Superelevation Criteria for Sharp Horizontal Curves on Steep Grades (2014) / Chapter Skim
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1 Superelevation Criteria for Sharp Horizontal Curves on Steep Grades Geometric design policy for horizontal curves is established by the American Association of State Highway and Transportation Officials (AASHTO) and published in A Policy on Geometric Design of Highways and Streets (referred to as the Green Book)
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2• Trucks: – Single-unit truck – Tractor semi-trailer truck – Tractor semi-trailer/full-trailer truck (double) The vehicle maneuver scenarios studied in this research for vehicles on curves include the following: • Vehicle maintains constant speed equal to the design speed of the curve (no deceleration, i.e., 0 ft/s2)
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3 • There is no concern of a passenger vehicle rolling over while traveling at the design speed on a sharp horizontal curve with a steep downgrade, when designed according to current AASHTO Green Book policy. • Based upon a review of the literature, the lowest rollover thresholds for tanker trucks (i.e., liquid-cargo tank trucks)
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4and transient bicycle models for a vehicle, when single-unit trucks must decelerate at a rate of -11.2 ft/s2 (stopping sight distance braking) or a rate equivalent to emergency braking (-15 ft/s2)
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5 of safety against skidding is negative in certain conditions, most notably situations when more aggressive braking is needed such as deceleration rates similar to those used to develop stopping sight distance or emergency braking design criteria (-11.2 or -15 ft/s2)
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6at -11.2 ft/s2 is unlikely to be required on upgrades of 4% or more (i.e., the available sight distance is greater than minimum stopping sight distance design values) , emax values up to 12% may be used for minimum-radius curves.
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