European design guidelines explicitly address horizontal design consistency for two-lane, rural roads in an attempt to promote smooth operating speed profiles and, in turn, safe operation. U.S. practice qualitatively advocates consistent alignment but provides little objective guidance to assure that consistency is achieved. This paper presents a procedure for measuring the consistency of horizontal design as defined by operating speed and accidents expected. Operating speeds and accident rates can be predicted for various lane widths based on degree of curve and posted recommended speeds, as derived from measurement of 261 sites in New York state. Guidelines for changes in operating speeds and acceptable accident rates for good, fair, and poor designs are suggested, and various nomographs are developed to evaluate roadway sections based on design parameters. In addition, an example application is provided to illustrate the case of fair design practices. It is concluded that such a procedure could readily be adapted by the design community in prescribing improvements to existing facilities or in fine tuning new highway design.
Abstract