The finished surface of the road should have sufficient crown to shed the rain water into the side ditches. There are in common use two methods of securing this crown. In one the earthen surface is made level, and the slope is given by a greater thickness of metaling at the center than at the sides; in the other, the slope or camber is given to the earth bed, and the metal has a uniform thickness. The advocates of the first system say that there is more wear at the center than at the sides, and that consequently the metaling should be thicker at the center. Those in favor of the uniform thickness say that as the pressure on the earth is practically the same at the sides as at the center, the thickness should be uniform, since the principal object of the layer of stone is to distribute the concentrated pres sure of the wheel over a greater surface of the earth bed. Both forms of construction are in common use, although the preference seems to be slightly in favor of making the subgrade parallel to the finished road-surface and the stone of uniform thickness. A level subgrade is slightly cheaper to form.
Tresaguet, a Frenchman, seems to have originated in 1764 the system of making the subgrade parallel to the finished road surface. Telford finished the subgrade level, while MacAdam made it parallel to the surface of the road. Tresaguet used the form of foundation now known as telford.
Fig. 45, page 198, shows a cross section of the celebrated Shrews bury and Holyhead road in the west of England, built by Telford
in 1815. The construction of this road, which formed a link in
the direct line of communication between England and Ireland, was made a national undertaking, and resulted in what was at that time one of the finest pieces of road construction in the world.
The Swiss roads shown in Fig. 55 and 56, page 210, have a level subgrade.
shows a Tresaguet or " modern telford " road as built in New Jersey. Notice that the base of the foundation is parallel to the surface of the finished road.
For examples of steep grades on broken-stone roads, see § 82.
Width. In view of the considerable cost necessarily in volved in constructing a first-class broken-stone road, it is impor tant to determine the width that should be paved. The width of way necessary for ordinary rural traffic is often over-estimated. Two wagons having a width of wheel base of 5 feet and a width of load of 9 feet can pass on a 16-foot roadway and leave 6 inches between the outer wheel and the edge of the paved way and a clearance of 1 foot between the inner edges of the loads. This extreme ease will rarely occur, and hence a width of 16 feet will certainly be enough unless there is considerable rapid traffic.
The Massachusetts Highway Commission carefully measured the vidtli of traveled way on numerous crushed-stone roads, and found that with an improved width of 15 to 24 feet,—the average being 1n.1 feet,—the maximum width of traveled way averaged
14.92 feet and the width commonly traveled averaged 11.05 feet.* On this evidence the Commission concludes that " a width of 15 feet is ample except in the vicinity of the larger towns, and that 12 feet is sufficient for the lighter traveled ways, but that 10 feet is too narrow unless good gravel can be obtained for the shoulders."t The average width commonly traveled on forty-six of the 15-foot roads was 9.58 feet.
In New Jersey the width for state-aid roads is 9 to 16 feet, mostly 10 to 12 feet. The improved width of French roads varies from 16 to 22 feet (see § 89); in Austria, from 14 to 26 feet; and in Belgium there are many roads only 81 feet wide.
The preceding discussion has referred only to the width of the paved portion; but there should be an additional width of earth sufficient to keep the broken stone in place, particularly while being rolled. This strip of earth is usu ally called a shoulder, but sometimes and improperly a wing (see § 322). The proper width of the shoulder will depend upon the soil, the climate, and the amount of rolling it receives. Usually 2 or 3 feet is sufficient, although 5 to 7 feet is frequently provided —see Fig. 46. The Swiss road shown in Fig. 55, page 210, has a shoulder of only 18 inches. Compare Fig. 50-54, pages 209-10, and Fig. 57 and 58, page 211. An excess width of shoulder adds greatly to the cost of the road when in excavation or on embankment. The surface of the shoulder should conform to the general curve of the finished roadway. The earth shoulder serves the double purpose of holding the broken stone in place and of affording room for vehicles in passing each other. To improve the shoulders for the second purpose, they are sometimes covered with a thin coat of gravel to harden the surface. Sand shoulders are speedily hardened by the infiltration of fine stone dust and dirt washed from the surface of the road. This effect is quite noticeable with coarse sand; and is appreciable even with fine sand.
The center of the road should be higher than the sides, so that the water from rains may flow rapidly into the side ditches. If originally too flat, the road is soon worn hollow, and the middle becomes a pool if on level ground, or a water course if on an inclination. In the former case the middle of the road is sloppy; and in the latter, the fine material washes away and leaves the larger stones bare. There has been much discussion both as to the proper amount of crown and the exact form of the transverse profile of the roadway.