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Michigan Urban Trunkline Segments Safety Performance Functions (SPFs) Development and Support


Co-principal investigators:

Project status


Start date: 08/16/15
End date: 07/31/16


Report: Michigan Urban Trunkline Segments Safety Performance Functions (SPFs) Development and Support (NA pdf) June 2015



About the research


This study involved the development of safety performance functions (SPFs) for signalized and stop-controlled intersections located along urban and suburban arterials in 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 that 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 the Michigan Department of Transportation (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 final report also documents procedures to maintain and calibrate these SPFs over time, allowing for consideration of general trends that are not directly reflected by the predictor variables.