Design Guidance for Intersection Auxiliary Lanes (2014) / Chapter Skim
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Pages 112-144
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From page 112... ...
112 Recommended Revisions to AAshto Green Book Overview This appendix presents the research team's suggested changes to the AASHTO Policy on Geometric Design of Highways and Streets (commonly known as the Green Book)
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113 9.3 Types and Examples of Intersections, 9.3.1 Three-Leg Intersections (pages 9-10 to 9-14) Proposed Revision to Green Book Channelized Three-Leg Intersections Channelization is often desirable for some reasons described in Section 9.6.2.
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114 flow capacity of the highway or where through and cross-traffic volumes are sufficiently high to warrant signal control. Auxiliary lanes are also desirable for lower volume conditions.
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115 side of the intersection." Speed-change lanes, it is assumed, refer to auxiliary lanes at interchanges, but no cross-reference is made; one could assume that instead of referring to intersection auxiliary lanes in Section 9.7, it refers to Tables 10-3 through 10-5 and/or the discussion on interchange auxiliary lanes on pp.
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116 is between 75 degrees and 105 degrees with a minimum of 70 degrees and maximum of 110 degrees. • Illinois DOT, Bureau of Local Roads and Streets Manual (111)
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117 the upstream or downstream end of the channelized right-turn lane. Similarly, a highway agency survey (11)
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118 Deceleration lanes -- Drivers making a right-turn maneuver at an intersection are usually required to reduce speed before turning. Significant deceleration that takes place directly on the through traveled way may disrupt the flow of through traffic and increase the potential for conflicts with through vehicles.
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119 of various design vehicles superimposed upon typical curb radii found in many U.S. cities.
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120 Widening on tangent alignment, even with flat curves, may produce some appearance of distorted alignment. Where the road is on a curve or on widening alignment, advantage should be taken of the curvature in spreading the traffic lanes without using reverse curves, as illustrated in sections C and D of Figure 9-37C.
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121 ratios should be evenly divisible by 5 (15:1, 20:1, etc.)
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122 9.7 Auxiliary Lanes, 9.7.1 General Design Considerations (pages 9-124 to 9-125) Proposed Revision to Green Book Deceleration lanes are advantageous on higher-speed roads, because the driver of a vehicle leaving the highway has no choice but to slow down on the through-traffic lane if a deceleration lane is not provided.
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123 could be offered as an alternative. While drivers may be able to negotiate the left-turn lane at higher deceleration rates, even up to the 11.2 ft/s2 rate described in Section 3.2.2 for stopping sight distance, a design that accommodates lower rates provides a more conservative design that is less demanding on drivers and contains more provision for storage of queues of left-turning vehicles.
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124 Figure A-2. Example graphic for replacing Green Book Figure 9-48.
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125 9.7 Auxiliary Lanes, 9.7.2 Deceleration Lanes (pages 9-127 to 9-130) Proposed Revision to Green Book Taper Length On high-speed highways it is common practice to use a taper rate between 8:1 and 15:1 (longitudinal:transverse or L:T)
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126 The longitudinal location along the highway, where a vehicle will move from the through lane to a full-width deceleration lane, will vary depending on many factors. These factors include the type of vehicle, the driving characteristics of the vehicle operator, the speed of the vehicle, weather conditions, and lighting conditions.
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127 Left-Turn Lane Peak-Hour Volume (veh/hr) Three-Leg Intersection, Major Urban and Suburban Arterial Volume (veh/hr/ln)
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128 (a) Three Legs (b)
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129 Table 9-23 is a guide to traffic volumes where left-turn facilities should be considered on two-lane highways. For the volumes shown, left turns and right turns from the minor street can be equal to, but not greater than, the left turns from the major street.
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130 divider to 1.8 to 2.4 m [6 to 8 ft] immediately before the intersection, rather than to align it exactly parallel with and adjacent to the through lane.
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131 left-turn lanes have also been used at locations with very high left-turn volumes. Double and triple turning lanes should only be used with signalization providing a separate turning phase.
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132 When considering construction or elimination of a median opening, the following guidance can be considered: • For median widths less than 9 m [30 ft] in width, median openings should be constructed opposite driveways and crossroads as appropriate.
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133 Discussion The phrase "care should be taken" is awkward and vague and the second sentence should be reworded to eliminate it. The statement that "care should be taken not to make the median opening longer than needed" should be expanded to give the reason for the guidance.
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134 for the farside far-side quadrant is less expensive than that for the nearside near-side quadrant. Vertical alignment and comparative grading costs may also influence the intersection quadrant where the turning roadway is placed.
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135 9.9 Indirect Left Turns and U-Turns, 9.9.2 Intersections with Jughandle or Loop Roadways (page 9-160) Proposed Revision to Green Book Additional information regarding design and operation of jughandle intersections is presented in Signalized Intersections: Information Guide (23)
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136 left turns on the major street only. The use of the CFI separates the conflict points for left-turning traffic from the primary intersection.
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137 A modification to the median U-turn intersection is the restricted crossing U-turn intersection. A figure showing the RCUT, such as Figure A-15, should be added along with a brief description.
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138 Conflict type Four-Leg Signalized Intersection Median U-Turn Configuration Restricted Crossing U-Turn Configuration Merging/diverging 16 12 18 Crossing (left turn) 12 0 2 Crossing (angle)
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139 and Alternative Intersections/Interchanges: Informational Report (104)
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140 In conditions where the U-turn crossover is unsignalized, provisions for sufficient gaps in the traffic stream through the use of an upstream signal or due to natural gaps appearing in the traffic stream due to low volumes may be needed. Further, when establishing the clearance intervals for the signalized crossover, it is essential to provide additional time to account for the extra travel distance required for drivers to navigate the loon.
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141 Discussion The Green Book states "For U-turn openings designed specifically for the purpose of eliminating left-turn movement at a major intersection, they should be downstream of the intersection, preferably midblock between adjacent crossroad intersections." This statement raises several issues: • Locating the crossover "midblock between adjacent crossroad intersections" implies that this application is in an urban setting. This should be specified.
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142 Discussion The use of roundabouts is evolving in the United States. Because of this, public outreach and education are still vital factors in realizing the improvements in traffic operations and safety that can be achieved with these designs.
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143 the turbo roundabout based on their destination. Because of this, turbo-roundabouts do not permit U-turn maneuvers.
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144 end of the Design Vehicle section. At a minimum, a list of things to "watch for" as a designer should be added (e.g., consider lowboy vehicle designs which could hang up, maximum height of splitter islands)
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