Evaluation of the 13 Controlling Criteria for Geometric Design (2014) / Chapter Skim
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From page 68... ...
68 S E C T I O N 6 This section of the report applies the existing traffic operational and safety relationships from Section 2 and the new relationships from the research presented in Section 4 to prioritize the 13 controlling criteria for specific roadway types. The discussion begins by presenting the ranking of the 13 controlling criteria from a recent survey of highway agencies, followed by presentation of the priorities developed in this research.
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From page 69... ...
69 width ranked as having the greatest importance and horizontal clearance and grade ranked as having the least importance. 6.2 Sensitivity Analysis of Key Controlling Criteria for Specific Roadway Types The research included a sensitivity analysis for both traffic operations and safety of the 10 key controlling criteria (excluding design speed, structural capacity, and vertical clearance for the reasons cited earlier)
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From page 70... ...
70 • The lower end of the range of interest, representing restrictive design, was set equal to the lowest value generally used in practice or a typical low value for each of the controlling criteria; for example, the lower end of the range of interest for lane width was set equal to 9 ft. Using the example of lane width, the sensitivity of traffic operations to lane width and the sensitivity of safety to lane width were each determined by reducing lane width from the upper end value of 12 ft to the midpoint value of 10.5 ft.
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Levels for Specific Controlling Criteria Midpoint between Typical or typical and Design criterion ideal design Restrictive design restrictive design Comment Lane width (ft) 12 9 10.5 Range from ideal to most restrictive lane width Shoulder width (ft)
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From page 72... ...
72 Controlling criterion Traffic operational effect Traffic safety effect Lane width Table 5 and Equation 1 Table 6 and Equation 2 Shoulder width Table 5 and Equation 1 Tables 13 and 14 and Equation 7 Bridge width Table 5 and Equation 1a No known effect based on Section 4.3 Horizontal curve radius Table 21 Equation 15 Sag vertical curve length No known effect based on Section 2.7 Equations 30 through 33 Grade Tables 23 through 31 and Equation 34 Table 32 and Equation 35 Stopping sight distance No known effect based on Section 2.9 Effect based on Section 4.7 if a hidden feature is present Cross slope No known effect based on Section 2.10 No known effect based on Section 2.10 Superelevation No known effect based on Section 2.11 Equations 36 through 38 Lateral offset Not applicable where shoulders are present Not applicable where shoulders are present a No additional effect beyond the effect of a narrower lane or shoulder, if present on bridge. Table 71.
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From page 73... ...
73 Design criterion Traffic safety effect: Percent change in fatal-and-injury crashes/mi/yeara in comparison to base condition Rank order of traffic safety effectb Scenarioc #1 Scenario #2 Scenario #3 Scenario #1 Scenario #2 Scenario #3 Combined Lane width 4.27 5.15 5.84 2 2 2 2 Shoulder width 5.24 6.62 7.51 1 1 1 1 Bridge width 0.00 0.00 0.00 7 7 7 7 Horizontal curve radius 0.88 1.06 1.20 4 4 4 4 Sag vertical curve length 0.00 0.00 0.00 9 9 9 9 Grade 0.97 1.17 1.33 3 3 3 3 Stopping sight distanced 0.00 0.00 0.00 10 10 10 10 Stopping sight distancee 0.03 0.03 0.02 6 6 6 6 Cross slope 0.00 0.00 0.00 8 8 8 8 Superelevation 0.66 0.80 0.91 5 5 5 5 Lateral offset 0.00 0.00 0.00 11 11 11 11 a Traffic safety effects are in comparison to crash frequency of 0.71 fatal-and-injury crashes/mi/year for the base condition in Scenario #1, 1.46 fatal-and-injury crashes/mi/year for base condition in Scenario #2, and 2.58 fatal-and-injury crashes/mi/year for base condition in Scenario #3. b Estimated based on literature review results and judgment for situations with zero effects.
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From page 74... ...
Levels for Specific Controlling Criteria Midpoint between Typical or typical and Design criterion ideal design Restrictive design restrictive design Comment Lane width (ft) 12 9 10.5 Range from ideal to most restrictive lane width Outside shoulder width (ft)
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From page 75... ...
75 Controlling criterion Traffic operational effect Traffic safety effect Lane width Table 7 and Equation 3 Table 8 or 9 and Equation 2 Shoulder width Table 16 and Equation 3 Tables 13 and 14 and Equation 7 or Table 17 Bridge width Table 16 and Equation 3a No known effect based on Section 2.4 Horizontal curve radius Equation 18 Equations 19 and 20 Sag vertical curve length No known effect based on Section 2.7 No known effect based on Section 2.7 Grade Tables 33 through 36 and HCM Equation 14-4 No known effect based on Section 2.8 Stopping sight distance No known effect based on Section 2.9 Effect estimated as equivalent to the rural two-lane highway effect in Section 4.7 if a hidden feature is present Cross slope No known effect based on Section 2.10 No known effect based on Section 2.10 Superelevation No known effect based on Section 2.11 No known effect based on Section 2.11 Lateral offset Not applicable where shoulders are present Not applicable where shoulders are present a No additional effect beyond the effect of a narrower lane or shoulder, if present on bridge Table 75. Estimation methods for traffic operational and safety effects for rural multilane highways.
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76 large to be quantified. It is possible that some of these design criteria have effects on crash frequency that are too small to be quantified, and it is likely that some design criteria, in fact, have no effect on crash frequency.
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From page 77... ...
Levels for Specific Controlling Criteria Midpoint between Typical or typical and Design criterion ideal design Restrictive design restrictive design Comment Lane width (ft) 12 9 10.5 Range from ideal to most restrictive lane width Outside shoulder width (ft)
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From page 78... ...
78 Controlling criterion Traffic operational effect Traffic safety effect Lane width Table 10 and Equation 4 Equations 5 and 6 Shoulder width Table 19 and Equation 4 Equations 8 through 13 Bridge width Table 19 and Equation 4a No known effect based on Section 2.4 Horizontal curve radius No known effect based on Section 2.6 Equations 22 through 25 Sag vertical curve length No known effect based on Section 2.7 No known effect based on Section 2.7 Grade Tables 39 through 42 and HCM Equations 11-2 through 11-4 No known effect based on Section 2.8 Stopping sight distance No known effect based on Section 2.9 Effect estimated as equivalent to the rural two-lane highway effect in Section 4.7 if a hidden feature is present Cross slope No known effect based on Section 2.10 No known effect based on Section 2.10 Superelevation No known effect based on Section 2.11 No known effect based on Section 2.11 Lateral offset Not applicable where shoulders are present Not applicable where shoulders are present a No additional effect beyond the effect of a narrower lane or shoulder, if present on bridge Table 79. Estimation methods for traffic operational and safety effects for rural freeways.
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From page 79... ...
79 fact, have no effect on crash frequency. A rank order for the remaining design criteria has been established based on literature review results and judgment.
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From page 80... ...
80 Priority rank Roadway type Rural two-lane highways Rural multilane highways Rural freeways TRAFFIC OPERATIONS 1 (highest priority) Shoulder width Lane width Lane width 2 Lane width Shoulder width Shoulder width 3 Horizontal curve radius Horizontal curve radius Horizontal curve radius 4 Grade Grade Grade 5 Bridge width Bridge width Bridge width 6 Superelevation Superelevation Superelevation 7 Stopping sight distancea Stopping sight distancea Stopping sight distanceb 8 Stopping sight distancec Stopping sight distancec Stopping sight distancec 9 Sag vertical curve length Sag vertical curve length Sag vertical curve length 10 Cross slope Cross slope Cross slope 11 (lowest priority)
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