Recent Roadway Geometric Design Research for Improved Safety and Operations (2012) / Chapter Skim
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Pages 12-19
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From page 12... ...
Therefore, for a loop ramp with a smaller radius (approximately 300 ft or less) , two signal indications are needed to satisfy the MUTCD's requirements, with one at the left side of the ramp curve to provide sufficient sight distance for the approaching vehicles and one at the right side of the ramp curve to provide sufficient viewing angle for the stopped vehicles.
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From page 13... ...
. TABlE 2 RECOMMENDED DESIgN CONTROlS FOR VERTICAl CURVES Initial Speed Stopping Sight Distance for Design Rate of Vertical Curvature, K (length per % of algebraic difference in grade A)
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From page 14... ...
(2002) studied circular horizontal curves on rural two-lane highways in Nebraska to determine relationships among design speed, operating speed, and posted speeds for developing horizontal alignment design guidelines.
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From page 15... ...
Finally, they found that as ADT increased, 95th percentile speed decreased; the authors speculated that roadways with higher ADT values may be perceived by drivers as having a higher likelihood of speed enforcement. The report contains their recommendation for using a series of equations to estimate mean, 85th percentile, and 95th percentile operating speeds at approach locations and midpoints of horizontal curves in Nebraska or curves with similar characteristics.
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From page 16... ...
His analysis results showed that the most desirable spiral length, which offered advantages in comparison with a tangent-to-curve transition, was equal to the distance traveled during the steering time. He developed a model to estimate the desirable spiral length for transitions of sharp horizontal curves on two-lane rural roads, based on the data collected in three studies.
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From page 17... ...
The results of their analysis indicated that modern headlamps provided significantly less light above the horizontal than sealed-beam headlamps, indicating a potential need to modify the design equations for sag vertical curves. According to their theoretical analysis, the upward divergent headlamp angle used in the sag curve design equation should be reduced from 1° to between 0.75° and 0.90°.
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From page 18... ...
Safety issues also point to [the need for] some minimum sight distance." For work zone design speeds less than 40 mph, the SSD values tabulated in the Green Book and corresponding to work zone design speed were recommended.
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From page 19... ...
vertical alignment • Current North American design practices might yield segments of the vertical curve where the driver's view is constrained to a distance shorter than the required SSD. An alternative design procedure is recommended based on a new model that incorporated longitudinal friction and acceleration, which produced new recommended values for minimum lengths of crest and sag vertical curves (Hassan 2004)
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