National Academies Press: OpenBook

Recent Roadway Geometric Design Research for Improved Safety and Operations (2012)

Chapter: Chapter Two - Design Controls and Criteria

« Previous: Chapter One - Introduction
Page 6
Suggested Citation:"Chapter Two - Design Controls and Criteria." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
×
Page 6
Page 7
Suggested Citation:"Chapter Two - Design Controls and Criteria." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
×
Page 7
Page 8
Suggested Citation:"Chapter Two - Design Controls and Criteria." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
×
Page 8
Page 9
Suggested Citation:"Chapter Two - Design Controls and Criteria." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
×
Page 9
Page 10
Suggested Citation:"Chapter Two - Design Controls and Criteria." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
×
Page 10
Page 11
Suggested Citation:"Chapter Two - Design Controls and Criteria." National Academies of Sciences, Engineering, and Medicine. 2012. Recent Roadway Geometric Design Research for Improved Safety and Operations. Washington, DC: The National Academies Press. doi: 10.17226/14661.
×
Page 11

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

7 Overview The advent of a new definition for design speed in the 2001 Green Book led to some ideas for research about the effects of that new definition. Researchers used that definition to inves- tigate relationships among design speed, operating speed, and posted speed. Also, multiple studies reviewed selections of the Green Book to determine if long-standing guidelines were still applicable to modern vehicles and drivers. Special atten- tion was paid to trucks, to determine if roadways primarily designed for passenger cars would still accommodate increas- ing numbers of heavy vehicles. Design vehicles Trucks are an important consideration in the geometric design of highways. Many highway geometric design policies are based on vehicle characteristics. Truck characteristics are often a key consideration in determining the recommended values of such criteria. Harwood et al. (2003b) conducted a research project to “review the characteristics of trucks in the current U.S. truck fleet, as well as possible changes to the truck fleet, and [recommend] appropriate changes to high- way geometric design policy to ensure that highways can reasonably accommodate trucks.” The authors recommended several changes in the design vehicles presented in the Green Book, specifically: • That the then “current WB-15 [WB-50] design vehicle be dropped because it [was] no longer common on U.S. roads.” • “The kingpin-to-center-of-rear-tandem distance for the WB-19 [WB-62] design vehicle be increased from 12.3 to 12.5 m [40.5 to 41 ft].” • “The WB-20 [WB-65] design vehicle should be dropped from the Green Book and the WB-20 [WB-67] design vehicle” (shown in Figure 1a) used in its place. • “A three-axle truck, the SU-8 [SU-25] design vehicle” (shown in Figure 1b), “and a Rocky Mountain Double, the WB-28D [WB-92D] design vehicle” (shown in Fig- ure 1c) be added to the Green Book. The researchers did not identify a need to update the Green Book design criteria for sight distance, lane width, horizon- tal curves, cross-slope breaks, or vertical clearance to better accommodate trucks. In each case, their evaluation deter- mined that the current geometric design criteria could rea- sonably accommodate trucks. The research did develop “a spreadsheet program, known as the truck speed profile model, [designed to] estimate the truck speed profile on any specified upgrade, considering any truck weight/power ratio, any initial truck speed, and any vertical profile. Field stud- ies were also conducted to better quantify the weight/power ratios of the current truck fleet; the results of [those] field studies [indicated] that trucks in western states have better performance than in eastern states and the truck population on freeways generally has better performance than the truck population on two-lane highways.” Easa and El Halim (2006) conducted research to establish minimum radius requirements on the basis of vehicle stabil- ity for trucks on three-dimensional (3-D) reverse horizontal curves with intermediate tangents. With vehicle simulation software, vehicle dynamics were recorded for the base case of two-dimensional (2-D) simple curves and for reverse curves superimposed with different vertical alignments (upgrade, downgrade, crest curve, and sag curve). They conducted sim- ulation for two maximum superelevation rates, three design vehicles, and different vertical grades. Two mathematical models were developed for flat and 3-D reverse curves. The models provided the minimum radius of the sharper arc of the reverse curve as a function of design speed, maximum super- elevation, ratio of flatter to sharper curve radius, design vehicle, and intermediate tangent length. Their results indicated that an increase in the minimum radius of existing design guides (between 5% and 27%) was required to compensate for the effects of reverse curvature and vertical alignment and main- tain the same comfort level specified in the design guides. They concluded that the required increase could be reduced by using longer intermediate tangents, and they presented design requirements for the spiral length of reverse curves. Design speeD Fitzpatrick and Carlson (2002) reviewed current practices for selection of design speed after the release of the 2001 Green Book and its revised definition of design speed. They found that practices varied widely, including the use of functional classification, consideration of location (i.e., rural or urban), terrain, Green Book procedure, legal speed limit (possibly with a value of 5 or 10 mph added), anticipated volume and/ or operating speed, adjacent development, costs, and design chapter two Design cOntrOls anD criteria

8 (a) (b) (c) FIGURE 1 Dimensions of recommended design vehicles: (a) Interstate semitrailer [WB-20 (WB-67)] design vehicle, (b) three-axle single-unit [SU-8 (SU-25)] design vehicle, (c) Rocky Mountain double combination [WB-28D (WB-92D)] design vehicle (Harwood et al. 2003b).

9 consistency. They found that as many as half of the states surveyed used posted speed or operating speed in their con- siderations, although the Green Book process did not explic- itly include them. Techniques they recommended for future revisions to the design speed selection process included: • Consideration of anticipated posted or operating speed; • A feedback loop; • Modifying values recommended for different functional classes, rural versus urban, or terrain; and • Explicit consideration of tangent length as a design element. Under NCHRP Project 15-18 (Fitzpatrick et al. 2003a), “the Texas Transportation Institute compiled and analyzed industry definitions for speed-related terms and recommended more consistent definitions for the Green Book and the MUTCD. The researchers surveyed state and local practices for establishing design speeds and speed limits and synthesized information on the relationships between speed, geometric design elements, and highway operations. Next, researchers critically reviewed geometric design elements to determine if they should be based on speed and identified alternative [design-element] selection criteria. Geom etric, traffic, and speed data were collected at numerous sites around the United States and ana- lyzed to identify relationships between the various factors and speeds on urban and suburban sections away from signals, stop signs, and horizontal curves (all elements previously found to affect operating speeds).” The work of the NCHRP 15-18 team was documented in NCHRP Report 504 (Fitzpatrick et al. 2003a) In addition to including the survey of practice and information on the relationships between speed and various geometric and traffic factors, the report lists suggested refinements to the Green Book in the following areas: design speed definitions, information on posted speed and its relationship with operat- ing speed and design speed, how design speed values were selected in the United States (noting that anticipated posted speed and anticipated operating speed were also used in addi- tion to the process in the then-current edition of the Green Book, which is based on terrain, functional class, and rural versus urban), changes to functional class material, and addi- tional discussion on speed prediction and feedback loops. Among the findings documented in NCHRP Report 504 are the following: • The “strongest relationship found in NCHRP Project 15-18 was between operating speed and posted speed limit. No other roadway variable [including design speed] was statistically significant at a 5 percent alpha level.” • “Design speed [appeared] to have minimal impact on operating speeds unless a tight horizontal radius or a low K-value [was] present. Large variance in operating speed was found for a given inferred design speed on rural two-lane highways.” • Other notable relationships between operating speed and roadway variables were identified as follows: – Access density showed a strong relationship with 85th percentile speed, with higher speeds being asso- ciated with lower access densities. – Lower speeds occurred as pedestrian activity increased. – The absence of either centerline or edgeline markings was associated with lower speeds. – Speeds were lower where on-street parking was permitted. – When no median was present, speeds were slightly lower than when a raised, depressed, or two-way left-turn lanes (TWLTL) median was present, with a few exceptions. – There was no evidence that the presence of curb and gutter resulted in lower speeds for a facility. • Results from a mailout survey indicated that most states used Green Book definitions in the design of roadways, “but far fewer respondents indicated that it was their preferred definition.” • Most design elements and their values were either directly or indirectly selected based on design speed. In several situations, the type of roadway was used to determine the design element value or feature; however, the type of roadway was strongly associated with the operating speed of the facility. • The relationship with operating speed was identified for several design elements. In some cases, such as for hori- zontal curves, the relationship was strong, and in other cases, such as for lane width, the relationship was weak. In all cases when a relationship between the design ele- ment and operation speed existed there were ranges when the influence of the design element on speed was minimal. • “While the relationship between a design element and operating speed may be weak, the consequences of selec- t ing a particular value may have safety implications. A safety review [indicated] that there [were] known relationships between safety and design [features] and that the selection of the design feature [varied] based on the operating speed of the facility. Therefore, the design elements investigated within this study should be selected with some consideration of the anticipated operating speed of the facility. In some cases the con- sideration would take the form of selecting a design ele- ment value within a range that has minimal influence on operating speed or that would not adversely affect safety, while in other cases the selection of a design ele- ment value would be directly related to the anticipated operating speed.” Based on their findings, researchers recommended the following changes to the Green Book in NCHRP Report 504: • “Add discussion on posted speed limit to encourage a better understanding of the relationship between 85th percentile speed and posted speed limit (i.e., posted

10 speed limits [were] generally set between 4 and 8 mph less than the measured 85th percentile speed, and only 23% to 64% of vehicles operated below the posted speed limit in urban areas in field studies).” • “Change text to recognize freeways as a unique func- tional class. Encourage the recognition that the look of a roadway (e.g., ramps, wide shoulders, and medi- ans) is associated with the anticipated speeds on the facility.” • “Add comments in the design speed discussion to iden- tify that the following may affect operating speed: radius, grade, access density, median presence, on-street parking, pedestrian activity, and signal density.” • “Add information on the state of the practice for select- ing design speed values, [because] anticipated operating speed and anticipated posted speed limit [were] being used by a notable percentage of the states [surveyed].” • “Introduce the concept of speed prediction and feedback loops, [with] reference to FHWA-[sponsored] work on the [Interactive Highway Safety Design Model] IHSDM.” Garrick and Wang (2005) examined context-based alter- natives to the use of design speed as a controlling criterion for design of streets and highways. They concluded that there were two main areas of concern—how to better define con- text and how to design for appropriate operations (including speed)—that must be addressed in developing a more coher- ent and context-based approach to design. They discussed the need for an overarching design framework that integrates all facets required for good design; their framework consisted of a four-step process: define the context, characterize the function, select the road typology, and determine the design details. They believed that this framework would be essential when designing truly context-based thoroughfares that facili- tate the operational and safety issues of all users and that also address the issues of context and livability that affect how well streets or roads function as places. Wang et al. (2006) investigated the relationship between the speed choices of drivers and their associated low-speed (e.g., speed limits ranging from 30 to 40 mph) urban roadway environments by analyzing second-by-second in-vehicle global positioning system data from more than 200 randomly selected vehicles in Georgia. The authors developed operating- speed models for low-speed urban street segments on the bases of roadway alignment, cross-section characteristics, roadside features, and adjacent land uses; their goal was that the model could “help highway designers and planners better understand expected operating speeds when they design and evaluate low-speed urban roadways.” The authors concluded that the following variables were significant at the 95th per- centile: number of lanes, the density and offsets of roadside objects, the density of T-intersections and driveways, raised curb presence, sidewalk presence, on-street parking, and land uses. They suggested that the posted speed limit not be included in the model because of its strong correlation to design speed, and that the posted speed limit was highly correlated with the intercept variable and the number of lanes variable in their model. In addition, they found that several significant variables in their tangent model became statisti- cally insignificant when posted speed limit was included. Their major findings included the following: • The number of lanes per direction of travel had the most significant influence on drivers’ speeds at tangent locations. • On-street parking and sidewalks were “the second and third [most] significant variables that [affected] drivers’ speeds on tangent” sections of low-speed urban streets. • Drivers selected lower speeds with an increase in the density of trees or utility poles, or with a decrease in their offsets. • Drivers tended to select lower speeds with an increase in density of driveways or T-intersections. Donnell et al. (2009) employed another term for practi- tioners to consider, “referred to as ‘inferred design speed.’ Inferred design speed is applicable only to features and ele- ments that have a criterion based on [a] designated design speed (e.g., vertical curvature, sight distance, superelevation).” The inferred design speed of a feature will be different from the designated design speed when the actual value is differ- ent from the criterion-limiting (minimum or maximum) value. For example, the inferred design speed for a combination of radius and superelevation is the maximum speed for which the limiting speed-based side friction value is not exceeded for the designed rate of superelevation and the inferred design speed; as such, it is determined through an iterative process. The inferred design speed for a horizontal curve may also be limited by horizontal offsets to sight obstructions on the inside of a horizontal curve. The inferred design speed for a crest vertical curve is the maximum speed for which the available stopping sight distance (SSD) is not exceeded by the required SSD. The inferred design speed may also be lim- ited by a combination of lane width and average daily traffic (ADT). The inferred design speed can be greater than, equal to, or less than the designated design speed. Design cOnsistency Under NCHRP Project 15-17, Wooldridge et al. (2003) “reviewed the domestic and international literature on geo- metric design consistency and developed a comprehensive list of geometric design features for high-speed, rural, two-lane roads that can reduce geometric consistency or violate driver expectancy. They then identified the most critical roadway features or combinations of features and considered how they might affect driver performance. A data collection and analy- sis plan was developed to formulate relationships between key parameters of the features and driver performance.” As part of the research, the team recommended a definition for design consistency: “Design consistency is the conformance

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

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

Next: Chapter Three - Elements of Design »
Recent Roadway Geometric Design Research for Improved Safety and Operations Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 432: Recent Roadway Geometric Design Research for Improved Safety and Operations reviews and summarizes roadway geometric design literature completed and published from 2001 through early 2011, particularly research that identified impacts on safety and operations.

The report is structured to correspond to chapters in the American Association of State Highway and Transportation Officials’ A Policy on Geometric Design of Highways and Streets, more commonly referred to as the Green Book.

NCHRP Synthesis 432 is an update of NCHRP Synthesis 299 on the same topic published in 2001.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!