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.