Safety Factors for Road Design: Can They Be Estimated?

When asked to state the safety of a particular design, highway engineers are at a loss to give a single meaningful measure, as is possible in structural or geotechnical engineering. The question of a meaningful road safety design measure led to a method to estimate the margin of safety and safety index for isolated highway components. The method uses the basic highway alignment design equations and, on the assumption that the variables are independent, normal, and random, the expected value of the mean and variance are estimated and the margin of safety and safety index are derived. The proposed measures of road geometric design safety are the margin of safety and safety index. The variables included in the safety measures represent the characteristics of driver, vehicle, and road surface. Calculations based on preliminary information indicate that the safety index is the most meaningful safety measure for road design. A general method to specify the design parameter's value is also proposed. This method is based on factors that represent the strategic importance of a road, the number of road users, the type of vehicles, the quality of the drivers, the expected environmental conditions, the terrain, and the general standard of design or construction. These factors are all found implicitly in current design procedures. The apparent advantage of the proposed method is that the designer must explicitly specify the importance of the modifying factors. Research is required to make the method useful. The research must clearly develop the mean, variance, and distribution of the variables used in the basic geometric cjesign equations. Further information will eventually be needed on the interaction of the variables, to remove the independence requirement and to permit an estimate of the road system reliability over a specified road link.