Superelevation Criteria for Sharp Horizontal Curves on Steep Grades (2014) / Chapter Skim
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10 S E C T I O N 2 This section summarizes the literature related to superelevation criteria for sharp curves on steep grades and also summarizes current practice on this issue. The topics are organized as follows: • Horizontal curve design • Heavy trucks • Driver comfort • Friction studies • Vehicle dynamics models • Current practice 2.1 Horizontal Curve Design Current AASHTO policy on horizontal curve design is based upon a point-mass model.
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11 tan 1 tan 100 1 100 (6)
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12 "rollover threshold" of the vehicle. Rollover thresholds are a characteristic of vehicle design and loading that can be estimated from static tests, but are best determined from dynamic tests.
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13 3 mph (5 km/h) was proposed to balance traffic flow and construction cost, thus allowable maximum side friction demands corresponding to the Va - Vc = 3 mph (5 km/h)
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14 friction values are conservative for contemporary passenger vehicles traveling at the design speed and recommended that the coefficient of friction at impending skid be revisited in the AASHTO Green Book to reflect current pavement design practices and performance. 2.2 Heavy Trucks Eck and French (2002)
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15 ideal for trucks; but the coefficient of friction is much less for natural rubber tires, with the result that trucks have a stopping distance of 1.65 to 2.65 times farther than a passenger vehicle, assuming both are using high-grip tires of good condition. More recent work examining stopping distance includes that of Olson et al.
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16 to pavement friction values in the lateral and longitudinal directions through a simple transformation since the friction factor is defined as force divided by vertical load. Specifically, the longitudinal and lateral friction demands are derived from the demanded tire forces as follows: Longitudinal Friction Factor (13)
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17 model versus the point-mass model is that it examines not only force balance, but also moment balance. The moment balance in particular prevents the vehicle from "spinning out" on a roadway.
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18 steeper than 5%. They pointed out that these maneuvers will be more critical for trucks since they have lower maximum side friction factors.
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19 The design manual for the Indiana Department of Transportation (INDOT) recommends the use of a higher speed in superelevation calculations than the design speed for the following conditions: • Transition area.
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