Turner-Fairbank Highway Research Center

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Evaluating the Interactive Effects of Traffic Volumes and Access Density on Crash Frequency

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Ali Mohammed Alsubeai




Control of access points (e.g., driveway density) is an important consideration in roadway access management. Research has shown that by limiting the number of access points, there is a reduction in the number of conflict points along a roadway, resulting in improved safety. The effects of managing access points on crash frequency are documented as Crash Modification Factors/Functions (CMF) in the Highway Safety Manual (HSM) and the federal Highway Administration’s CMF Clearinghouse. The CMFs from the HSM indicate that the impacts of Access Density (AD) on crash frequency are a function of both AD and traffic volume (for rural roads). It does not include traffic volume in the CMFs for urban roads. Although the function could be calculated manually, both AD and traffic volume data from research in this thesis were compared to the ranges provided on the CMF Clearinghouse website. The function is available from the CMF Clearinghouse website, but it does not show the interactive effects of AD and traffic volume, and the majority of CMFs do not provide associated confidence intervals. The objectives of the current research were to develop CMFs for AD, as well as associated confidence intervals through time for various towns and cities in Minnesota. The data used was collected and provided by the following: (1) Highway Safety Information Systems (HSIS) over a period of five years at the same sites, (2) the Institute of Transportation Engineers (ITE), (3) and Google Earth. The methodology used in this study was cross-sectional longitudinal with multivariate statistical analysis. Multiple functional classifications of urban roadways were considered, with a focus on major/minor collectors and minor arterials. The CMFs were developed for total number of crashes, fatal, injury, rear-end, and side-swipe crashes. The interaction between AD and traffic volume was considered. Confidence intervals for the resulting CMFs were determined. The results of this research will be useful for engineers and planners in determining when AD should be changed or limited.

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