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7 CHAPTER 1 Introduction and Research Approach Problem Statement allow flexibility. For most control values, the Green Book indi- cates that the recommended ranges provide a safe, comfortable, Every roadway design starts with the intention to provide a and aesthetically pleasing roadway; however, there are cases safe facility that addresses mobility concerns, accommodates where additional flexibility is necessary and, therefore, the the physical and social environment, and is financially feasi- design exception process is available. This process allows the ble. To achieve such designs, engineers rely on guidelines and designer to consider other design values for a specific element policies, which aim to address these goals. Sometimes, how- in order to better fit the design to the situation. In many cases, ever, it may not be practical to conform to the highest possible there is little research that quantifies such decisions and their values of all these guidelines. For example, adherence to a impact on safety (2, 3). certain geometric specification may create environmental The concept of guidelines was emphasized even more in concerns, affect historical structures, be economically unfea- Flexibility in Highway Design (4), a recent publication by U.S. sible, or otherwise affect a community in an undesirable way. DOT, and is further stressed in AASHTO's Guide for Achieving To address such issues, the roadway design may need to deviate Flexibility in Highway Design (5). The previously used approach from the prevailing optimum value expressed in guidelines and typically deemphasized the design's impact on human and policies. An understanding of the impacts of such alternative natural environments, increasing the possibility of creating designs on both the safety and the operational character of the wide swaths of pavement cutting through communities and roadway is essential to making an informed choice among natural resources. This approach was typically justified by possible designs. arguing that it results in designs with increased safety, but this The AASHTO publication A Policy on Geometric Design result was not always achieved. A critical review of design of Highways and Streets (commonly referred to as the Green guidelines by Hauer (6) stated that several design guidelines Book) provides guidance to the designer by referencing a are based on empirical data from decades ago, with some not recommended range of values for critical dimensions of the validated through research. Also, research demonstrated that design of new roadway alignments and those undergoing major other design values lower than those suggested in the Green reconstruction (1). The Green Book provides guidelines--not Book work well to achieve flexibility in design while balancing standards--that permit sufficient flexibility to encourage the concerns of safety and capacity. While safety must always distinctive independent and appropriate designs for specific be considered when selecting design values, the ramifications situations. However, such flexibility can introduce uncertainty of cost restrictions and environmental concerns might warrant since there may be insufficient data to quantify potential consideration of a reduced value for a design element. Proper trade-offs when evaluating design variations. Even though the designs should assess competing constraints and create a Green Book indicates that the referenced guidelines provide solution that meets mobility and safety objectives. for a safe, comfortable, and aesthetically pleasing roadway, designers may have little information regarding the safety and Research Objectives and Approach operational consequences that can result if they deviate from the recommended range of values or select one value from This research brought together existing research knowledge over another within the range. and project experience to provide a reference guide for planners The Green Book provides control values for the design of new and designers. This research effort will provide the highway alignments or those undergoing major reconstruction that design community with information resources and decision