The 2009 MUTCD1 contains a standard on the application of a speed limit sign, stating “speed zones (other than statutory speed limits) shall only be established on the basis of an engineering study that has been performed in accordance with traffic engineering practices.” The MUTCD does not provide a precise definition of an “engineering study.” Further, the 2009 MUTCD2 provides guidance that “when a speed limit within a speed zone is posted, it should be within 5 mph of the 85th-percentile speed of free-flowing traffic.” FHWA subsequently offered information through a guidance memorandum clarifying that setting speed limits does not require using the 85th percentile methodology3.
The consequences of performing a substandard engineering study, or not being able to produce documentation on the study, can be severe for transportation agencies. Liability concerns are of particular interest, which can involve concerns about whether the study that sought to set a speed limit had been performed adequately and the availability of supporting documentation. Consistently performing engineering studies to set speed limits and thoroughly documenting the results of those studies should result in better driver compliance, easier enforcement, and fewer crashes. The objectives of this project were 1) to provide more precise guidelines to the NCDOT on how its engineers should conduct speed limit studies for various roadway settings and 2) to recommend ways by which the NCDOT can document those studies.
This research project led to the development a number of related products. NCDOT can use the developed research products to provide consistency to the studies that engineers conduct to support recommended speed limits. The substantive portions of this report are included as appendices:
Speed Limit Review Documentation Forms (Appendix A)
Data Collection Terms (Appendix B)
Summary of Speed-Related Research (Appendix C)
Background and Example Forms for Roadway Speed Limit Review (Appendix D)
State Speed Study Practices (Appendix E)
A speed limit study can be fully documented with the form provided in this research study (Appendix A). Further appendices provide supporting information and guidance for completing the form and determining an appropriate speed limit. To realize the benefits of a consistent and comprehensive system for studying speeds, a storage system must be used. Each study should be stored at the Division or Region office in a format and system that is efficient and manageable.