Proposed Horizontal Curve and Vertical Grade Encroachment Adjustment Factors

The probability of a vehicle encroaching onto the roadside is influenced by several geometric characteristics of the roadway. The horizontal curvature and vertical grade encroachment adjustment factors (EAFs) used in the third version of the Roadside Safety Analysis Program (RSAPv3) were adopted from adjustment factors introduced in 2003 in NCHRP Report 492 during the original development of RSAP. These adjustments were developed by interpreting earlier work on curves and grades. Given the weaknesses and limitations of the earlier study, it was suggested that a larger-scale study with larger sample size and more representative crash severities would be appropriate. This paper describes the development of new horizontal curve and vertical grade EAFs for rural areas. EAFs were developed from a cross-sectional model of crash data that controlled for geometric influences, traffic volumes, segment length, and highway type. The proposed EAFs are not identical to the current factors, which is not surprising, bec...