Grade of this road?

[TEG24601]

SR-526, here in Washington, has a section, while also SW 84th St. that it 9% and that road feels like you are going to slide out of your seat sometimes.  This road does not appear to be anywhere near that steep.

[empirestate]

http://bit.ly/16yu4Wu

Your argument is invalid.

[wxfree]

My vote is 15%, because unless there's some solid reason not to, I'm going to agree with the professionals who built the road.

[DSS5]

My vote is 15%, because unless there's some solid reason not to, I'm going to agree with the professionals who built the road.

It is a privately built and maintained road. The 15% is just what the company who built the complex originally said it was going to be.

[J N Winkler]

It is a privately built and maintained road. The 15% is just what the company who built the complex originally said it was going to be.

I would be inclined to accept that as credible since, in order to build the road, the development company would have had to obtain a permit from the county, and the application for it would have been accompanied by a set of construction plans for the road.  It would raise uncomfortable questions for the company if it went out in public saying "15%" when the construction plans gave a much higher value.

[2Co5_14]

I have some experience comparing roadway plans on paper with the actual field conditions, and slopes typically appear twice as steep as they actually are.  So a slope that measures 2:1 (horizontal:vertical) will look like it is 1:1.  That hillside slope that looks like a 45 degree angle (1:1) is actually more like 26.6 degrees (2:1), which would sound more realistic to me.  I know on GA state highways we use maximum side slopes of 2:1 because that is the steepest slope that will still remain stable for grass and other vegetation to take root and not get washed away.  All that being said, the builder's number of 15% is realistic.  It's still going to be a steep road and potentially dangerous in icy conditions.

[Alps]

As for how engineers measure grade, it's simple - you draw a line down the middle of the road (or a lane line, shoulder line, gutter, etc., but let's use the middle for simplicity, since it's what's usually used for a 2-lane road like this). Your line would not follow every pebble, but at the same time, you wouldn't cut through the top of a hill. Think of it as dropping a 2-inch thick rubber hose on the ground. The shape the hose takes will be a smooth series of curves and tangents, and that's your best approximation of grade.

That's how you measure grade at a specific point.  But when a sign says a road has a grade of X%, is that the grade at a specific point, or is it the average grade over a section? And if the latter, how do they decide what section?

Typically, when I see a maximum grade posted, it's the maximum grade achieved on that section (and I don't mean instantaneously on top of a pebble). I've seen "15% grade next 2 miles" (exaggeration) and you maybe have one 1/4 mile downgrade at 15% and even some flat sections in those 2 miles.