"percent Grade" ratings

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JerryP

JerryP

Well-known member
Joined
Feb 18, 2013
Posts
71
Location
Virginia
I get the idea of grade ratings, such as 6%, 9%, etc. and the higher %, the steeper incline. However, how are they derived, and why aren't angles used? Does the percent grade indicate the ratio of elevation change (in feet, for example) over a horizontal distance (also in feet)? Such as 90 ft rise over 900ft is a 10% grade?
What are the steepest grades allowed on Interstate Highways, and other US Routes?
 
An 8% grade means the highway rises 8 feet in 100 feet or horizontal run, for example.

Interstates designs attempt to limit grades to 6%.
 
Thanks. I thought 6% was attempted max, but I know of a couple Interstates in VA that are 7% ( e.g. I77 at Fancy Gap).
 
Grade percentage is an angle - it's just expressed differently.  Civil engineers use grade percentage more often than degrees for terrain angles because inclines are often not constant angles. A roadbed may change degree of angle numerous times over the distance from bottom to top, but the grade percentage will express the total change. Thus a 6% hill climb may in fact be composed of greater and lesser angles at different points along the way.

Besides, grade percentage is easy to calculate when surveying.  If the ground "over there" is 12 feet higher than "here", and "there" is 240 feet away, then the grade is 12/240 or 5%. Calculating degrees is more arduous!  :)
 
Trigonometry is more difficult than arithmetic :)
 
Ned said:
Trigonometry is more difficult than arithmetic  :)
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Used to be...then they invented the modern day calculator like the HP Scientific.  With that introduction, they made arithmetic as difficult as trigonometry for me, anyway. :)
 
Being able to do the calculations doesn't mean the user understands the results.  Why don't we measure grades in radians? :D
 
I had an algebra teacher in high school that for the test would give you the question and the answer.  You had to show the work, and then explain what it meant.  It's a wonder I graduated.

I always wondered how grades were figured too.  Always just had a feel for how steep the climb was based on the number, but not the science behind it.....or the math behind it rather.
 
gwcowgill said:
If it hadn't been for my oldest daughter, I would never had made it through college algebra
Click to expand...


I made it through a few semesters of college algebra and advanced math but when my oldest daughter hit  high school advanced math the result of me trying to help her understand the solution always ended up "We don't do it that way Dad." After taking my solution to school she admitted the teacher said it was "An alternative way to find the answer."
 
For you geometry and trigonometry fans, the % grade expresses the tangent of the angle of climb. It uses percentage rather than a decimal, but it's the same number.

A grade of 6% (tangent .06) is 3.43363036 degrees. See why highway engineers like to use percent rather than degrees?
 
Gary RV Roamer said:
A grade of 6% (tangent .06) is 3.43363036 degrees. See why highway engineers like to use percent rather than degrees?
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It doesn't sound so steep when you put it in degrees.
 
That's why the signs read;  ie.  6% OVER three miles.  Good explanation Gary, as always.
 
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