Reducing lane and shoulder width to permit an additional lane on a freeway a technical report.

As the demand for additional capacity on urban Texas freeways continues to increase, there is a need to better understand how to optimize the lane and shoulder configurations to improve capacity without adversely impacting the operating speed or increasing crashes. The overall objective for this research effort was to identify the operational and safety implications of using reduced lane and shoulder widths for a variety of freeway configurations. To effectively assess the influence of the geometric features, the operational analysis only incorporated uncongested speed data. The research team used detailed speed data, acquired from a variety of sensors located along freeway corridors in Dallas, Houston, and San Antonio. In addition, the evaluation of the safety implications of narrowed freeway lane and shoulder widths used crash data that extended from 2010 to 2013. Using exploratory analysis and statistical evaluations, the research team ultimately assessed how influential the various lane and shoulder widths can be on the overall corridor. This report includes a literature review (Chapter 2) of operational and safety studies related to freeway lane and shoulder widths. Included in the literature review is a summary of typical lane and shoulder width values followed by a review of the studied operational effects of lane and shoulder widths. Similarly, the corresponding safety effects are then summarized. In some cases, the operational and safety effects have been jointly evaluated, so the literature review concludes with a summary of this literature. Chapter 3 identifies the candidate data collection elements followed by a review of site identification and selection for this study. The detailed analysis of operational and safety effects of freeway lane and shoulder widths are then included in Chapter 4 and 5, respectively. The report concludes with a summary of findings (Chapter 6). This concluding chapter also highlights the individual findings and presents equations that can be used to assess unique lane and shoulder width configurations. A companion spreadsheet is available to simplify these calculations. (Author/publisher)