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.”
Clear Vision.
—Clear vision is another thing that should be insisted upon as a means for safety. Weeds, brush and trees are all too frequently allowed to obscure the sight. With ordinary brakes on smooth roads from 100 to 150 feet is needed to stop a car moving at 30 miles per hour. With first-class brakes this might be decreased, but since it usually takes a driver a short period to react from the time a car heaves in sight or he sees a break in the pavement or some other obstruction, there should be allowed 150 feet to stop if the roads are smooth and hard. In order that there might be a good factor of safety it is desirable to get at least 250 feet clear vision. This will require for a 200-ft. radius that the brush, trees and so on should be cut back about 27 feet from the traveled way. As the radius becomes larger the distance necessary to clear back becomes less: 300-ft. radius, clearance, 12 ft.; 400-ft. radius, 7 ft.; 500-ft. radius, 4 ft. In case the curve is in a cut the bank on the inside at the height of the eye should be excavated far enough back to give the necessary clear vision.