Why Does A Six Percent Grade Seem So Steep?

Question

Interstate Highways are allowed up to 6% grade when going up or down hills. 6% grade is only 3.42 degrees. It makes complete sense that an Interstate has grades of up to 6 percent because I have measured the angle/grade my self and it came close to 6%. When going down hills on Interstates (and I am positive they are Interstates), it seems like they are MUCH steeper than 6%. What is the reason for this optical illusion? Here is a picture of I-70 in Missouri going down a 6% or 3.42 degree hill. http://www.flickr.com/photos/20518990@N08/2518953051/

Answer 1

I've been asking the same thing.  NO ONE seems to be answering your question!  I guess no one knows!  :-(

When you figure it out let me know.

Answer 2

the grade is feet per 100 feet traveled.  for example, you said 6% grade, that would be going up or down 6 feet in elevation for every 100 feet that you travel.

it may seems so much of a slope because many are such a great distance.  you look at a handicapped ramp, it doesn't look like much of a slope, but they have a slope of 1 inch for each 12 inches of length is about 8% grade

Answer 3

Some cars and especially trucks have a hard time maintaining the speed limit on a six percent grade...anything steeper than six has to be marked to warn trucks of steep grade ahead so they can select the right gear to conserve their brakes going down hill...

Answer 4

There are several spots on I-70 through the Colorado Rockies where the grade is 6% (i thought it was steeper). There are "runnaway truck ramps" in these areas for semi trucks who loose their brakes. The max speed for trucks in these areas is also lower (I think 45mph). Of course these same heavy trucks are always seen creeping uphill at about 20-30mph.