| Description |
In 2014, there were 17,791 fatalities as a result of roadway departure crashes in the U.S., representing 54% of all traffic fatalities in the U.S. Roadway departure crashes account for approximately 52%of traffic fatalities in the state of Utah. A significant number of roadway departure crashes occur on horizontal curves along rural, two-lane highways. Previous research has indicated that providing "consistent" designs that are compatible with driver expectations and capabilities can reduce the number of roadway departure crashes at these locations. Various measures of design consistency have been proposed to quantify the levels by which a road design meets driver expectations and capabilities, including speed differentials, alignment indices, and visual demand/work load estimates. Among them, alignment indices have been proven as direct design consistency measures to analyze crash frequency. The objective of this research was to estimate relationships between the expected frequency of horizontal curve roadway departure crashes and geometric design consistency, characterized by using alignment indices along rural, two-lane highways in Utah. Negative binomial and zero-inflated negative binomial regression models were estimated which relate expected frequencies of roadway departure crashes to design and traffic characteristics of the rural, two-lane road segments. The dataset consists of 578 horizontal curves with corresponding design and traffic information, as well as characteristics of the upstream and downstream tangents and curves. Horizontal alignment indices, curve lengths, average daily traffic volumes (ADTs), and general geometric variables were tested in the model specifications. To build the dataset for model estimation, roadway features were gathered along rural, two-lane state routes in Utah using the Utah Department of Transportation's LIDAR files. Crash data were also provided by the Utah Department of Transportation for these same routes and spanned the years 2008 through 2014. Eventually, the best two models were explored in this study. One model included the following parameters: the natural logarithm of average annual daily traffic, the changed radius rate, vertical curvature change rate, maximum change in degree of curvature, indicator variable for the presence of a vertical curve on a horizontal curve, and average grade. The other model had the same variables as the first model, but the ratio of average radius over radii replaced the changed radius rate and the average change in degree of curvature replaced the maximum change in degree of curvature. |
