Using spatial interpolation to determine impacts of annual snowfall on traffic crashes for limited access freeway segments.

Snowfall affects traffic safety by impacting vehicle performance, driver behavior, and the transportation infrastructure. Depending on intensity snowfall can reduce visibility, pavement friction performance, vehicle stability and maneuverability. Based on this premise, the objective of this study was to use spatial interpolation to analyze the effects of annual snowfall on crash occurrence at non-interchange freeway segment locations in Michigan. Using the geostatistical method of Ordinary Kriging, site specific historical snowfall values were estimated based on data obtained from a series of weather stations during the primary winter months (December, January, and February) for the years of 2004 through 2014 along Michigan's entire limited access freeway network. These weather data were spatially matched with historical crash data and roadway inventory data for each freeway segment. A Negative Binomial regression model was developed to quantify the effects of snowfall on crashes. Explanatory variables included annual average daily traffic, segment length, horizontal curvature, and snowfall. The results indicated that annual snowfall has a statistically significant positive effect on winter crashes for all types of crashes analyzed. With respect to vehicle type, crashes involving a truck or bus experienced the strongest relationship with annual snowfall. Considering crash severity, property damage crashes possessed a stronger relationship with snowfall as opposed to injury crashes.

Driver speed selection on high-speed two-lane highways: Comparing speed profiles between uniform and differential speed limits.

OBJECTIVE: Although a considerable amount of prior research has investigated the impacts of speed limits on traffic safety and operations, much of this research, and nearly all of the research related to differential speed limits, has been specific to limited access freeways. The unique safety and operational issues on highways without access control create difficulty relating the conclusions from prior freeway-related speed limit research to 2-lane highways, particularly research on differential limits due to passing limitations and subsequent queuing. Therefore, the objective of this study was to assess differences in driver speed selection with respect to the posted speed limit on rural 2-lane highways, with a particular emphasis on the differences between uniform and differential speed limits. METHODS: Data were collected from nearly 59,000 vehicles across 320 sites in Montana and 4 neighboring states. Differences in mean speeds, 85th percentile speeds, and the standard deviation in speeds for free-flowing vehicles were examined across these sites using ordinary least squares regression models. RESULTS: Ultimately, the results of the analysis show that the mean speed, 85th percentile speed, and variability in travel speeds for free-flowing vehicles on 2-lane highways are generally lower at locations with uniform 65 mph speed limits, compared to locations with differential limits of 70 mph for cars and 60 mph for trucks. CONCLUSIONS: In addition to posted speed limits, several site characteristics were shown to influence speed selection including shoulder widths, frequency of horizontal curves, percentage of the segment that included no passing zones, and hourly volumes. Differences in vehicle speed characteristics were also observed between states, indicating that speed selection may also be influenced by local factors, such as driver population or enforcement.

Assessing characteristics related to the use of seatbelts and cell phones by drivers: application of a bivariate probit model.

INTRODUCTION: The effects of cell phone use and safety belt use have been an important focus of research related to driver safety. Cell phone use has been shown to be a significant source of driver distraction contributing to substantial degradations in driver performance, while safety belts have been demonstrated to play a vital role in mitigating injuries to crash-involved occupants. METHOD: This study examines the prevalence of cell phone use and safety belt non-use among the driving population through direct observation surveys. A bivariate probit model is developed to simultaneously examine the factors that affect cell phone and safety belt use among motor vehicle drivers. RESULTS: The results show that several factors may influence drivers' decision to use cell phones and safety belts, and that these decisions are correlated. PRACTICAL APPLICATIONS: Understanding the factors that affect both cell phone use and safety belt non-use is essential to targeting policy and programs that reduce such behavior.

Driver behavior during bicycle passing maneuvers in response to a Share the Road sign treatment.

The interaction of motorists and bicyclists, particularly during passing maneuvers, is an area of concern to the bicycle safety community as there is a general perception that motor vehicle drivers may not share the road effectively with bicyclists. This is a particular concern on road sections with centerline rumble strips where motorists are prone to crowd bicyclists during passing events. One potential countermeasure to address this concern is the use of a bicycle warning sign with a "Share the Road" plaque. This paper presents the results of a controlled field evaluation of this sign treatment, which involved an examination of driver behavior while overtaking bicyclists. A series of field studies were conducted concurrently on two segments of a high-speed, rural two-lane highway. These segments were similar in terms of roadway geometry, traffic volumes, and other relevant factors, except that one of the segments included centerline rumble strips while the other did not. A before-and-after study design was utilized to examine changes in motor vehicle lateral placement and speed at the time of the passing event as they relate to the presence of centerline rumble strips and the sign treatment. Centerline rumble strips generally shifted vehicles closer to the bicyclists during passing maneuvers, though the magnitude of this effect was marginal. The sign treatment was found to shift motor vehicles away from the rightmost lane positions, though the signs did not significantly affect the mean buffer distance between the bicyclists and passing motorists or the propensity of crowding events during passing. The sign treatment also resulted in a 2.5miles/h (4.0km/h) reduction in vehicle speeds. Vehicle type, bicyclist position, and the presence of opposing traffic were also found to affect lateral placement and speed selection during passing maneuvers.