The slope from the center to the side should be at least half an inch to a foot, or 1 foot in 24 feet; and it should not be more than 1 inch to a foot, or 1 foot in 12 feet. If the surface is well cared the former is better than the latter; but in no case is it wise to ex ceed the latter slope.
There is considerable difference of opinion as to the exact form to be given to the surface of a roadway (see § 308-12). Some claim that it should be the arc of a circle, and others that it should consist of two planes meeting at the center and having their junction rounded off with a short curve. The first form is shown in Fig. 9 and the second in Fig. 10. Great refinement in this matter is neither possible nor important. The proper crown can be easily and cheaply constructed with the road machine or scraping grader (§ 142).
The drainage of the surface of a road is chiefly a matter of main tenance (see Art. 2 of the present chapter); and one of the most common defects of maintenance is the failure to fill the ruts and keep the surface smooth so that the water will be promptly dis charged into the side ditches. A comparatively shallow rut will nullify the effect of any reasonable amount of crown, and wear deeper and deeper with each passing vehicle. Seldom is a mile of road seen which does not have a number of ruts and saucer-like depressions which catch and hold the water. On undulating roads, ruts and holes are naturally drained; and this is the reason why undulating roads are better than perfectly flat ones (see Minimum Grade, § 86).
The crown should be greater on steep grades than on the more level portions, since on the grade the line of steepest descent is not perpendicular to the length of the road, and consequently the water in getting from the center of the road to the side ditches travels obliquely down Idle road. If the water once commences to run down the center of the roadway on a steep grade, the wheel tracks are quickly deepened, stones are loosened or uncovered, and the road becomes rough and even dangerous. Under these circumstances, it is necessary to construct catch-waters, " water-breaks, " "hum mocks," or " thank-you-marms " at intervals to catch the water which runs longitudinally down the road, and convey it to the side ditches, thereby preventing the formation of gullies in the road surface. These catch-waters way be either broad shallow ditches or low flat ridges constructed across the road; and they may slope toward one or both side ditches. In the former case, they should cross the road diagonally in a straight line; and in the latter case, in plan they should be a broad angle with the apex at the center of the road pointing up hill. There is little or no difference
between the merits of the ditch and the ridge, unless the bottom of the former is paved with gravel, broken stone, or cobbles. The ridges are more common, but usually are so narrow and so high as to form a serious obstruction to traffic. However, neither the ditches nor the ridges should be used except on steep grades where really necessary, since either form is at best an obstruction to travel. The angle that the catch-waters shall have with the axis of the road should be governed by the steepness of the grade—the steeper the grade the more nearly should the catch-waters run down the road. They should have a considerable breadth so that wheels may easily ascend them and horses will not stumble over them.
Catch-waters should also be constructed in a depression where an ascending and a descending grade meet, in order that they may collect the water that runs down the traveled way and convey it into the side ditches. These catch-waters should run square across the road, and should be quite shallow ditches, the bottom of which is hardened with gravel, broken stone, or cobbles.
Some writers recommend that a surface of the road on the face of hillsides should consist of a single slope inclining inwards (see Fig. 11). This form of surface is advisable on sharp curves, but is of doubtful propriety elsewhere. The only advantage of this form is that the water from the road is prevented from flowing down the outer face of the embankment; but the amount of rain water falling upon one half of the road can not have a very serious effect upon the side of the embankment. With a roadway raised in the center and the water draining off to either side, the drainage will be more effectual and speedy than if the drainage of the outer half must pass over the inner half. If the surface is formed of one plane, as in Fig. 11, the lower half of it will receive the greater share of the travel; and as it will be more poorly drained, it is nearly certain to wear hollow. This will interfere with the surface drainage; and consequently a road with this section will require excessive attention to keep it in good condition. Figs. 55 and 56, page 210, show two forms of Swiss hillside roads having the center higher than either side.
Whatever the form of the road surface, if the hillside is steep there should be a catch-water above the road to prevent the water from the hillside above from flowing down on the road. Fig. 11 shows such a catch-water. It should be, say, 6 feet back from the excavation, and should have a width and depth according to the amount of water to be intercepted.