Design Guidelines for Horizontal Sightline Offsets (2019) / Chapter Skim
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10 2.1 Current SSD Design Criteria This section presents an overview of the current AASHTO SSD design criteria and their application to vertical and horizontal sight restrictions. The review of SSD design criteria presented here is based on the 2011 edition of the Green Book.
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Design Criteria for HSOs 11 The second term in the SSD model (1.075 V2/a) is the distance needed for the vehicle to come to a stop once the driver begins to apply the brakes.
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12 Design Guidelines for Horizontal Sightline Offsets the full distance given by the DSSD criteria presented in Table 1. This requires minimum crest vertical curve length given by: when DSSD is less than L, ( )
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Design Criteria for HSOs 13 Figure 7 shows the parameters considered in determining the length of a crest vertical curve to provide SSD. Figure 8 shows the minimum lengths of crest vertical curves for specific values of algebraic difference in grade (A)
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14 Design Guidelines for Horizontal Sightline Offsets trees, bushes, crops, or other vegetation are present on the inside of a horizontal curve, a driver's ability to see a sufficient distance ahead may be limited. The smaller the radius of a horizontal curve, the larger the distance from the roadway, or HSO, that should be clear of obstructions for the driver to have available at least the applicable DSSD specified in the Green Book (AASHTO 2011)
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Design Criteria for HSOs 15 2.2.2 Initial Speed at Beginning of Braking Maneuver In deriving SSD values, the first term of Equations (2)
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16 Design Guidelines for Horizontal Sightline Offsets of the roadway (except on sag vertical curves where an overhead structure may be present)
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Design Criteria for HSOs 17 Figure 9. SSD on horizontal curves (AASHTO 2011)
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18 Design Guidelines for Horizontal Sightline Offsets (a)
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Design Criteria for HSOs 19 stopped temporarily on the shoulder) , the value of HSO, m, is only of practical importance to design where: > + 2 (7)
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20 Design Guidelines for Horizontal Sightline Offsets Liu and Wang (2012)
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