Design of Highways
The earliest tracks, apart from those used by the Maoris, were probably made through the bush to enable kauri spars to be hauled down to the beaches for trading and whaling ships. As settlement grew, tracks were developed further afield to provide a rudimentary form of communication by foot or horse. With the discovery of the goldfields and the Maori Wars of the sixties, roadmaking assumed a new importance. The fact that many of these routes are still in use is a tribute to the skill of the early surveyors and engineers who carried out their work under most difficult conditions. Today the problem of the surveyor is less hazardous, owing to the use of a new aid called photogrammetry. By this method plans can be developed from aerial photographs which reveal contours at close intervals, thus enabling road location to be undertaken with sufficient accuracy for contracts to be let.
Before 1930 road engineers had largely to rely for earthmoving on such primitive methods as the pick and shovel, wheelbarrow, and horse-drawn cart, though motor lorries were used to carry away a great deal of the spoil. The coming of the crawler tractor, which was about 100 horsepower, rapidly changed the technique of roadmaking. Larger cuts and fills became both possible and economical and unit costs remained almost stationary, A further advance came with the introduction of the modern tractor (over 400 horsepower), so powerful that hard rock can be ripped up for handling without the use of explosives. Large rubber-tyred scrapers have also been developed as complementary units for moving large quantities of soil and rock at high speeds.
When traffic was horse drawn, speeds were really not of great importance; thus the curvature of the early roads is generally quite unacceptable by modern standards. Improvement of alignment is a matter of cost and the tendency today is to group substandard curves and to change speed value in steps. For many years the National Roads Board has used portion of a spiral or lemniscale for this purpose. In order to improve the riding qualities of highway curves, transitions have been introduced which change the curvature gradually and introduce superelevation. But good road designing is not in itself the answer to all the traffic problems of today. The human factor must never be overlooked. Thus special traffic signs warn motorists of the correct speed for taking curves, and “no passing” lines along certain sections give warning that overtaking is dangerous.
Easy grades were of great importance to the horsedrawn vehicle but were not always possible of attainment. But grades are not nearly so important to the modern high-powered motor cars, though on very steep inclines some commercial vehicles may be forced to drop to a low speed, thereby delaying other traffic. To avoid such congestion and to carry such slow-moving traffic, road engineers in Australia and the United States have evolved the system of an extra up-hill lane. This is now being used with considerable success in New Zealand on the Bombay Hills north of Pokeno and on the new Taihape deviation.