Again the duplicate carriageway lends itself to progressive development. In the first instance, while the country alongside the road remains open, a single carriageway can be formed and will meet travellers' needs until such time as population and traffic increase, when the second carriageway can be constructed without interfering with the road which is already in use. The general tendency of road practice to assimilate railway methods, encourages the view that separate carriageways (up and down lines) are likely to prevail in the case of new arterial roads.
In the older and more settled countries where a tolerably com plete network of old highways already exists, the road engineer's task is usually confined to the improvement of these, rather than to the construction of entirely new communications. Due to the steady increase in funds available through taxation it became possible to launch in Great Britain in 1925 the so-called "trunk road programme" for the systematic transformation of the most important through routes of the country. The expenditure incurred or approved for this purpose had reached by March 1928 a total of about £6,500,000. In their unimproved state these roads usually possess a carriageway varying from 14 ft. to 24 ft. in width, with little, if any, footway. Generally speak ing, where improvement is now undertaken, sufficient land is being acquired to allow a dimension of 6o ft. between fences. This affords space for a carriageway 3o ft. wide, two 8 ft. foot ways and two 7 ft. verges. In more rural areas the carriageway is limited for the present to 20 ft., only one footway is constructed, and the verges are widened to absorb the additional space.
forests makes an excellent track for all kinds of traffic. Upon that side of the space devoted to the road, which the heavy traffic leading to a town will use, two parallel rows of sills are laid longi tudinally flatwise, the earth being well packed and rammed to the level of their faces. The joints are not opposite; a short piece of sill is put either under or by the side of each joint. Cross-boards are laid down loosely, so that groups of four boards together will project on alternate sides of the road forming a shoulder to enable vehicles to get on to the track at any point. The remainder of the road space is formed as an earthen track, for light vehicles. Its slope outwards may be 1 in 16, that of the plank road 1 in 32. If the soil is too bad for the earthen track, short lengths of plank road of double width are made at intervals to form passing places. The cross boards are spiked down on five sills, and are sprung so as to give a fall both ways.
The log road is formed across swamps by laying young trees of similar length close together. This is ridiculed as a "corduroy" road, but it is better than the swamp. Good temporary roads may be made by laying down half logs roughly squared upon the ground, close together or with spaces between of a couple of inches, into which earth is well rammed.
The mode of carrying a road across a bog upon a foundation of faggots or brushwood is well known. In India the native roads have been made equal to heavy traffic by laying branches of the mimosa across the track. And in the great plains, where the soil, when dry, would otherwise lie deep in dust, this is entirely prevented by laying across the track a coarse reed or grass like the pampas-grass, and covering it with 3 or 4 in. of loam.