Safety Effects of Horizontal Curve and Grade Combinations on Rural Two-Lane Highways

The safety effects of horizontal curves and grades on highways have been quantified separately, but it is not currently known whether or how the safety performance of horizontal curves interacts with that of grades. Although the first edition of the AASHTO Highway Safely Manual provides crash modification factors for the safety effects of horizontal curvature and percent grade on rural two-lane highways, the manual does not have a method for accounting for the interactions between these effects. In other words, in the Highway Safely Manual procedures for rural two-lane highways, the safety effect of a horizontal curve is the same whether the curve is located on a level roadway, a straight grade, or a vertical curve. Similarly, the safety effect of a straight grade is the same whether it is located on a tangent roadway or horizontal curve. Researchers have always supposed that there are interactions between the safety effects of horizontal and vertical alignments, but these interactions have not been demonstrated in a form useful for safety prediction. The results of research undertaken to quantify the safety effects of five types of horizontal and vertical alignment combinations based on Highway Safety Information System data and crash records from 2003 to 2008 in Washington State are summarized. The outcome is a set of safety prediction models for fatal and injury and property-damage-only crashes. To present the results in a form suitable for incorporation into the Highway Safety Manual, crash modification factors representing safety performance relative to level tangents were developed from these models for each of the five combinations.