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To be strictly correct, it's the vertical distance divided by the horizontal distance, not the slant distance; but for the small angles you'll see on the streets, there's not much difference. Mathematically, it's the difference between the tangent of the angle and the sine of the angle. Slope in percent is the tangent of the angle multiplied by 100. An angle of 45° would rise 1 unit for every unit traveled horizontally, for a slope of 100%, but you'd travel 1.414 units along the road. A 10% slope would be 5.71° and you'd travel 1.005 units on the road for each unit horizontally. A 20% slope would be 11.31° and you'd travel 1.02 units on the road for each unit horizontally.
