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Michigan urban trunkline segments safety performance functions (SPFs) : final report.
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2016-07-01
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[PDF-2.78 MB]
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Alternative Title:Michigan urban trunkline segments safety performance functions (SPFs) development and support.
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Edition:Draft final report, 09/30/2013 - 09/30/2016
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Abstract:This study involves the development of safety performance functions (SPFs) for urban and suburban trunkline segments in the
state of Michigan. Extensive databases were developed through 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 an 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 statewide models that consider only annual average
daily traffic (AADT), as well as regionalized models which take into account regional differences in traffic patterns and roadway
geometry. More detailed models were also developed, which considered additional geometric factors such as lane width, right
and left shoulder widths, median width, driveway density, on-street parking driveway density by land use, school count, posted
speed limit, and intersection and crossover density. 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 two-way arterials and oneway
arterials. Additionally, severity distribution functions (SDFs) were estimated, which can be used to predict the proportion of
injury crashes experienced. The SDFs may include various geometric, operation, and traffic variables that will allow the
estimated proportion to be specific to an individual segment. 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 a consideration of general
trends that are not directly reflected by the predictor variables.
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