The existing posted speed limit on a section of highway is 55 mi/h and studies have shown that that the current 85th percentile speed is 65 mi/h. If the posted speed limit is to be increased to the current 85th percentile speed, what should be the increase in the radius of a curve that is just adequate for the existing posted speed limit? Assume a superelevation rate of 0.08 for the existing curve and for the redesigned curve.
What will be an ideal response?
For the existing curve, use Equation 3.34 to determine the radius.
R = u2/15(e + fs)
R = (55)2/15(0.08 + 0.13)
R = 960 ft
Similarly, determine the radius for the curve to be redesigned.
R = (65)2/15(0.08 + 0.11)
R = 1482 ft
The increase should then be 1482 – 960 = 522 ft
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