Evaluation of the 13 Controlling Criteria for Geometric Design (2014) / Chapter Skim
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From page 5... ...
5 S E C T I O N 2 This section presents the results of the review of design criteria, traffic operational and safety effects, and mitigation strategies for the 13 controlling criteria. This information concerning each of the controlling criteria is presented in Sections 2.1 through 2.13.
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6only an indirect effect. Once a design speed for a project is selected, however, that design speed influences the values (or value ranges)
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7 design speeds should not be less than the low range presented in Exhibit 10-56, with other specific guidance offered for particular types of ramps (loops as well as direct and semidirect connections)
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8accommodate truck offtracking; a formal design exception is not required where lane widening is not provided on a horizontal curve (7)
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9 CMF is calculated using the equations shown in Table 6 based on the lane width and the average annual daily traffic (AADT)
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10 the default distribution of crash types presented in HSM Table 10-4. This default crash type distribution and, therefore, the value of pra may be updated from local data as part of the calibration process.
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11 2.2.3 Urban and Suburban Arterials Design Criteria AASHTO policy provides substantial flexibility in the use of 10- to 12-ft lanes on urban arterials. In particular, Chapter 7 of the Green Book includes the following guidance: • Lane widths of 12 ft are most desirable and should be used, where practical, on higher speed, free-flowing, principal arterials.
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From page 12... ...
12 • Lane widths of 10 ft may be used in highly restricted areas having little or no truck traffic. Left-turn and combination lanes used for parking during off-peak hours and for traffic during peak hours may be 10 ft in width.
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13 signalized intersection approaches and not to midblock sections of arterials, it is not presented in this report, since intersection design criteria are outside the scope of the research. Traffic Safety Effects Chapter 12 (Urban and Suburban Arterials)
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14 2.3 Shoulder Width Shoulder width affects both capacity and safety on roadways. A wide shoulder increases capacity by reducing lateral friction between traffic and roadside objects and thereby increasing driver comfort.
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15 SOURCE: Based on HSM Figure 10-8. Figure 4.
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16 2.3.2 Rural Multilane Highways Design Criteria The Green Book states that the design criteria for shoulder width on rural two-lane highways presented in Table 12 are generally applicable to rural undivided multilane arterials, as well. For rural divided multilane arterials, the shoulder widths presented in Table 15 are recommended.
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From page 17... ...
17 is not one of the factors that influences free-flow speed. This suggests that shoulder width either has no effect on the freeflow speed on an urban street segment or an effect that is very small in comparison to the factors that are in the procedure (see HCM Exhibit 17-5)
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18 • For property-damage-only single-vehicle crashes on horizontal curves, CMF exp 0.0840 W 10 (11)
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19 narrowed cross section as well as decreased driver comfort on the bridge. 2.4.1 Rural Two-Lane Highways Design Criteria The minimum lane widths and shoulder widths shown in Tables 4 and 12, based on Green Book Exhibit 7-3, serve as the recommended minimum bridge widths for rural twolane arterials.
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20 2.4.3 Urban and Suburban Arterials Design Criteria Chapter 7 of the Green Book states that the minimum clear width for new bridges should be the same as the minimum curb-to-curb distance of the roadway for general conditions. For bridges that exceed 200 ft in length, the offsets to parapets, rails, or barriers may be reduced to 4 ft where shoulders or parking lanes are provided on the arterial.
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From page 21... ...
21 2.6.1 Rural Two-Lane Highways Design Criteria The design criteria for minimum curve radius presented in Table 20 apply to rural two-lane highways. Traffic Operational Effects Chapter 15 (Two-lane Highways)
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22 that Table 21, as it appears in the original research, uses metric units for speed and curve radius. Traffic Safety Effects Chapter 10 (Rural Two-Lane Highways)
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23 • For property-damage-only crashes, exp 0.040 0.13 ln 2 5730 3.80 1 1 exp 0.040 1.0 (17) ,CMF G R R L for horizontal curves G for tangents on nonlevel grades for level tangents base condition SG PDO C ( )
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24 concluded that the role of horizontal curvature in safety for this type of road may need reconsideration. There are several CMFs for horizontal curve radius in the FHWA CMF Clearinghouse, but none of these is specifically applicable to urban and suburban arterials.
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25 SOURCE: Based on Green Book Figure 3-41. Figure 5.
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26 Traffic Operational Effects Chapter 15 (Two-Lane Highways) of the HCM provides a methodology for adjusting the LOS boundaries on rural two-lane highways to account for vertical alignment, considering general terrain classes or specific grades, as well as the percentages in the traffic flow of two types of heavy vehicles (trucks and recreational vehicles)
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From page 27... ...
27 • For property-damage-only crashes, exp 0.0046 5730 1.0 1 1.0 (27) 1,CMF R L K for horizontal curves for tangents at Type crests for level tangents base condition C PDO VC( )
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28 and recreational vehicles)
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29 establish critical lengths of grade that would produce a differential of 15 mph or more between the minimum speed of trucks and the average speed of traffic. Depending on traffic and truck volumes, locations with critical length of grade may warrant the addition of truck climbing lanes.
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30 The heavy vehicle adjustment factor, fHV, accounts for heavy vehicles traveling more slowly on grades than passenger cars. A larger value of the passenger-car equivalence factors for heavy vehicles, ET or ER, results in a higher demand flow rate.
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31 fg = grade adjustment factor from Table 23 or 24 fHV = heavy vehicle adjustment factor from HCM Equations 15-4 or 15-5, which utilize data from Tables 25 through 27 The demand flow rate in the opposing direction is determined in a manner entirely analogous to Equation 34. The service measure average travel speed, which is one of two measures used to determine LOS, is then determined with HCM Equation 15-6.
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32 The underlying research (34, 35) presents the CMF as a continuous function rather than a step function, as follows: CMF (1.0 0.016 G)
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33 case, the use of a climbing lane would be considered a mitigation strategy and not part of the controlling criterion. Traffic Operational Effects According to Chapter 17 (Urban Street Segments)
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34 Traffic Safety Effects Chapter 12 (Urban and Suburban Arterials) of the HSM does not include a CMF for grade on urban and suburban arterials.
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35 • Reducing the frequency or severity of lane-departure crashes (enhanced pavement markings; delineation; shoulder, painted edgeline, or centerline rumble strips; paved or partially paved shoulders; safety edge treatment; clear recovery area; traversable slopes; breakaway safety hardware; and barrier where appropriate)
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From page 36... ...
36 of the controlling criteria; no design exceptions are required for decision sight distances less than the Green Book criteria. The HCM does not include any effect of stopping sight distance on LOS for any roadway type.
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37 safety. There are also no safety effects found in the FHWA CMF Clearinghouse.
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38 on operations by damaging overpasses or other structures that result in extended road closures. Special attention is given to vertical clearance on Interstate freeways to maintain the integrity of the system for national defense purposes.
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39 2.13.3 Urban and Suburban Arterials Design Criteria The design criterion for lateral offset on urban and suburban arterials in the 2006 RDG (39) , and previous editions, is 1.5 ft.
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40 Design criterion Roadway type Traffic operational effects Design speed All No direct effects.a Lane width Rural two-lane highways See Table 5 (based on HCM Exhibit 15-7) and Equation 1.
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From page 41... ...
41 Design criterion Roadway type Traffic safety effects Design speed All roadway types No direct effects.a Lane width Rural two-lane highways See Equation 2 and Table 6 (based on HSM Equation10-11 and Table 10-8)
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