Further Development of the Safety and Congestion Relationship for Urban Freeways

Abstract

Strategic Highway Research Program 2 (SHRP 2) Reliability Project L07, Identification and Evaluation of the Cost-Effectiveness of Highway Design Features to Reduce Nonrecurrent Congestion, provided (1) general guidance on the range of design elements that could be used by transportation agencies to improve travel time reliability and reduce nonrecurrent congestion on urban freeways and (2) the Analysis Tool for measuring operational and safety effectiveness and calculating a life-cycle benefit–cost value. This value can be used to support decision making about the possible use of individual treatments to address actual nonrecurrent traffic conditions. The tool is a Visual Basic for Applications (VBA) interface overlaying a Microsoft-based Excel 2007 spreadsheet. Analysts can input data about a highway such as geometrics, volumes, and crash totals, and the tool computes delay and reliability indicators resulting from various design treatments and translates those results into life-cycle costs and benefits. For the safety-effectiveness analysis, a new relationship between safety and congestion was explored, and a mathematical model was developed to quantify crash frequency at various levels of traffic density. This supplemental report presents the research findings on the effort to further develop and refine the original safety and congestion relationship model using two additional independent freeway data sets. The results of this additional research confirmed the graphical relationship between crash frequency and traffic density developed in the original research. The crash rate on urban freeways varies with traffic density in a U-shaped curve. The lowest crash rates occurred at medium traffic densities, with slightly higher crash rates (single-vehicle- dominant) recorded at lower traffic densities and much higher crash rates recorded at higher traffic densities (multiple-vehicle-dominant). Therefore, if a design treatment is effective in reducing nonrecurrent congestion conditions at higher levels of service, it should also be effective in reducing crashes, resulting in a safety benefit.

Original document

The different versions of the original document can be found in:

• http://dx.doi.org/10.17226/22283

• http://pdfs.semanticscholar.org/6e8a/eacae0e4c8c6de5b08caa585736006967dad.pdf

• https://trid.trb.org/view.aspx?id=1326386,

https://www.nap.edu/catalog/22283,

https://academic.microsoft.com/#/detail/358562196