Design Guidance for Freeway Mainline Ramp Terminals (2012) / Chapter Skim
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Pages 16-20
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From page 16... ...
16 This section presents background information on the conceptual models utilized by AASHTO in determining minimum recommended acceleration and deceleration lane lengths for entrance and exit terminals, respectively. The focus of this research is on evaluating the appropriateness of the existing AASHTO models given current driver behavior and vehicle performance characteristics.
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From page 17... ...
17 tion rates used to generate those values, based on Equation 1. Overall, the pattern is a decreasing acceleration rate as the initial speed increases, but there are situations when this overall pattern is not followed.
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18 Va = Speed after tn s of deceleration without brakes, mi/h Vr = Entering speed for controlling exit ramp curve, mi/h tn = Deceleration time without brakes (assumed to be 3 s) , s dn = Deceleration rate without brakes, ft/s2 dwb = Deceleration rate with brakes, ft/s2 The 1965 Blue Book provides the most detailed information on how the current design values in the 2004 Green Book are determined, but it should be noted that the 2004 Green Book deceleration lane lengths differ slightly from the 1965 Blue Book values.
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From page 19... ...
19 Exhibit 10-73, based upon the assumptions described above. The overall pattern shows higher rates of deceleration for higher speeds.
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From page 20... ...
20 randomly, and may not occur when the entering driver is prepared to merge. As a result, drivers routinely are faced with judging gaps, adjusting their speeds, and often aborting their merge or producing extreme acceleration and/or speed behavior.
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