United States Department of Transportation
i
Michigan urban trunkline intersections safety performance functions (SPFs) development and support.
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2015-06-01
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[PDF-3.10 MB]
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Abstract:This study involves the development of safety performance functions (SPFs) for signalized and stop-controlled intersections
located along urban and suburban arterials in the state of Michigan. Extensive databases were developed that resulted in the
integration of traffic crash information, traffic volumes, and roadway geometry information. After these data were assembled, an
exploratory analysis of the data was conducted to identify general crash trends. This included assessment of the base models
provided in the Highway Safety Manual (HSM), as well as a calibration exercise, which demonstrated significant variability in
terms of the goodness-of-fit of the HSM models across various site types. Michigan-specific SPFs were estimated, including
simple models that consider only annual average daily traffic (AADT). More detailed models were also developed, which
considered additional geometric factors, such as posted speed limits, number of lanes, and the presence of medians, intersection
lighting, and right-turn-on-red prohibition. Crash modification factors (CMFs) were also estimated, which can be used to adjust
the SPFs to account for differences related to these factors. Separate SPFs were estimated for intersections of only two-way
streets and for those where at least one of the intersecting streets was one-way as the factors affecting traffic safety were found to
vary between these site types. Severity distribution functions (SDFs) were also estimated, which can be used to predict the
proportion of injury crashes which result in different injury severity levels. The SDFs may include various geometric, operation,
and traffic variables that will allow the estimated proportion to be specific to an individual intersection. Ultimately, the results of
this study provide MDOT with a number of methodological tools that will allow for proactive safety planning activities,
including network screening and identification of high-risk sites. These tools have been calibrated such that they can be applied
at either the statewide level or within any of MDOT’s seven geographic regions, providing additional flexibility to accommodate
unique differences across the state. The report also documents procedures for maintaining and calibrating these SPFs over time,
allowing for consideration of general trends that are not directly reflected by the predictor variables.
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