CHAPTER XII.

WIDTH AND TRANSVERSE CONTOUR.

611. 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.

612. 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 centre) 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.

613. 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 16 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.

614. 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.

615. The cost of maintaining a mile of road depends more upon the extent of the traffic 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.

616. The width of land appropriated for road purposes varies in the United States from 494 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 8 feet seems to be the regular width, while in Austria it varies from 14 to 26 feet.

Figs. 85-92, page 342, show the subdivision of the roadway into wheelway, sidewalks, and ditches.

617. Width of Mountain Roads.-Mountain roads should be proportioned in width to the amount of traffic; they should be neither too wide nor too narrow. If of excessive width, the cost of construction is increased; if too narrow, traffic will be interrupted by blockades. An economical width is 24 feet, and the stone covering should extend from gutter to guttre. If the center only is covered, the road will soon be destroyed, as, by reason of the curves on a mountain side predominating over straight reaches, the traffic will hug either one side of the road or the other.

Table LXIII shows the number of acres required per mile for different widths of roadway.

TABLE LXIII.

ACRES REQUIRED PER MILE FOR DIFFERENT WIDTHS OF ROADWAY.

618. Transverse Contour. The centre 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 the following table:

619. 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, but the best form for streets will be found to be a parabolic curve starting from the edge of the gutter next the carriageway or one foot from the curb line. Fig. 54 shows this form, which is obtained in the following manner: Divide the ordinate or the width between the edge of the gutter and the centre of the street into 10 equal parts, and raise perpendiculars the length of which will be determined by multiplying the rise at the center by the respective number of each perpendicular in the diagram. The amounts thus obtained can be added to the rod readings, and the stakes set at the proper distance across the street with their tops at this level will give the true curve.

620. For country roads a curve of suitable convexity may be obtained as follows: Give of the total rise at the width from the centre to the side, and g of the total rise at the width (Fig. 55). 621. Excessive height and convexity of cross-section contract the width of the wheelway, by concentrating the traffic at the

centre, that being the only part where a vehicle can run upright. The force required to haul vehicles over such cross-sections is increased, because 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 continuous travel of vehicles in one track soon wears ruts which retain water, and the surface is not so dry as with a flatted section, which allows the traffic to distribute itself over the whole width.

622. Sides formed of straight lines are also objectionable. They wear hollow, retain water, and defeat the object sought by raising the centre.

623. Concave Form.-In Triest, Austria, the early pavements were laid concave, i.e., inclining to the middle, along which, under the surface-canals or sewers extended with gratings at intervals for the admission of surface-drainage. The same method, but with open channels through the centre, is practised in several South American towns. Experience has proved that this plan is not desirable or convenient for traffic.

624. The required convexity should be obtained by rounding the formation surface, and not by diminishing the thickness of the covering at the sides.

625. On hillside and mountain roads it is generally recommended 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, viz., that described in Art. 619.

626. With a roadway raised in the centre 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 road is usually subjected 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. When heavy rain falls it is seriously damaged.