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9 CHAPTER 2 Literature Review Roadway projects where design elements trade-offs are four-lane roadways. Thus, these relationships and their areas considered typically incorporate a full range of geometric and of application must be further examined. traffic operational problems, coupled with increasingly restric- Another Green Book topic requiring additional background tive environmental constraints. These problems may require information for designers centers on the relative importance variation from the normally used guidance values or tradi- of various geometric elements on safety. It is apparent that not tional solutions. Moreover, every project is unique in terms of all geometric elements have the same impact on safety and the geometric conditions, traffic, safety history, purpose and operational effectiveness, and the selected design value can need, project context, community character, and public pri- affect additional elements. For example, the choice of a design orities. What is reasonable or may work in one location may speed of 45 mph or less for a road allows the designer to use not be appropriate in another for any number of technical or a smaller minimum curve radius, a narrower clear zone, a context-sensitive reasons. The literature review conducted for shorter vertical curve, and shorter sight distances than those for this research examined safety implications from geometric a higher design speed. Here, the impact is significantly greater element trade-offs, and the findings are presented herein. In than when selecting a single design element to be adjusted. addition, NCHRP Synthesis of Highway Practice 299: Recent Moreover, roadway elements can exert varying degrees of Geometric Design Research for Improved Safety and Operations influence even through a single element. For example, lane presents an extensive literature review on geometric design width will exert an impact on a two-lane roadway that will elements for improving safety and operations (8). The follow- be different from that exerted on a four-lane roadway. There- ing section presents first an overview of roadway design issues fore, a prioritized list is needed to identify the relative signif- and then the findings on the effects of specific cross-section icance of each geometric element. Given the current definition elements for multilane highways. of design speed, it is probably the most critical design element to be selected since it has the potential to impact the values used for almost all other design elements (1, 5). Roadway Design Issues Most studies dealing with safety and speed typically con- The Green Book lacks background information sufficient sidered speed limit and so little is known about the influence for understanding the safety and operational implications of design speeds on safety. It could be assumed that there is of combinations of critical geometric features. The recently some relationship between design speeds and speed limits, published Guide for Achieving Flexibility in Highway Design but because of the methods used to establish speed limits in provides some information on these areas, but also lacks any many states, it is not feasible to develop a systematic relation- quantifiable relationships for the values of various design ship between the two (10). Current highway design approaches elements (5). There are several geometric features that have a emphasize speed as a surrogate for quality and efficiency. This greater effect when combined than when considered alone-- approach is probably reasonable for rural areas where high for example, Zegeer and Deacon (9) showed that the combined speeds are frequently desirable, but not for roads in urban or lane and shoulder width has a greater impact on the safety level suburban areas. of two-lane rural roadways than does lane or shoulder width Several studies have examined cross-section elements and alone. At the same time, there are cases where these combi- attempted to develop models or relationships that could esti- nations have little or no impact. The same combination of mate safety implications from varying individual components. lane and shoulder width has a small to possibly no impact on The work of Zegeer et al. (1113) identified the relationship