Slope on road signs.

[Xeranox]

Road signs alerting about steep hills often give a percentage (at least they do in Norway). Is this simply 90 degrees divided into 100% (so that 45 degrees = 50%), or is it the mathematical average slope of the hill (rise over run , so that 45 degrees = 100%)? Or is it something else?

[tomd]

Straight out the highway code (�1.49 at most post offices, good to keep in the car to prove point during road rage):

 

Gradients shown as ratio ie. 20% = 1:5

 

So yes, rise over tread. That means that a 20% hill has an angle to the horizontal of 11.3 degrees, and a 100% gradient is 45 degrees from either.

[brachiopod]

It's the gradient ratio expressed as a percentage.

So, a 1 in 4 hill (pretty steep) is 25%

ie (1 � 4)  x 100

[Tim Baxter]

It's something else.  It actually refers to the Tangent of the angle of the slope.  If you can imagine a right-angled triangle lying on its back the tangent is the slope, the height is the opposite side and the length travelled over the ground is the adjacent side.  Hence the opp/adj = rise/base line, may = 1/4 but is expressed as a %age, i.e. 25%.  How this is more helpful to the layman than 1 in 4 is beyong me!

[dougielogan]

A 1 in 4 hill is if a rise of one foot every 4 feet traveled

[tomd]

Tim - the tangent of the angle of the slope is just rise over tread, but a very complicated was of expressing it.

Think toa - tan = opp / adj = rise / tread.

[Xeranox]

Okay, thanks all. So it is, indeed, possible to have a 1000% gradient. Although I wouldn't try to climb it in a car :)