Institute for Transportation

About the research

The relationship between speed and safety continues to be a high-priority research topic as numerous states consider speed limit increases. This study leveraged data from the Second Strategic Highway Research Program (SHRP2) Naturalistic Driving Study (NDS) to examine various aspects of driver behavior, including speed limit selection and engagement with in-vehicle distractions, as well as the impacts of these behaviors on crash risk while controlling for the effects of traffic, geometric, and environmental conditions. High-resolution time-series data were analyzed to examine how drivers adapt their speed on roadways with different posted limits, in speed limit transition areas where increases or decreases occur, as well as along horizontal curves, both with and without posted advisory speeds.

The research also involved an investigation of the circumstances under which driver distraction is most prevalent. The factors associated with crash and near-crash events were compared with similar data from normal, baseline driving events across various scenarios to improve understanding of the nature of the precipitating events. Driver responses, including reaction times and deceleration rates, were examined during the course of crash and near-crash events to determine how driver response varied across various scenarios.

Ultimately, this research provided important insights as to how drivers adapt their behavior and how these behaviors, in turn, influence the likelihood of being crash involved.

Funding Sources:
Midwest Transportation Center
USDOT/OST-R ($50,000.00)
Total: $50,000.00

Contract Number: DTRT13-G-UTC37

About the research

This study involved the development of safety performance functions (SPFs) for rural road segments and intersections in the state of Michigan. The facility types included two-lane and four-lane state trunklines (divided and undivided), rural county roadways (paved and gravel), signalized intersections, and minor-road stop controlled intersections. Data were compiled from several sources for thousands of rural road segments and intersections statewide. These data included traffic crashes, traffic volumes, roadway classification, geometry, cross-sectional features, and other site characteristics for the period of 2011-2015. These data were assembled into separate files based on the facility type, jurisdiction, and federal aid status. The Highway Safety Manual (HSM) base SPFs were then calibrated using the Michigan-specific data, which demonstrated significant variability in terms of the goodness-of-fit of the HSM models across various site types, due in part to the very high proportion of deer crashes on Michigan’s rural highways. Consequently, Michigan-specific SPFs were estimated, including simple statewide models that considered only annual average daily traffic (AADT), as well as regionalized models that accounted for regional differences in drivers, weather, topography, and other characteristics. More detailed models were also developed, which considered additional factors such as shoulder width, driveway density, horizontal curvature, median presence, road surface type, and intersection skew. Crash modification factors (CMFs) were estimated, which are used to adjust the SPF crash estimates to account for differences related to the site characteristics. Methods for prediction of crash frequency by collision type and injury severity were also established. Depending on the facility type, this was performed either by using separate SPFs, severity distribution functions (SDFs), or crash distributions. Ultimately, the results of this study provide a number of tools that 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 either at the statewide level or within any of MDOT’s seven geographic regions to accommodate unique differences across the state. The report also documents procedures for maintaining and calibrating these SPFs over time to account for temporal changes that occur across the network.

About the research

The purpose of this research was to assist in determining the potential impacts of implementing a proposed 65 mph speed limit on non-freeways in Michigan. Consideration was given to a broad range of performance measures, including operating speeds, traffic crashes and crash severity, infrastructure costs, fuel consumption, and travel times. Specifically, a prioritization strategy was developed to identify candidate MDOT non-freeway road segments possessing lower safety risks and potential infrastructure costs associated with raising the speed limit from 55 to 65 mph. Ultimately, approximately 747 miles of undivided and 26 miles of divided 55 mph non-freeways were identified as lower risk candidates, representing approximately one-eighth of the MDOT systemwide mileage posted at 55 mph. An economic analysis of the anticipated costs and benefits associated with the proposed speed limit increase was performed for these lower risk candidate segments, in addition to a systemwide estimate. As the travel time savings were expected to outweigh the fuel consumption costs, it was necessary to determine if these net operational benefits outweighed the expected infrastructure upgrade costs and increased crash costs. For roadways possessing horizontal and/or vertical alignments that are not compliant with a 65 mph speed limit, an unfavorable benefit/cost ratio would likely result due to the excessive infrastructure costs incurred during 3R (resurfacing, restoration, rehabilitation) or 4R (reconstruction) projects. Crashes were expected to increase for all implementation scenarios, with a particular increase in the risk of fatal and incapacitating injuries. Due to the substantially large infrastructure costs, application of the 65 mph speed limit is specifically not recommended for non-freeway segments requiring horizontal or vertical realignment to achieve design speed compliance. Even for segments where compliance with the increased design speed is maintained, careful consideration must be given to the potential safety impacts –particularly to fatal and injury crashes – that may result after increasing the speed limit.

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.

About the research

The objectives of this five-year Transportation Pooled Fund study are to evaluate conventional and new technologies and procedures for testing concrete and concrete materials to prevent material and construction problems that could lead to premature concrete pavement distress, and to develop a suite of tests that provides a comprehensive method of ensuring long-term pavement performance. A preliminary suite of tests to ensure long-term pavement performance has been developed. Shadow construction projects are being conducted to evaluate the preliminary suite of tests. A mobile concrete testing laboratory has been designed and equipped to facilitate the shadow projects. The results of the project are being compiled in a user-friendly field manual, which will be available by summer 2006.