforcing roadway concepts.
  
  
• Chapter 6 discusses implementation of the self-enforcing road concepts.

Readers interested in applying self-enforcing, or self-explaining, roadway design methods in practice are recommended to focus on chapters 4 and 5. Those interested in learning additional technical details underlying the self-enforcing roadway methods described in this report are recommended to read chapters 3 through 5.