Selecting Ramp Design Speeds, Volume 1: Guide (2021) / Chapter Skim
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3 Section 1. Introduction 1.1 Background In the AASHTO A Policy on Geometric Design of Highways and Streets, commonly referred to as the Green Book, design speed is defined as a selected speed used to determine the various geometric design features of the roadway (AASHTO, 2018)
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4 The primary components of a ramp at a service interchange include the freeway mainline ramp terminal (i.e., an acceleration or deceleration lane) , the ramp proper, and the crossroad ramp terminal; and they must be considered together with the two intersecting roadways during the design of a ramp (see Figure 1)
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5 Figure 2. Primary Components of a Ramp at a System Interchange The design guidelines presented herein provide further detail for selecting an appropriate ramp design speed than presented in the 2018 Green Book, to address several overarching challenges that may lead to confusion or inconsistent interpretation of existing AASHTO guidance for selecting an appropriate ramp design speed.
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6 configurations separately, Green Book Table 10-1 provides upper-, middle-, and lowerrange values for ramp design speeds. AASHTO policy then provides further guidance regarding desired, practical, and/or minimum ramp design speeds corresponding to the upper-, middle-, and lower-speed ranges for different ramp configurations.
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7 should be applicable to other ramp components, and not simply the controlling curve on the ramp proper. • There is no consensus on whether the ramp design speed should be a single value or should vary along the ramp: Again, the Green Book states that the guide values in Green Book Table 10-1 are applicable to the sharpest, or controlling, curve on the ramp, usually on the ramp proper, and do not pertain to the ramp terminals; but as speeds may vary along a ramp, it is not directly evident whether a single ramp design speed should be used or the ramp design speed should vary along the ramp.
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8 higher (Leisch, 2005)
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9 speeds of vehicles exiting the freeway mainline ramp terminal will be consistent with the anticipated operating speeds entering the controlling curve.
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10 Figure 4. Illustration of Dividing the Ramp Proper of an Exit Ramp into Individual Tangents and Curves 3.
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11 be used to design the curve rather than the anticipated speeds at the beginning or end of the curve. The basic curve equation is: 𝑅 = ( .
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12 does not provide an explicit definition, but rather the definition or meaning of the term is understood. For these terms an explicit definition of the term is provided herein.
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13 Figure 5. Freeway Mainline Ramp Terminals (Entrance and Exit Ramps)
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14 Ramp design speed Ramp design speed is a selected speed used to determine the various geometric design features of the ramp. Where the horizontal alignment of the ramp is curvilinear, the ramp design speed applies to the controlling curve on the ramp proper.
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15 Ramps Configurations: Diagonal ramp (Figure 6) A diagonal ramp is a turning roadway that connects two intersecting roadways at an interchange.
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16 Direct connection (Figure 8) A direct connection is defined as a ramp that does not deviate greatly from the intended direction of travel.
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17 Acceleration length (LAcc Length) (Figure 5)
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18 Taper length (LTaper) (Figure 5)
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19 Painted nose (Figure 12) The painted nose is a point, having no dimensional width, where the pair of solid white pavement edge markings from the ramp and intersecting roadway meet.
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20 Crossover crown line At the boundary between the right lane of the freeway and the adjacent auxiliary lane of a freeway mainline ramp terminal, the cross slope rates of the freeway lane and auxiliary lane may differ. This boundary location is referred to as the crossover crown line.
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21 Superelevati on Superelevation is the rotation of the pavement on the approach to and through a horizontal curve. Superelevation is intended to assist the driver by counteracting the lateral acceleration produced by tracking the curve (TRB, 2010)
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22 Horizontal sight distance Sight distance is the length of the roadway ahead visible to the driver. On horizontal curves, the obstruction that limits the driver's sight distance may be some physical feature outside of the traveled way, and on a crest vertical curve the obstruction may be the road surface at some point.
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