Superelevation.
—The superelevation of the outer side of a curved roadway can not at one and the same time be made suitable for all rates of speed. Works on mechanics give the formula for the elevations of the outer edge as
e = av2 gR
| where | e | = | the elevation in feet; |
| a | = | the width of road in feet; | |
| g | = | acceleration of gravity in ft. per sec. per sec.; | |
| v | = | velocity in ft. per sec.; | |
| R | = | radius in feet. |
Or if the velocity, V, is given in miles per hour and the elevation, E, in inches this reduces to
E = 121aV2 150R = 0.807aV2R
© Underwood and Underwood
A DANGEROUS CURVE AT LOOKOUT MOUNTAIN
A DANGEROUS CURVE MADE SAFE BY AN ARTISTIC CONCRETE WALL
The Tennessee State Highway at Lookout Mountain, Built of Cemented Concrete.
The country pavements are usually superelevated for 12 miles per hour. Even this with the minimum radius gives a surface so tipped that it is difficult for horse-drawn wagons to remain on it unless the horses trot around the curve. Substituting for a velocity of 12 miles per hour, a width of 20 feet, and a radius of 200 feet there results
E = 11.6 inches.
The following table gives the necessary elevation in inches that there shall be no side thrust at various speeds, for a road one foot wide. To get the elevation for any width multiply by the width.
Table of Superelevation in Inches per Foot of Width
| Radius Feet | Speed in Miles per Hour | |||||
|---|---|---|---|---|---|---|
| 10 | 12 | 15 | 20 | 30 | 40 | |
| Inch | Inches | Inches | Inches | Inches | Inches | |
| 100 | 0.81 | 1.16 | 1.82 | 3.23 | 7.26 | 12.91 |
| 200 | .40 | .58 | .91 | 1.61 | 3.63 | 6.46 |
| 300 | .27 | .38 | .61 | 1.08 | 2.45 | 4.30 |
| 400 | .20 | .29 | .45 | .81 | 1.81 | 3.23 |
| 500 | .16 | .23 | .36 | .64 | 1.45 | 2.58 |
Unless the road is intended for a speedway, 12 miles per hour would be about the right speed to use. In rounding a curve of radius 200 feet at a speed of 30 miles per hour, superelevated as shown in the table for 12 miles per hour the coefficient of friction would have to be about one-fourth to prevent skidding. In dry weather this would practically always be exceeded. A committee of the National Highway Traffic Association, 1922, recommends “that on all curves of more than three degrees the pavement and inner-half of the earth shoulder should be banked. This superelevation should vary from 0 for a 3-degree curve to 1 inch per foot of width for curves of 20 degrees or sharper.”