The objective of the research was to develop guidelines that designers can use to improve the geometric design consistency on high-speed, non-urban, two-lane roadways. The research scope was intended to complement work done by FHWA that focused primarily on speed consistency. The inclusion of other geometric design elements (e.g., lane width reductions, lane drops, and driveways) addresses a broader range of inconsistencies. Because “design consistency” is not defined in AASHTO design guidelines, a review of the literature was undertaken, and state DOTs and the research panel were surveyed to determine how they would define “design consistency.” Based on the findings of the http://gulliver.trb.org/publications/nchrp/nchrp_rpt_502.pdf This report may be accessed by Internet users at survey and the literature review, a recommended definition was developed: Design consistency is the conformance of a highway’s geometric and operational features with driver expectancy. A telephone survey of design engineers, consultants, law enforcement personnel, and accident reconstructionists was used to evaluate roadway features for their potential to influence design consistency. Using the findings of the survey and a review of the literature, geometric features were selected for potential inclusion in design consistency rules. Geometric features or elements having the potential to violate driver expectancy or design consistency principles were evaluated for use in a rules-based consistency check. Several approaches have been used in evaluating geometric design consistency; among these approaches have been speed consistency, positive guidance, and driver workload. Based on these approaches and the principles of design consistency reviewed in the literature, the basic structure of the consistency rules was established. The structure was developed to include a brief background section on the issue, necessary models or algorithms, warning levels, and text warnings (including potential remedial measures) for the designers. Based on the structure developed, design consistency rules were developed to notify designers and engineers when the potential for driver expectancy violations is present. Following initial development, the design consistency rules were reviewed by the research staff in a series of meetings, reviewed by the research panel, evaluated in three case studies, and revised to their final recommended form. (Author/publisher)
Abstract