United States Department of Transportation
[PDF-3.39 MB]
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Edition:May 2019–August 2022
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Abstract:Posted speed limits (PSLs) are a highly complex issue involving engineering, human factors, and political and societal concerns. On a national level, recent research along with calls to change how speed limit are set, especially for city streets, have generated extensive discussion on future speed limit setting procedures. Within this TxDOT project, the Research Team conducted dialogs with TxDOT districts to learn about the practices and procedures being used, developed a number of products designed to increase the understanding of operating speed and of PSLs, and performed new research into operating speed relationships with roadway characteristics. The developed communication products include videos (one for engineers and one for the public), a pamphlet for public distribution, answers to common questions about speed and speed limits, and a workshop on state and national speed limit setting practices. Review of the literature along with this study’s evaluations confirmed that an increase in the PSL is typically associated with a smaller magnitude increase in operating speed and that higher operating speeds are associated with higher PSLs. Freeway geometric factors, including left and right shoulder widths and distance to downstream and upstream ramps by ramp type (entrance or exit), demonstrated to have a significant impact on operating speed. This study estimates that a strategy that entails modifying geometry, changing the PSL, and varying the level of law enforcement presence within the ranges included in this study may impact freeway operational speeds up to 6.2 mph (depending upon existing conditions along with the changes in the geometry, PSL, and enforcement). An evaluation of speeds on a sample of Texas non-access-controlled highways used speed study data collected by TxDOT and the Research Team within the past 20 years. For all development levels, speeds increased with outside shoulder width and overall roadway width. Suburban roadway factors resulting in reduced operating speed included driveway/access point density, signal density, presence of sidewalk, and higher traffic volumes or truck percentages.
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