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 If a highway with traffic normally running at 65 mph has an inclined section with a 3% grade, how much can the elevation of the roadway increase before the speed of the larger vehicles is reduced to 55 mph? [Solution Shown Below]
Solution Looking at the table in the Ascending Grades module, we can see that a 3% grade causes a reduction in speed of 10 mph after 1400 feet. To find the exact increase in the elevation of the highway we would need to employ some simple trigonometry. But, since the angle of a 3% grade is small, we can just estimate the elevation increase by multiplying the length of the grade by the grade itself. This yields 1400*0.03 = 42 ft. The elevation of the roadway can only be increased by about 42 feet before heavy vehicles are reduced to a speed of 55 mph.
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