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Publication Number:  FHWA-HRT-17-098     Date:  January 2018
Publication Number: FHWA-HRT-17-098
Date: January 2018


Self-Enforcing Roadways: A Guidance Report


Reducing the number of fatal and injury crashes in the United States is a high priority among Federal, State, and local transportation agencies. Crashes occurring in rural areas comprise most of the total crashes, many of which can be attributed to speeding. Due to the substantial number of speeding-related crashes in rural areas, traffic safety improvement programs should focus on crash-reduction strategies in these rural areas, with the most effective speed-management programs focusing on reducing speeding-related crashes on moderate- and high-speed, two-lane rural highways. Therefore, this guidance document focuses on methods to mitigate speeding-related crashes on two-lane rural highways with posted speed limits of 35 mph (56.4 km/h) or greater.

Because speeding is a complex problem that involves the interaction of many factors, successfully mitigating speeding-related crashes requires integration and coordination among engineering, enforcement, and education. From an engineering perspective, a design concept referred to as “self-enforcing roadways” has been developed to guide appropriate road-user behavior. 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. To affect speed compliance, designers typically use geometric elements that encourage drivers to select operating speeds that are appropriate for the intended purpose of the roadway. The ideal is for operating speeds and posted speed limits to be in harmony with the roadways geometric design speed.

This guidance report identifies methods that may produce self-enforcing, or self-explaining, roadways during the geometric design process. While safety performance associated with these methods is not yet well understood, an implied outcome of effective speed management is that less severe crashes will result via the application of self-enforcing, or self-explaining, road-design principles. This report identifies and describes six self-enforcing road concepts and the processes needed to implement these concepts when designing or evaluating existing two-lane rural highways. It is anticipated that the concepts may be used to design roadways that produce operating speeds consistent with the desired operating speeds of the roadway. The six concepts, further described below, include (1) the speed feedback loop process, (2) the inferred design speed approach, (3) design consistency methods, (4) the application of existing geometric design criteria, (5) the combination of signs and pavement markings, and (6) the setting of rational speed limits:

  1. The speed feedback loop process requires checking for consistency among geometric design elements and anticipated operating speeds to determine an appropriate posted speed limit. Use of a speed feedback loop during the design process involves speed prediction during preliminary engineering and compares the expected operating speed to the geometric design speed. A desirable outcome occurs when the expected operating speed is equal to, or nominally lower than, the designated design speed.

  2. The inferred design speed approach evaluates the geometric design of a roadway by determining the maximum speed for which all design speed-related criteria are met. The inferred design speed is then compared to the anticipated or measured operating speeds and the posted speed limit using graphical methods.

  3. Design consistency methods, such as the application of operating speed prediction models, may be used to establish posted speed limits based on predicted operating speed. Operating speed models aid in determining the effects that geometric design features have on driver speed choice along a roadway. Design consistency methods can be applied either manually, using a series of equations, or using a computer model, such as the Design Consistency Module of the Federal Highway Administrations (FHWA) Interactive Highway Safety Design Model. (FHWA 2016a)

  4. The application of existing geometric design criteria includes the use of features with a known relationship to operating speed. Understanding how the design values affect speed, including the spatial relationships between different geometric elements, may affect driver speed choice along an alignment and promote operations that are consistent with the posted speed limit.

  5. The combination of signs and pavement markings can be used to manage speeds on existing roadways when major reconstruction of the road is not feasible. Traffic control devices, such as signs and pavement markings, communicate information to drivers regarding speed choice and may encourage drivers to select speeds that are more in harmony with the posted speed limit on the roadway. Many signs and pavement markings have been evaluated and are associated with driver speed choice. Several examples of these signs and pavement markings that are known to reduce operating speeds on two-lane rural highways are described in this report.

  6. The setting of rational speed limits consists of choosing a posted speed limit that is reasonable, rational, and consistent with the features of the roadway. A Web-based tool that can be used to provide guidance regarding appropriate posted speed limits is the FHWAs USLIMITS2. (FHWA 2016b) The program determines rational speed limits using an expert system, which includes a series of decision rules and procedures that are applied based on user input.

These methods can be applied individually or in combination for planned and existing two-lane rural highways. For new or major reconstruction activities, the speed feedback loop process, the inferred design speed approach, the design consistency method, and the application of existing geometric design criteria should be considered the most effective self-enforcing, or self-explaining, design concepts. These methods should be incorporated early in the planning and design process to ensure that roads are designed to produce operating speeds similar to the intended operating speeds on the roadway. However, when major reconstruction activities cannot be—or are not—planned to occur on a roadway, existing roadways can be retrofitted with a combination of signs and pavement markings to produce a self-enforcing, or self-explaining, roadway. The setting of rational speed limits may be considered for both planned and existing roadways.

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