Turnpike accident data are analyzed to show that curves of low curvature have higher than average accident involvement histories. Horizontal curvature and grade when in combination are also shown to exert independent influences on accident statistics. Tire traction is shown to be substantially degraded at water depths well below those needed for hydroplaning. Pavement drainage and hence water depth on the road surface is seen to be primarily influenced by road width, superelevation, and rainfall rate, and to be essentially independent of grade. Analytical analyses of the steady turning performance of a vehicle are used to show that cornering on a horizontal curve is essentially the same as cornering on a curve constructed on a grade, within the range of grades in common use. Computer simulation analyses involving parametric variations of vehicles, tires, road surfaces, curvature, superelevation of grade, and maneuvers are used to define specific limiting velocity boundaries for vehicle handling performance. Such boundaries are shown to be well below those which are assumed when using the AASHTO curve design formula. The accident, traction, drainage, and vehicle performance analyses are then used to formulate an expression for required pavement skid number. The formulation, including the influences of curvature, grade, pavement drainage, legal tire tread depth, skid number gradient and roadway design velocity, is developed in the appendix. Examples are used to illustrate applications.
Abstract