Design Guidance for Freeway Mainline Ramp Terminals (2012) / Chapter Skim
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From page 8... ...
8This section first summarizes the current AASHTO design policy for freeway mainline ramp terminals and then summarizes existing literature on geometric design and safety performance, vehicle performance characteristics, human factors considerations, and operational performance issues related to freeway mainline ramp terminals. 2.1 AASHTO Design Policies for Speed-Change Lanes and Freeway Mainline Ramp Terminals AASHTO defines an SCL as an auxiliary lane, including tapered areas, primarily for the acceleration or deceleration of vehicles entering or leaving the through-traffic lanes.
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9 which is the difference between the average running speed on the mainline and the speed on the sharp or controlling terminal curve on the turning roadway. The average running speed is used instead of the design speed, based on the assumption that the exiting drivers travel at the average running speed when highway volumes are low.
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10 achieve this speed is referred to as the acceleration length and is measured from the end of the governing curve on the ramp proper to where the right edge of the SCL and through lane are 12 ft apart. Adjustment factors are used to increase the recommended acceleration lane lengths for ramps with positive grades and decrease acceleration lane lengths for ramps with negative grades.
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11 2.1.4 Sight Distance Sight distance along a ramp should be at least as great as the design stopping sight distance. Sight distance for passing is not needed.
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12 • One study found the safety of entrance terminals is enhanced when 800-ft or longer acceleration lanes are provided (Twomey et al., 1993)
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13 practices of freeway entrance ramp meters and HOV bypass lanes and noted that the designs performed well operationally. Hunter et al.
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14 available, drivers may accelerate to create a gap or decelerate to wait for a later gap. Choudhury et al.
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15 acceleration. Lighter vehicles merge much sooner than heavy trucks, and average speed differences between the mainline traffic and entering traffic at the end of the acceleration lane range from 6.5 to 10.2 mi/h.
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