A practical procedure was developed that explicitly considers the driving behavioral and safety rules of a horizontal alignment for the evaluation of roadway new designs; redesigns; and projects of rehabilitation, restoration, and resurfacing. Design classes were developed to classify, from a traffic safety point of view, roadway sections as good, fair, or poor; these design classes are associated with three safety criteria to develop an overall quantitative safety evaluation procedure for two-lane rural roads. The safety criteria are introduced to analyze and evaluate by ( a) Safety Criterion (SC) I: the difference between design speed and actual driving behavior as expressed by variations in observed 85th percentile speed; ( b) SC II: the difference between observed 85th percentile speeds on successive design elements; and ( c) SC III: the difference between side-friction assumed and side-friction demanded at the 85th percentile speed level on curves. Furthermore, the issues discussed include design speed, operating speed, sound tangential, and side-friction factors, as well as the application of tangents in the design process. A comparative analysis of the actual accident situation with the results of the safety criteria reveals a convincing agreement. Thus, the great advantage of the new concept is that as early as in the design stages, safety criteria can predict the endangerment (low, medium, and high) for new alignments or allow statements about the safety conditions of existing (old) roadway sections or whole road networks.

More than 50% of traffic fatalities occur on two-lane rural roads, and more than half of these fatalities occur on curved roadway sections. A large body of research can be used to analyze and evaluate the fundamental relationships between accident situation, highway geometric design, driving behavior, and driving dynamics. These factors form the basis for the development of three quantitative safety criteria used to evaluate the hazards of two-lane rural roads with respect to new designs; redesigns; restoration, rehabilitation, or resurfacing projects; and existing alignments. The safety criteria support the design engineer in classifying new or old roadway sections according to good (sound), fair (tolerable), and poor (dangerous) design practices. On the basis of observation of the actual variation in the accident rate with respect to road alignment, a fuzzy model was developed to classify roadway elements by using these safety criteria to obtain a more careful evaluation of highway design inconsistencies. For each criterion, the inconsistencies were included in three fuzzy sets (good, fair, poor) with differing degrees of membership. By defining linear membership functions, it was possible to obtain good results to classify road sections and then to determine a prioritization scale of maintenance interventions. The procedure can be applied to large databases of road networks to identify the more dangerous design elements that need interventions to improve highway safety and to allocate resources under limited budget conditions.

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Safety issues affecting modern highway geometric design of roads in non-built-up areas are discussed. The status of knowledge of Safety Criteria I and II (achieving operating speed consistency and design consistency) for two-lane rural roads is briefly reviewed. Considerations for design consistency are also extended to multiple-lane rural and suburban road design to evaluate good, fair, and poor design levels for these road categories. Utilization ratios (n) for maximum permissible side friction factors are developed for different road categories, topography levels, and maximum and minimum superelevation rates. Relevant minimum radii for curve design are established and compared with present AASHTO values insofar as possible. Safety Criterion III (achieving driving dynamic consistency) is further developed, through which reliable and quantitative ranges between the side friction assumed and the actual friction demand at curved sites could be established. AASHTO's design policy is evaluated on the basis of Criteria II and III. The evaluation showed, for lower design speeds (30, 40, and 50 mph), an unbalanced degree of curve and superelevation rate ranges, which lead to poor design practices from a safety viewpoint. By heeding the three safety criteria and the recommended side-friction utilization ratios, sound alignments can be established for the various design cases in future highway geometric design and redesign and for resurfacing, restoration, and rehabilitation strategies.