Side-Friction Factors in the Design of Highway Curves

In order to provide an evaluation of the current AASHTO curve design Standards and to provide the information necessary to substantiate any proposed changes in these standards, a study was made of (part of the complex system of interaction between driver, vehicle, and pavement) those physical characteristics of the vehicle and the pavement that influence the critical cornering maneuver. The study consisted of: the use of mathematical simulations of motor vehicles traversing highway curves; the evaluation of mathematical simulations by means of full- scale vehicle tests; and use of the mathematical simulations in parametric studies to evaluate current AASHTO curve design standards. A modified version of an advanced simulation model was used. Full- scale tests with two vechiles, a 1964 Ford and a 1971 Pontiac, were used to validate the simulation. Details of the vehicle/time combinations are outlined. A modified point-mass equation, similar to the classic equation upon which current AASHTO desing standards are based, was used in the analysis. Results of the study indicate that the current AASHTO geometric design policy will in most cases provide safe, conservative designs for highway curves. The specific findings of this study which are classified as 'direct' and 'indirect' are listed. A major finding of this study is that the skid number (SN) as measured with an ASTM locked-wheel skid trailer equipped with highway- type tires, is a reasonable indicator of the average lateral pavement skid resistance available to cornering vehicles using the same tires when traversing long-radius highway curves (oner 800-ft. radius).