SIGHT DISTANCE EVALUATION ON COMPLEX HIGHWAY VERTICAL ALIGNMENTS

Sight distance (stopping, passing, and decision) is a key element in h ighway geometric design. Existing models for evaluating sight distance on vertical alignments are applicable only to simple, isolated elemen ts such as a crest vertical curve, a sag vertical curve, and a reverse vertical curve (a sag curve following a crest curve, or vice versa). This paper presents an analytical methodology for evaluating sight dis tance on complex vertical alignments that involve any combination of v ertical alignment elements. The methodology can be used for evaluating passing sight distance on two-lane highways, and stopping sight dista nce and decision sight distance on all highways. Sight distance contro lled by the headlight beam can also be evaluated. The locations of sig ht-hidden dips, which may develop when a sag vertical curve follows a crest vertical curve with or without a common tangent, can be determin ed. Also, sight distances obstructed by overpasses are evaluated. A pr ofile of the available sight distance can be established and used to e valuate sight distance deficiency and the effect of alignment improvem ents. A software was developed and can be used for determining the ava ilable sight distance accurately. The software may replace the current field and graphical practice for establishing the no-passing zones an d evaluating stopping and decision sight distances on complex vertical alignments.