THE EVOLUTION OF THE MOTOR ROAD.
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2.—The Modern Surface that Has Overcome All ,Troubles.
THERE CAN BE no doubt that the old macadam roads throughout the county, with wood paving and compressed asphalt for city streets,. would have-continued to satisfy us to this day but for the coming of the motor. Its arrival immediately created a number of new problems for road engineers, and, in their solution, the roads of the country have undergone a vast and radical improvement.
Motor traffic was found to have a twofold effect upon macadam roads, corresponding to the division
• of motor vehicles into two main classes i.e., touring cars -fitted with pneumatic tyres and heavy commercial vehicles fitted with solid tyres. A further subdivision of the commercial vehicle section represents a proportion, numerically small but of importance in this connection, of heavy tonnage fitted with iron shod wheels. In the first class the action of the pneumatic tyre upon ordinary macadam roads is of a suction character, lifting. and dispersing the fine dust which forma a matrix holding together the particles of stone. By thus dispersing the binding material, a disintegration of the road was set up and the dust problem was created.
The dust nuisance, beside being dangerous to health, became so acute that property adjoining the main roads depreciated some 20 or 25 per cent, in
value. So marked was the "lifting " or suction action of tyres that it was common, in early motoring days to see a road with two big grooves worn in it, sometimes 6 ins, thick, full of sharp particles of flint, while the hedges on each side were white with dust. The action of heavy vehicles, while not dispersing the dust to such an extent, was equally serious in affecting the foundations. The problem was a difficult and _pressing one, but British road engineers have succeeded in mastering it. For the dust trouble, tar .spraying was adopted and, although only a temporary palliative, it cured the worst of the trouble. Then, certain Chemicals were found to wash out from the tar with the rain, and to poison the fish in neighbouring streams. Other preparations, including liquid bitumen, were marketed which were demonstrated to be free from this drawback. But, at the best, tirring cured only the dust and did nothing to prevent the destruction of both surface and foundations by heavier vehicles. The solution of this problem called for reconstruc'
ton. Wood blocks, granite setts or compressed asphalt would have answered the purpose, but their cost was prohibitive and rendered them impossible of application save in city streets. It was evident that the roads needed to be reconstructed. The new traffic called for new material and new methods. What was needed was a hinder, some sort of cement which would resist the suction and hold the stones in position. Tarred and bituminous slag and bituminous macadam roads have accordingly been adopted, and along these linh suikess has been . found.
There are certain features essential for a durable, dustless motor road. The cementing material must be *Waterproof, hygienic, and not liable to melt or " bleed " in hot weather, nor to crack in the frosts of winter. Such a material has been found in bitumen, and it is safe to say that all future progress in road paving lies in developments of the use of this material. .
What is .bitumen? It is a hard, glossy substance like pitch, having -great adhesive power so that it binds the road firmly together.It is capable of being readily •melted in suitable boilers, and of resisting the heat of an average summer. It is hard enough to stand heavy traffic, yet is sufficiently resilient not to crack under it. And this ideal material for the modern motor road is a product of petroleum. As petrol is the first fraction in the distillation of petroleum, so bitumen is its final product. It is fnost interesting to note that petroleum, from which is produced the petrol that created the road problem., has also furnished the ideal material to solve that problem. The work of the road engineer is ably suppleinented by that of the chemist, and road making is now an exact science.
The stones or Mineral aggregate to be employed are carefully tested, classified, and graded, and the bitu men used is analysed for purity and tested for density at different temperatures by an ingenious machine known as a penetration machine, and for its cementing value by a " ductility " machine. In the laboratory an electric graduated furnace is employed for determining the resistance of road mixtures to heat, electric globes being devised to reproduce as nearly as possible the light and heat of the sun. Two classes of bituminous roads are built to-day known respectively as the grouting and mixing methods. In the former, the clean granite is rolled and the hot bitumen poured on, and the road again rolled. In the mixing process, carefully graded stone and sand are mixed in a special machine with the hot liquid bitumen and the mixture spread and rolled in one or two coats on the excavated road. Great pro gross was being made along these lines all over th43 country prior to the war by our local authorities, assisted by grants from the Road Board.
One of the most recent and promising developments is the use of crushed and graded destructor clinker, in the place of stone and sand. This enabled a local authority to utilize the waste clinker from its dust destructor, and saves the cost of carting away the clinker from the destructor works, which has previously been an expensive item. Probably one of the big schemes Of employment on cliemobilising our Army will be to take up the -work that has perforce been mostly neglected during the past three years, and, with renewed and ample supplies of petrol, will also come new roads from Land's End to John o' Groats, ove? which the now forbidden passenger car will roam again, and, what is more important nationally, thousands of public service vehicles and motor lorries will move, including many of those now on "active service " at the Front, as ev'denee of the renewed and expanded industrifil activity which we all look forward to after the war.