Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report (2021) / Chapter Skim
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Pages 102-153
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From page 102... ...
102 Appendix A Suggested Text for Future Edition of AASHTO Green Book This appendix provides potential changes recommended for consideration to the next edition of the AASHTO Green Book, based upon the findings and conclusions of this research.
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103 Figure 10-60. General Types of Ramps The different ramp patterns of an interchange (i.e., the different types of interchange configurations)
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104 10.9.6.2 General Ramp Design Considerations 10.9.6.2.1 Ramp Design Speed The ramp design speed is a selected speed used to determine the various geometric design features of a ramp. The ramp design speed should be a logical one with respect to the type of intersecting highways, area type (i.e., urban or rural)
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105 acceleration or deceleration along the ramp is expected by the driver; while if the intersecting crossroad is a minor arterial or collector, moderate acceleration or deceleration is expected by the driver along the ramp. In urban or constrained areas where the intersecting facility is a primary highway or major street, moderate to significant acceleration or deceleration along the ramp is expected by the driver; while if the intersecting road is a minor arterial or collector, significant acceleration or deceleration is expected by the driver along the ramp.
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106 Table 10-1. Range of Guide Values for Ramp Design Speed as Related to Highway Design Speed U.S.
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107 Metric Context Ramp configuration Highway design speed (km/h) 50 60 70 80 90 100 110 120 130 140 Ramp design speed (km/h)
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108 The application of values in Table 10-1 to various conditions and ramp types is discussed below. 10.9.6.2.2 Portion of Ramp to Which the Ramp Design Speed Is Applicable The values in Table 10-1 apply to the controlling feature on the ramp proper.
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109 ramp proper (i.e., the ramp design speed) should be consistent with the operating conditions at the crossroad ramp terminal.
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110 used at service interchanges, and in these situations the ramp design speeds may range from 30 to 65 mph [50 to 100 km/h]
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111 compound curvature to prevent unexpected and abrupt speed adjustments. Additional design information regarding the use of compound curves is presented in Section 3.3.7.
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112 Figure 10-61. Ramp Shapes In Figure 10-61A, the loop and the outer connection are separated, as is generally desirable.
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113 The shape of a semidirect connection (Figure 10-61D) is influenced by the location of the terminals with respect to the structures, the extent to which the structure is widened, and the curve radii needed to maintain a desired turning speed for an important left-turning movement.
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114 ramp terminal facilities, short upgrades of 7 to 8 percent permit good operation without unduly slowing passenger cars. Short upgrades of as much as 5 percent do not unduly interfere with truck and bus operation.
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115 10.9.6.2.14 Superelevation and Cross Slope The following guidelines should be used for cross-slope design on ramps: 1. Superelevation rates, as related to curvature and design speed on ramps, are given in Tables 3-8 through 3-12.
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116 Figure 10-62. Development of Superelevation at Free-Flow Ramp Terminals
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117 Figure 10-62B shows a parallel-type exit from a tangent section that leads into a flat exiting curve. At point b, the normal cross slope of the through roadway is projected onto the auxiliary lane.
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118 point upstream from the gore, having some dimensional width that separates the roadways. The neutral area refers to the triangular area between the painted nose and the gore nose and incorporates the physical nose.
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119 Although the term "gore" generally refers to the area between a through roadway and an exit ramp, the term may also be used to refer to the similar area between a through roadway and a converging entrance ramp. At an entrance terminal, the point of convergence (beginning of all paved area)
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120 Figure 10-64C presents a major fork, with neither diverging roadway having priority. The offset is equal for each roadway, and striping or rumble strips are placed upstream from the physical nose.
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121 Figure 10-66. Gore Area, Single-Lane Exit Source: Arizona DOT Figure 10-67 shows a gore at a major fork between two freeways.
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122 Whereas Figure 10-68 shows a gore at a two-lane exit from a freeway, Figure 10-69 shows a typical gore and ramp terminal for a ramp entering a freeway. Figure 10-68.
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123 10.9.6.3 Ramp Traveled-Way Widths 10.9.6.3.1 Width and Cross Section Ramp traveled-way widths are governed by the type of operation, curvature, and volume and type of traffic. It should be noted that the roadway width for a turning roadway includes the traveled-way width plus the shoulder width or equivalent offset outside the edges of the traveled way.
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124 also begin within the entrance terminal, with the transition to the narrower ramp shoulder accomplished gradually on the ramp end of the terminal. Abrupt changes should be avoided.
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125 10.9.6.4.1 Left-Side Entrances and Exits Left-side entrances and exits are contrary to driver expectancy when intermixed with right-side entrances and exits and should be avoided, where practical. Left-side entrances and exits for managed lane facilities are also contrary to driver expectancy.
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From page 126... ...
126 ascending grade, the portion of the ramp intended for acceleration and the ramp terminal should closely parallel the through-lane profile to permit entering drivers to have a clear view of the through road ahead, to the side, and to the rear. It is desirable that profiles of highway ramp terminals be designed with a platform on the ramp side of the approach nose or merging end.
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127 but as a general rule, the entering driver's view is obstructed by the crest of the profile at an overpass and by the columns, abutments, and approach walls at an underpass. The conditions for determining the distance between a structure and the far side approach nose are similar to those discussed for speed-change lanes.
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128 300 ft [90 m] is recommended between the end of the taper (as shown in Figure 10-72)
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129 Where an entrance ramp is followed by an exit ramp, the absolute minimum distance between the successive noses is governed by weaving considerations. The spacing policy for EN-EX ramp combinations is not applicable to cloverleaf loop ramps.
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130 there are mainline volumes that meet or exceed capacity, parallel type entrances allow additional flexibility to drivers in selecting a merge location. See Section 9.7 for discussion of SCLs applicable to at-grade intersections.
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131 Notes: 1. LAcc Length is the recommended acceleration length as shown in Table 10-4 or as adjusted by Table 10-5.
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132 application, this point of convergence of the left edge of the ramp and the right edge of the through lane may be assumed to occur where the right edge of the ramp traveled way is 12 ft [3.6 m] from the right edge of the through lane of the freeway.
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133 the freeway without using much of the gap acceptance length. This behavior results in undesirable merging operations.
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134 Table 10-4. Minimum Acceleration Lengths for Entrance Terminals with Flat Grades of Less Than Three Percent U.S Customary Highway Acceleration Length, LAcc Length (ft)
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135 Metric Highway Acceleration Length, LAcc Length (m) , for Design Speed (VDS)
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136 Table 10-5. SCL Adjustment Factors as a Function of Grade U.S.
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137 Table 10-5. SCL Adjustment Factors as a Function of Grade (Continued)
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138 Minimum deceleration lengths for various combinations of design speeds for the highway and for the ramp roadway are given in Table 10-6. Grade adjustments are given in Table 10-5.
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139 well in the length-width superelevation adjustments to obtain a ramp cross slope different from that of the through lane. The width of the recovery area or the distance between the inner edges of the diverging lanes at the ramp nose is usually 20 to 30 ft [6.0 to 9.0 m]
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140 Figure 10-73. Exit Ramps -- Single Lane
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141 Table 10-6. Minimum Deceleration Lengths for Exit Terminals with Flat Grades of Less Than Three Percent U.S.
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142 Minimum deceleration lengths for exit ramps are given in Table 10-6, and adjustments for grades are given in Table 10-5. For a parallel-type exit ramp, the deceleration length begins where the width of the auxiliary lane increases to 12 ft [3.6 m]
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143 Wherever a part of a tapered speed-change lane falls on curved alignment, it is desirable that the entire length be within the limits of the curve. Where the taper is introduced on tangent alignment just upstream from the beginning of the curve, the outer edge of the taper will appear as a kink at the point of curvature.
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144 Figure 10-74b. Layout of Taper-Type Terminals on Curves (Metric)
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145 At ramp terminals on relatively sharp curves, such as those that may occur on freeways having a design speed of 50 mph [80 km/h] , the parallel type of speed-change lane has an advantage over the taper type.
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146 motorists entering directly onto the through lanes. The taper at the end of the acceleration lane should be long, preferably about 300 ft [90 m]
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147 approximately 2,500 ft [750 m] or more downstream from the entrance.
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148 uniformity. Either form of two-lane entrance is satisfactory if used exclusively within an area or a region, but they should not be intermixed along a given route.
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149 10.9.6.6.6 Two-Lane Exits Where the traffic volume leaving the freeway at an exit terminal exceeds the design capacity of a single lane, a two-lane exit terminal should be provided. To satisfy lane-balance needs and not reduce the basic number of through lanes, it is usually appropriate to add an auxiliary lane upstream from the exit.
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150 total length from the beginning of the first taper to the point where the ramp traveled way departs from the right-hand through lane of the freeway should range from 2,500 ft [750 m] for turning volumes of 1,500 veh/h or less upward to 3,500 ft [1,000 m]
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151 Figure 10-78. Major Forks In the case of a two-lane roadway separating into two, two-lane routes, there is no interior lane.
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152 the painted nose should be accomplished in a continuous sweeping curve with no reverse curvature in the alignment of the roadway edges. A branch connection is defined by (1)
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From page 153... ...
153 considerations should also be given to the fact that the outer lane from the roadway entering from the right is the slow-speed lane for that roadway, whereas the opposite is true for the roadway entering from the left. If the traffic volumes per lane are about equal, it would be proper to terminate the lane on the right, as shown in Figure 10-79B.
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