WIDTH AND TRANSVERSE CONTOUR.
A road should be wide enough to accommodate the traffic for which it is intended, and should comprise a wheelway for vehicles and a space on each side for pedestrians.
The wheelway of country highways need be no wider than is absolutely necessary to accommodate the traffic using it; in many places a track wide enough for a single team is all that is necessary. But the breadth of the land appropriated for highway purposes should be sufficient to provide for all future increase of traffic. The wheelways of roads in rural sections should be double; that is, one portion paved (preferably the center), and the other left with the natural soil. The latter if kept in repair will for at least one-half the year be preferred by teamsters.
The minimum width of the paved portion, if intended to carry two lines of travel, is fixed by the width required to allow two vehicles to pass each other safely. This width is 1p feet. If intended for a single line of travel, 8 feet is sufficient, but suitable turnouts must be provided at frequent intervals. The most economical width for any roadway is some multiple of eight.
Wide roads are the best; they expose a larger surface to the drying action of the sun and wind, and require less supervision than narrow ones. Their first cost is greater than narrow ones, and that nearly in the ratio of the increased width.
The cost of maintaining a mile of road depends more upon the extent of the trafiic.than upon the extent of its surface, and unless extremes be taken, the same quantity of material will be necessary for the repair of the road whether wide or narrow, which is subjected to the same amount of traffic. The cost of spreading the materials over the wide road will be somewhat greater, but the cost of the materials will be the same. On narrow roads the traffic, being confined to one track, will wear more severely than if spread over a wider surface.
The width of land appropriated for road purposes varies in the United States from 49> feet to 66 feet; in England and France from 26 to 66 feet. And the width or space macadamized is also subject to variation; in the United States the average width is 16 feet; in France it varies between 16 and 22 feet; in Belgium 84 feet seems to be the regular width, while in Austria from 141 to 264 feet.
Transverse Contour. The center of all roadways should be higher than the sides. The object of this is to facilitate the flow of the rain water to the gutters. Where a good surface is.maintained a very moderate amount of rise is sufficient for this purpose. Earth
roads require the most and asphalt the least. The rise should bear a certain proportion to the width of the carriageway. The most suitable proportions for the different paving materials is shown in table 10.
Form of Transverse Contour. All authorities agree that the form should be convex, but they differ in the amount and form of the convexity. Circular arcs, two straight lines joined by a circular arc, and ellipses, all have their advocates.
Kind of Surface. Proportions of the Carriageway. Width.
Earth Rise at center Ti Gravel Broken Stone For country roads a curve of suitable convexity may be obtained as follows: Give a of the total rise at a the width from the center to the side, and of the total rise at the width (Fig. 9).
Excessive height and convexity of cross-section contract the width of the wheelway, by concentrating the traffic at the center, that being the only part where a vehicle can run upright. The force required to haul vehicles over such eross-sections is increased, be cause an undue proportion of the load is thrown upon two wheels instead of being distributed equally over the four. The continual tread of horses' feet in one track soon forms a depression which holds water, and the surface is not so dry as with a flat section, which allows the traffic to distribute itself over the whole width.
Sides formed of straight lines are also objectionable. They wear hollow, retain water, and defeat the object sought by raising the center.
The required convexity should be obtained by rounding the formation surface, and not by diminishing the thickness of the covering at the sides.
Although on hillside and mountain roads it is generally recom mended that the surface should consist of a single slope inclining inwards, there is no reason for 'or advantage gained by this method. The form best adapted to these roads is the same as for a road under ordinary conditions.
With a roadway raised in the center and the rain water draining off to gutters on each side, the drainage will be more effectual and speedy than if the drainage of the outer half of the road has to pass over the inner half. The inner half of such a road is usually sub jected to more traffic than the outer 'half. If formed of a straight incline, this side will be worn hollow and retain water. The inclined flat section never can be properly repaired to withstand the traffic. Consequently it never can be kept in good order, no matter how constantly it may be mended. It is always below par and when heavy rain falls it is seriously damaged.