Fig. 5.—Drop Inlet Culvert
Drop Inlet Culverts.—In some locations erosion has begun in the fields adjacent to a culvert and it will probably continue until the stream above the culvert has eroded to about the level of the floor of the culvert. This is a reason for placing the culvert as high as the roadway will permit, so long as the area above the culvert will be properly drained. Considerable reclamation of land is possible if the culvert is constructed with a box at the inlet and as shown in Fig. 4. The area up-stream from the culvert will not erode below the level of the top of the box at the inlet end.
Where the stream crossing the road has eroded to considerable depth or has considerable fall, as would sometimes be the case on side hill roads, the culvert barrel would follow the general slope of the ditch but should have a drop inlet. This type of culvert is shown in Fig. 5.
Chapter IV
ROAD DESIGN
Necessity for Planning.—Sometimes highway improvement is the result of spasmodic and carelessly directed work carried out at odd times on various sections of a road, finally resulting in the worst places being at least temporarily bettered. The grade on the steepest hills is probably reduced somewhat and some of the worst of the low lying sections are filled in and thereby raised. Short sections of surfacing such as gravel or broken stone may be placed here and there. From the standpoint of the responsible official, the road has been "improved," but too often such work does not produce an improvement that lasts, and sometimes it is not even of any great immediate benefit to those who use the roads. In nearly every instance such work costs more in money and labor that it is worth.
Lasting improvement of public highways can be brought about only through systematic and correlated construction carried on for a series of years. In other words, there must be a road improvement policy which will be made effective through some agency that is so organized that its policies will be perpetuated and is clothed with enough authority to be capable of enforcing the essential features of good design and of securing the proper construction of improvements.
Details of highway construction and design must vary with many local conditions and types of surface. The limits of grades and the many other details of design may properly be adopted for a specific piece of work only after an adequate investigation of the local requirements and in the light of wide experience in supervising road improvement.