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OCR for page 10
11 of a highway's geometric and operational features with driver also seek road environments that are self-explaining, quickly expectancy." understood, and easy for users to act upon. The 15-17 project team then developed a set of rules for The HFG recommends that the highway designer and the evaluating the design consistency of selected conditions. Fol- traffic engineer examine the road environment in incremen- lowing their evaluation of several case studies, they devel- tal steps similar to those steps taken by a road user to ensure oped a list of data needs for future evaluations of selected that the user will not be overloaded with temporal tasks and design elements, as follows: decisions. In short, good human factor principles must be integrated into the design of the road system. The sizes of Cross section, the iterative and incremental steps are not going to be the Horizontal alignment, same for all road environments, and they will vary depend- Vertical alignment, ing on the road user, the type of highway, the operations, and Railroad grade crossings, the environment. The iterative steps, however, must over- Narrow bridges, lap from one section to the next to ensure continuity of the Driveways, travel path and that no potentially meaningful information Preview sight distance, for road users will be overlooked. Highway designers and Climbing and passing lanes, and traffic engineers must jointly examine the road environment; Frequency of decisions that is, lane alignment (roadway and intersections), signing (advisory, regulatory, and guidance), and operations (normal The data needed to evaluate a roadway design using the and work zones) relative to the likelihood users will be able developed design consistency rules largely consisted of infor- to perform the required tasks safely and efficiently within the mation the researchers deemed to be readily available to the time and space available. The HFG discusses these elements designer, through field measurements and speed models. In primarily in terms of the driver, but similar principles are also some cases, however, additional information may be neces- discussed in relation to the nonmotorized road user. No specific sary to evaluate older alignments. All of the rules, data needs, recommendations were given for changes in Green Book meth- and the research team's related recommendations for revisions odology, but the HFG provides additional guidance based on to the 2001 Green Book are summarized in NCHRP Report 502 empirical data and expert judgment. (Wooldridge et al. 2003). Work Zone Considerations Cafiso et al. (2005) developed a model based on fuzzy logic techniques to classify roadway elements by using three NCHRP Report 581 (Mahoney et al. 2004) discusses the safety criteria (design consistency, operating speed consis- procedure for establishing an appropriate design speed for tency, and driving dynamics) to obtain a more careful evalu- work zones, which they define as "a selected speed used to ation of inconsistencies between highway design elements determine [specific work zone] geometric design features." for redesigns, 3R projects, and existing alignments. For each A value equal to or slightly greater than the target speed criterion, the inconsistencies were included in three fuzzy (i.e., the desirable free-flow operating speed) is appropri- sets (good, fair, poor), with differing degrees of member- ate for work zone design speed. In the report, work zone ship. By defining linear membership functions, the research- design speed is applicable to radius of curvature and super- ers classified road sections and then determined a prioritiza- elevation and, when the work zone design speed is less than tion scale of maintenance interventions. Their procedure was 40 mph, it is also used to determine appropriate sight distance. intended to be applied to large databases of road networks to Work zone design speed may also be used in computing the identify the more dangerous design elements that need inter- minimum length of sag vertical curves. Other speed param- ventions to improve highway safety and to allocate resources eters (e.g., speed limit and anticipated 85th percentile speed) under limited budget conditions. are also referenced in some design guidelines. The authors conclude that the establishment of a target speed and work zone design speed, design of temporary traffic control, and Driver Characteristics potential selection of speed management measures are related. It is important that speed-related decisions within specific NCHRP's Human Factors Guidelines for Road Systems domains (i.e., design, regulatory, and speed management) be (HFG) (Campbell et al. 2008) states that designers and traffic consistent with an overall strategy. engineers need to examine the roadway environment for infor- mation conflicts that may mislead or confuse road users. They must anticipate what information the road user requires and Summary of Key Findings where it is needed so that appropriate design elements or traffic control can be integrated into the design and operational plans. This section summarizes key findings from this chapter. This Missing information is not helpful to the road user. As stated is an annotated summary; conclusions and recommendations another way in the report, designers and traffic engineers must are those of the authors of the references cited.

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12 Design Vehicles limit. Design speed appeared to have minimal impact on operating speeds unless a tight horizontal radius or a Dimensions of commonly used trucks have changed in low K-value was present (Fitzpatrick et al. 2003a). recent years, prompting recommendations to revise the Multiple studies examined the possibility of selecting dimensions of those vehicles in the Green Book (Har- a design speed based more heavily on the context of wood et al. 2003b). the environment in which the roadway was located. A Along with the changes in dimensions have come primary area of concern, however, was how to define changes in performance; however, research indicated the context to be considered (Garrick and Wang 2005; that design criteria for sight distance, lane width, hori- Wang et al. 2006). zontal curves, cross-slope breaks, and vertical clear- ance were sufficient to accommodate the performance of trucks (Harwood et al. 2003b). Driver Characteristics A study of human factors related to the driving task sug- Design Speed gested that designers and traffic engineers must exam- ine the roadway environment for information conflicts A review of design speed practices indicated that posted that may mislead or confuse road users (Campbell et al. speed limit and anticipated operating speed were fre- 2008). quently associated with the selection of design speed The study concluded that designers and traffic engi- (Fitzpatrick and Carlson 2002). neers must also seek road environments that are self- Observation of driving behavior revealed that the explaining, quickly understood, and easy for users to strongest indicator of operating speed was posted speed act up (Campbell et al. 2008).