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7 chapter two DEsign Controls and Criteria Overview mined that the current geometric design criteria could rea- sonably accommodate trucks. The research did develop The advent of a new definition for design speed in the 2001 "a spreadsheet program, known as the truck speed profile Green Book led to some ideas for research about the effects of model, [designed to] estimate the truck speed profile on any that new definition. Researchers used that definition to inves- specified upgrade, considering any truck weight/power ratio, tigate relationships among design speed, operating speed, and any initial truck speed, and any vertical profile. Field stud- posted speed. Also, multiple studies reviewed selections of ies were also conducted to better quantify the weight/power the Green Book to determine if long-standing guidelines were ratios of the current truck fleet; the results of [those] field still applicable to modern vehicles and drivers. Special atten- studies [indicated] that trucks in western states have better tion was paid to trucks, to determine if roadways primarily performance than in eastern states and the truck population designed for passenger cars would still accommodate increas- on freeways generally has better performance than the truck ing numbers of heavy vehicles. population on two-lane highways." Design Vehicles Easa and El Halim (2006) conducted research to establish minimum radius requirements on the basis of vehicle stabil- Trucks are an important consideration in the geometric design ity for trucks on three-dimensional (3-D) reverse horizontal of highways. Many highway geometric design policies are curves with intermediate tangents. With vehicle simulation based on vehicle characteristics. Truck characteristics are software, vehicle dynamics were recorded for the base case of often a key consideration in determining the recommended two-dimensional (2-D) simple curves and for reverse curves values of such criteria. Harwood et al. (2003b) conducted a superimposed with different vertical alignments (upgrade, research project to "review the characteristics of trucks in downgrade, crest curve, and sag curve). They conducted sim- the current U.S. truck fleet, as well as possible changes to the ulation for two maximum superelevation rates, three design truck fleet, and [recommend] appropriate changes to high- vehicles, and different vertical grades. Two mathematical way geometric design policy to ensure that highways can models were developed for flat and 3-D reverse curves. The reasonably accommodate trucks." models provided the minimum radius of the sharper arc of the reverse curve as a function of design speed, maximum super- The authors recommended several changes in the design elevation, ratio of flatter to sharper curve radius, design vehicle, vehicles presented in the Green Book, specifically: and intermediate tangent length. Their results indicated that an increase in the minimum radius of existing design guides · That the then "current WB-15 [WB-50] design vehicle (between 5% and 27%) was required to compensate for the be dropped because it [was] no longer common on U.S. effects of reverse curvature and vertical alignment and main- roads." tain the same comfort level specified in the design guides. · "The kingpin-to-center-of-rear-tandem distance for the They concluded that the required increase could be reduced by WB-19 [WB-62] design vehicle be increased from 12.3 using longer intermediate tangents, and they presented design to 12.5 m [40.5 to 41 ft]." requirements for the spiral length of reverse curves. · "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. Design Speed · "A three-axle truck, the SU-8 [SU-25] design vehicle" (shown in Figure 1b), "and a Rocky Mountain Double, Fitzpatrick and Carlson (2002) reviewed current practices for the WB-28D [WB-92D] design vehicle" (shown in Fig- selection of design speed after the release of the 2001 Green ure 1c) be added to the Green Book. Book and its revised definition of design speed. They found that practices varied widely, including the use of functional The researchers did not identify a need to update the Green classification, consideration of location (i.e., rural or urban), Book design criteria for sight distance, lane width, horizon- terrain, Green Book procedure, legal speed limit (possibly tal curves, cross-slope breaks, or vertical clearance to better with a value of 5 or 10 mph added), anticipated volume and/ accommodate trucks. In each case, their evaluation deter- or operating speed, adjacent development, costs, and design
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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).
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9 consistency. They found that as many as half of the states · Other notable relationships between operating speed surveyed used posted speed or operating speed in their con- and roadway variables were identified as follows: siderations, although the Green Book process did not explic- Access density showed a strong relationship with itly include them. Techniques they recommended for future 85th percentile speed, with higher speeds being asso- revisions to the design speed selection process included: ciated with lower access densities. Lower speeds occurred as pedestrian activity increased. · Consideration of anticipated posted or operating speed; The absence of either centerline or edgeline markings · A feedback loop; was associated with lower speeds. · Modifying values recommended for different functional Speeds were lower where on-street parking was classes, rural versus urban, or terrain; and permitted. · Explicit consideration of tangent length as a design When no median was present, speeds were slightly element. lower than when a raised, depressed, or two-way left-turn lanes (TWLTL) median was present, with Under NCHRP Project 15-18 (Fitzpatrick et al. 2003a), "the a few exceptions. Texas Transportation Institute compiled and analyzed industry There was no evidence that the presence of curb and definitions for speed-related terms and recommended more gutter resulted in lower speeds for a facility. consistent definitions for the Green Book and the MUTCD. The · Results from a mailout survey indicated that most states researchers surveyed state and local practices for establishing used Green Book definitions in the design of roadways, design speeds and speed limits and synthesized information on "but far fewer respondents indicated that it was their the relationships between speed, geometric design elements, preferred definition." and highway operations. Next, researchers critically reviewed · Most design elements and their values were either directly geometric design elements to determine if they should be or indirectly selected based on design speed. In several based on speed and identified alternative [design-element] situations, the type of roadway was used to determine selection criteria. Geom etric, traffic, and speed data were the design element value or feature; however, the type of collected at numerous sites around the United States and ana- roadway was strongly associated with the operating speed lyzed to identify relationships between the various factors and of the facility. speeds on urban and suburban sections away from signals, stop · The relationship with operating speed was identified for signs, and horizontal curves (all elements previously found to several design elements. In some cases, such as for hori- zontal curves, the relationship was strong, and in other affect operating speeds)." cases, such as for lane width, the relationship was weak. In all cases when a relationship between the design ele- The work of the NCHRP 15-18 team was documented ment and operation speed existed there were ranges in NCHRP Report 504 (Fitzpatrick et al. 2003a) In addition when the influence of the design element on speed was to including the survey of practice and information on the minimal. relationships between speed and various geometric and · "While the relationship between a design element and traffic factors, the report lists suggested refinements to the operating speed may be weak, the consequences of selec Green Book in the following areas: design speed definitions, ting a particular value may have safety implications. information on posted speed and its relationship with operat- A safety review [indicated] that there [were] known ing speed and design speed, how design speed values were relationships between safety and design [features] and selected in the United States (noting that anticipated posted that the selection of the design feature [varied] based speed and anticipated operating speed were also used in addi- on the operating speed of the facility. Therefore, the tion to the process in the then-current edition of the Green design elements investigated within this study should Book, which is based on terrain, functional class, and rural be selected with some consideration of the anticipated versus urban), changes to functional class material, and addi- operating speed of the facility. In some cases the con- tional discussion on speed prediction and feedback loops. sideration would take the form of selecting a design ele- Among the findings documented in NCHRP Report 504 are ment value within a range that has minimal influence the following: on operating speed or that would not adversely affect safety, while in other cases the selection of a design ele- · The "strongest relationship found in NCHRP Project ment value would be directly related to the anticipated 15-18 was between operating speed and posted speed operating speed." limit. No other roadway variable [including design speed] was statistically significant at a 5 percent alpha level." Based on their findings, researchers recommended the · "Design speed [appeared] to have minimal impact on following changes to the Green Book in NCHRP Report 504: operating speeds unless a tight horizontal radius or a low K-value [was] present. Large variance in operating · "Add discussion on posted speed limit to encourage a speed was found for a given inferred design speed on better understanding of the relationship between 85th rural two-lane highways." percentile speed and posted speed limit (i.e., posted