Effects of Geometric Characteristics of Rural Two-Lane Roads on Safety

The study investigated the safety effects of geometric and other roadway characteristics of rural two-lane roads by collecting and analyzing data and developing crash prediction models, crash reduction factors (CRFs), and cost-effectiveness ratios (CERs). On the basis of these results, and the existing inventory of roads and their geometric deficiencies, physical and monetary needs assessments were carried out under alternative budgeting and crash costing scenarios for the rural two-lane state highway network. It was found that influential crash factors at such roads include section length, traffic volume, lane width, shoulder width, pavement condition, average horizontal curve radius, and grades of vertical slopes. For county roads, additional influential variables are shoulder type, driveway density, and the presence of speed limit posting. The percentage reduction in crashes due to the various safety countermeasures was found to vary by crash severity type, highway class, level of deficiency, and the extent to which the deficiency is addressed. CRFs were developed for lane and shoulder widening, minor realignment and earthworks that increase horizontal curve radius or to decrease in vertical curve slope, pavement friction enhancement (for rural major collectors and minor arterials) and pavement condition improvement (for rural principal arterials). CERs were developed for each countermeasure, crash type and road functional class. CERs are useful for safety project evaluation particularly when INDOT/LTAP seeks to compare the safety benefit per dollar across mutually exclusive safety countermeasures at a specific problem area, or when a specific area suffers from multiple safety problem types and can therefore receive many different countermeasures only a few of which can be funded. For rural two-lane state highway sections that are deficient-but-not-necessarily-hazardous and both-hazardous-and-deficient sections, it was found that $530 million and $55 million, respectively, would be needed to address the engineering safety needs. Given an annual expenditure of $1M over a 5-year period, approximately 170 crashes (translating to $8-15 million, depending on crash costing method) could be saved. If the annual expenditure is increased to $2M over the same period, approximately 240 crashes (translating to $12-26 million) could be saved. The study results also suggest that there seems to exist a ceiling of effectiveness for engineering safety countermeasures, and therefore non-engineering countermeasures such as safety education and enforcement are also vital to further reduce crashes related to drivers, vehicles, and the driving environment. Implementation of the study would involve application of the study products for safety management at both state and local levels to make cost-effective decisions that will reduce highway crashes and thereby accrue the secondary and indirect benefits of increased mobility, enhanced geriatric driving, reduced tort liability, and enhanced economic productivity