What is the grade/slope/angle?
Asked by
Trillian (
21153
)
October 26th, 2010
Not that important of a question really. On my way home I get off the road by following a curve around to the right that ends up going over the road I was just on. It doesn’t seem like that steep of a slope, but when I’ve gone all the way around, there I am on the via duct (?) which is about 16 feet higher that the road.
The more I think about the roads and highways, the more ignorant I feel.
Those surveyors and engineers have really got some serious planning to do to make the roads work and come out correctly. It’s actually fairly amazing.
So, how many degrees of an angle is it? Is there a standard angle that they try to stick to or do they adapt to fit the terrain?
These are the things that I think about in the long hours of the night. Sad, huh?
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AFAIK, civil engineers try to accommodate both the terrain and the tolerances on cars.
Calculating the slope is just plain geometry (we know that the road rises a total of about 15 feet, but we need to know how long the stretch is). Wikipedia suggests that there is more to grading than just geometry, since one must also figure in the speed of the vehicle.
The standards for highways are less steep than for county roads and much less than city streets. and civil engineers have suggested slopes. for instance, you don’t want an interstate too steep or trucks will not get up it easily. (If it is too steep, they end up building a truck lane.)
that being said, the specs are generally in ranges rather than specific slopes.
@zenvelo can you expand on the term “ranges”?
I work at a engineering company as a Land Surveyor that specializes in transportation engineering, Not being an engineer, but watching them and working with them regularly; there are so many constraints in play an ordinary person may not consider. I know speed, visual distance (parabolas for good reason for most vertical curves), stopping distance, centrifugal force (spiral curves, several varieties) are some of the few considerations along with right-of way (my side) these are some key elements.
As to your grade question, funny, my son and I were just arguing about this, goes back to simple geometry-rise over run, so 5 feet of rise in a hundred feet is 5% which can be translated to the arcsin of 5 over 100 which is what in degrees..?
@Trillian for instance, the specs for ADA complaint wheelchair ramps (and overcrossings) say the slope must be in the range between 1:20 (one foot rise over a 20 foot run) and 1:16 (one foot rise over a 16 foot run).
Highway specs say that for different ranges of steepness, (like 0 to 2.5%, 2.5% to 4%, etc,) you need to take in account room for slow vehicles climbing and room for descending vehicles braking.
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