Calibrating Road Design Guides Using Risk-Based Reliability Analysis

Existing geometric design guides provide deterministic design criteria for highway elements that ignore the uncertainty associated with many design parameters. Reliability analysis has been advocated as an approach to account for this uncertainty and to evaluate the risk associated with a particular design feature. This paper discusses one important application of reliability analysis: the calibration of geometric design models to yield consistent safety (risk) levels. The paper provides calibrated design charts for the middle ordinate M, defined as the lateral distance between edge of median barriers and centerline of the adjacent traffic lane, at different probability of noncompliance levels. The results show that the calibrated values of M are generally lower than those derived from the AASHTO design guide. The calibrated design charts can offer designers dealing with highways with constricted right-of-way an option to use lower middle ordinate values and enable them to estimate the safety consequences of their decisions. Overall, the calibrated charts can aid the decision maker in determining the safety implications of deviating from geometric design standards and quantifying the safety level built in design values that are deemed acceptable.