Jonathan Kay

Effects of Public Rest Areas on Fatigue-Related Crashes

Fatigue-related crashes account for 2.2% to 2.6% of all fatal crashes in the United States on an annual basis. These types of crashes are prevalent in rural areas, and public roadside rest areas were developed largely to alleviate motorist fatigue and reduce the opportunity for such crashes by providing safe parking areas for tired drivers. However, research about the safety effects of rest areas has been limited. Results are presented of a spatial analysis that investigates how a road segments proximity to a rest area affects the frequency of fatigue-related crashes. Poisson and negative binomial models are estimated for freeways and two-lane highways to isolate the effects of proximity while controlling for other relevant factors, such as traffic volume. The results of these models indicate that the proximity of a road segment to the nearest rest area significantly influences crash frequencies on both freeways and two-lane highways, where traffic volumes tended to have similar effects, although the effects of proximity were slightly more pronounced on two-lane highways. The study results suggest that roadside rest areas provide a safety benefit, and the crash prediction models developed as a part of this research provide a starting point for quantifying these impacts.

Driver Speed Selection on High-Speed Two-Lane Highways: Comparing Speed Profiles Between Uniform and Differential Speed Limits

ABSTRACT 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.

Safety impacts of a statewide centerline rumble strip installation program

Lane departures are a precipitating event in most fatal crashes in the United States. This problem is particularly pronounced on high-speed undivided highways, which are prone to cross-centerline crashes. A common countermeasure to reduce such crashes involves installing centerline rumble strips (CLRS), which provide an audible and tactile warning to alert drivers of an impending lane departure event. This study assessed the safety effects of a statewide CLRS implementation program conducted by the Michigan Department of Transportation (DOT) between 2008 and 2010. This program included the installation of CLRS across the Michigan DOT–maintained network of rural, high-speed nonfreeway facilities. Shoulder rumble strips (SRS) were installed in combination with the CLRS at locations with paved shoulders at least 6 ft in width. The empirical Bayes method was used to assess the effectiveness of 4,077 mi of two-lane highways where CLRS were installed as part of this initiative. CLRS were found to reduce target cross-centerline crashes by 27.3% and by 32.8% when used in combination with SRS. Rumble strips were also effective in reducing crashes that occurred under adverse pavement conditions or involved passing maneuvers or driver impairment. This study also provides important insights as to the identification of target (i.e., cross-centerline) crashes. A comprehensive manual review of more than 70,000 crash report forms demonstrated that approximately 10% of target crashes were misclassified by crash type, which is the predominant field typically used to identify such crashes.

Empirical Models of Demand Levels and Turn-In Rates at Roadside Rest Areas

Roadside rest areas provide valuable and convenient services to road users as well as tangible safety benefits. However, these benefits are not always enough to warrant the costs, and therefore these facilities are not always economically justified. As such, understanding the factors that affect rest area demand is important for transportation agencies to design and maintain these facilities efficiently within their jurisdictions. The effects of nearby comparable facilities and rest areas on the demand characteristics of roadside rest areas have not been well analyzed in previous literature. This paper presents the results of binary logistic regression and negative binomial regression models on the demand characteristics of 47 roadside rest areas in Michigan. These models were developed separately for passenger cars and commercial trucks because the decision-making processes for these types of road users were likely different. Key results include the finding that greater distances between rest areas and nearest upstream rest areas or comparable facilities lead to increases in both the number and proportion of vehicles entering roadside rest areas. In addition, as volumes increase on the adjacent roadway, the proportion of vehicles entering roadside rest areas decreases despite a greater number of entering vehicles.

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 Michigans 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.

Economic Analysis of 65 mph Speed Limits on Rural Highways

Speed management has been a long-standing concern of transportation agencies. Recently, nationwide speed limit policy modifications have resulted in a general upward trend in many states. In Michigan, legislation was proposed to raise the maximum speed limits on state-owned, rural, nonfreeway highways from 55 to 65 mph. In response to this proposed speed limit policy change, research was performed to assess the associated economic impacts through the use of benefit–cost analysis. Several potential implementation scenarios of the proposed speed limit policy were investigated; these included a limited subset of roadways with favorable geometry and full statewide implementation. A primary research task was to estimate the costs for infrastructure upgrades necessary to achieve compliance with state and federal design speed requirements for the cohort of roadways included within each 65 mph implementation scenario. User benefits and disbenefits associated with the proposed speed limit increase also were estimated, which included reduced travel times, increased fuel consumption, and increased traffic crashes. Although the travel time savings were estimated to exceed the fuel consumption costs, the resulting benefit was far outweighed by crash-related disbenefits and agency infrastructure costs for roadways that required major geometric modifications (e.g., vertical or horizontal realignment). Consequently, an increase in the speed limit to 65 mph on rural, nonfreeway highways should be considered only if the critical geometric elements can maintain design speed compliance without major modification. However, even for roadways on which design speed compliance is maintained, careful, site-specific consideration must be given to the potential safety impacts, particularly with respect to fatal and injury crashes, which may result after the speed limit is increased.