The design speed of a multilane highway is 60 mi/h. Determine (a) the minimum stopping sight distance that should be provided for a level roadway, and (b) the minimum stopping sight distance that should be provided for a roadway with a maximum grade of 7 percent. Note: the term a/g in the appropriate equation is typically rounded to 0.35 for calculation purposes. Assume perception reaction time = 2.5 sec.
The minimum sight distance required in these cases is the stopping sight distance (SSD), given by Equation 3.27:
where u = design speed (mi/h)
t = perception-reaction time (sec)
a = rate of deceleration (taken as 11.2 ft/sec2)
g = gravitational acceleration (taken as 32.2 ft/sec2)
G = grade
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(a) Determine the minimum sight distance that should be provided for a level roadway.
Since the roadway is level, G = 0.
S = (1.47)(60)(2.5) + (60)2 / (30)(0.35 + 0)
S = 220.50 + 342.86
S = 563.36 feet
Therefore, the minimum sight distance for this horizontal roadway is 564 feet.
(b) Determine the minimum sight distance that should be provided for a roadway with a maximum grade of 7 percent.
Since this roadway has a maximum grade of 7 percent, G = –0.07. The downgrade (negative) case provides the most conservative (higher) value for design.
S = 1.47(60)(2.5) + (60)2 / 30(0.35 – 0.07)
S = 220.50 + 428.57
S = 649.07 feet
Therefore, the minimum sight distance for this roadway should be 650 feet.
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