Sometimes wide boulevards, with curved profile and main tained by continuous repairs, have a crown of one sixtieth of the width, or a rise of 0.4 inch per foot from side to center, or an average slope of 1 in 30. The French roads, which have a curved profile and are maintained by the system of continuous repairs, have a crown of one fiftieth of their width, or a rise from side to center of 0.5 inch per foot or a slope of 1 in 25. Many of the better cared for streets and park drives have a crown of one fortieth, or a rise from side to center of 0.6 inch per foot or an average slope of 1 in 20. On the state-aid roads in Massachusetts (narrow roads and continuous repairs), the surface consists of two planes meeting in the center, the transverse slope being f inch to a foot or 1 in 16. Broken-stone roads made of soft stone and maintained by periodic repairs frequently have an original crown of one twelfth—an average slope of 1 inch to 1 foot or 1 in 12.
In Providence, R. I., the following relation between the grade and the crown has been established: With a broken-stone road, the method of making repairs has more weight in determining the amount of the crown than in the case of either an earth road or a gravel road. The earth road is easily and cheaply maintained by what may be called the system of continuous repairs with the scraping grader, which restores or rather maintains the crown. With a gravel road, when it is necessary to restore the crown by adding more gravel, it is usually sufficient to put on only a thin layer and wait a com paratively short time for traffic to consolidate it. With a broken stone road, if the crown or rather the surface is to be perpetually maintained, it is necessary to keep a man upon a short stretch of the road practically all of the time, adding thin patches of stone in first one place and then another, a method so expensive that it is practiced in this country only on park drives, boulevards, etc.; and if the crown is to be restored periodically, it is necessary to add a considerable layer of stone and consolidate it by long con tinued and expensive rolling and sprinkling, and on account of the expense of this operation and the .obstruction to traffic it is customary to lay such a thickness of stone and to give the surface such a crown as not soon to require a repetition of the process. Therefore it happens that broken-stone roads are often built with a crown nearly, if not quite, equal to that of good earth roads, and with more perhaps than is given to good gravel roads.
There is a slight advantage of a very high crown for a broken stone road, particularly for one that is not frequently cleaned. If
the crown is great, the rains will the better wash the surface clean. Dirt upon the surface is not only unsightly, but is also detrimental since it holds the water and softens the surface. Of course the material washed by rains into the gutter must eventually be re moved; but this can be removed more cheaply from the gutter with a scraping grader at comparatively long intervals, than from the surface with brooms or scrapers at short in tervals. The practice of making a high crown is somewhat corn mon in villages using soft road metal and having earth gutters and only surface drainage.
This advantage of a high crown is less for a country road than for a village street, since the wind usually gets a better sweep at the former than at the latter.
Road Level. The transverse curvature of the surface of the road may be tested by the use of the instrument shown in Fig. 49. It consists of a straight edge, E F, about 8 feet 6 inches long made of a 1-inch by 6-inch pine plank, surmounted by a frame which serves as a handle and to support the plumb-line.
The general construction is sufficiently shown in Fig. 49. The plumb-bob can be bought at any hardware store for a few cents. On the front side of the piece A B, is screwed a piece C D, the middle half of the back of which is cut out just enough to leave room for the plumb-line to swing.
To adjust the instrument proceed as follows: Drive two nails 8 feet 4 inches apart into the floor with their heads at approxi mately the same level. Set the straight edge E F upon the nails, and make a temporary mark on the upper edge of A B or C D to indicate the position of the plumb-line; then reverse the straight edge end for end, and again mark the position of the plumb-line. Make a permanent mark square across the top of both A B and C D midway between the two temporary marks, and drive down the higher nail until the plumb-line hangs opposite this mark; then the lower edge of E F is exactly level.
To fit the above instrument for use in testing the crown of a road, it is necessary either (1) to fasten upon the piece E F a board whose lower edge is cut to the proper curve, or (2) to attach strips the ends of which indicate points on the curved profile. If the first method is employed, the curvature of the lower edge of the template may be determined by the method of § 310; and if the second method is preferred, the amount that the several strips should project beyond the edge of E F may be computed by the method of § 310. If the strips are used, it is most convenient to slot them and attach them with round-headed wood screws, so that the strips may be more accurately adjusted to position.