Self-Enforcing Roadways: A Guidance Report

CHAPTER 6. CONCLUSIONS

Speeding is cited as a contributory factor in nearly one-third of all fatal crashes reported in the United States. A significant number of these incidents occur on rural roadways with posted speed limits that exceed 40 mph. As a result, managing speeds on two-lane rural highways is likely to be an effective safety-management strategy. Application of self-enforcing, or self-explaining, roadways is one possible approach to manage speeds. In this context, a self-explaining or, self-enforcing, roadway is defined as one that encourages driver speed choice that is compliant with the regulatory speed limit.

This guidance report offers the following six conceptual approaches to design self-enforcing, or self-explaining, roadways:

1. Speed feedback loop in the geometric design process.
2. Inferred design speed approach.
3. Application of design consistency methods.
4. Application of existing geometric design criteria.
5. Use of signs and pavement markings.
6. Setting rational speed limits.

These methods can be applied individually or in combination for planned or existing two-lane rural highways. Example implementation methods are offered in this report, including two case studies of existing two-lane rural highways.

As the application of self-enforcing, or self-explaining, roadway design concepts becomes more commonplace in the United States and elsewhere, it is recommended that future research be undertaken to evaluate the effects of these practices on speed and safety. This report considers the relationship between various speed concepts and safety based on two case studies; however, these relationships should be explored across a variety of geographic locations to determine how speed concepts relate to safety performance on two-lane rural highways. Continuous, real-time speed information is available from data sources, such as the Second Strategic Highway Research Program (SHRP2) Naturalistic Driving Study and traffic monitoring systems, that can be used to link operating speeds to safety and geometric design features. The SHRP2 Roadway Information Database is another potential source for identifying geometric design features of roadways. Research using this information would be valuable in furthering the self-enforcing or self-explaining road concepts described in this guidance report. Lastly, driving simulators offer the opportunity to better understand how drivers select their speed in a controlled roadway environment. This affords the opportunity to study how combinations of pavement markings and signs, or altering geometric design criteria, may affect operating speeds.