How to Calculate the Percent of a Highway Slope

Highway slope affects the load on your car's engine.

Highway slope percentage is calculated just as one would calculate any other slope percentage--dividing rise, or change in vertical distance, by the run, or change in horizontal distance between any two given points on the highway. For your calculations of highway slope to be valid, they must be taken from two points on the same side of the same hill; if you measure two points with both up- and down-slopes between them, instead of just one or the other, your figures won't reflect the physical reality.

Things You'll Need

• Calculator

Instructions

1. Measure the horizontal distance from Point A to Point B on the highway. Note that the horizontal distance will only be the same as the actual highway distance if the highway is perfectly flat; if the highway rises at all between Points A and B, you must factor out the extra vertical distance traveled.

2. Measure the difference in altitude from Point A to Point B on the highway. You can use an altimeter to do this, look for road signs identifying altitude, consult a map, or use the altimeter function on a wristwatch or GPS unit.

3. Divide the difference in altitude from Step 2 by the horizontal distance in Step 1. For example, if the highway rises 50 feet from Point A to Point B, and there is a horizontal distance of 1,000 feet between Points A and B, you have 50 / 1000 = .05. To convert this to a percentage, just multiply by 100; so .05 * 100 = 5.0; there is a 5 percent slope on the highway from Point A to Point B.

Tips & Warnings

• If you know the difference in altitude between Points A and B, and the actual highway distance between Points A and B, you can extrapolate the horizontal distance between Points A and B. Just square the actual highway distance and the vertical change between Points A and B, then subtract the squared vertical change from the squared actual distance. Then take the square root of the result to find the horizontal distance between these points. So if the actual highway distance was 1,001.25 feet and the vertical distance between Points A and B was 50 feet, you'd have 1,001.25 squared (times itself), which is 1,002,501.563. 50 squared is 2,500. Subtracting the latter figure from the former, we have 1,000,0001.563; taking the square root of this with a calculator we get just over 1,000 feet for the horizontal distance between Points A and B.

References

• BookRags: Highway Grades