The smooth flow of traffic on modern highways is dependent on the weather of the changing seasons. The main weather hazards are ice, fog, floods and strong cross-winds.
Their effects can be minimised by meteorological surveys carried out when the roads are being designed and built, so that proposed stretches likely to be particularly vulnerable are identified and avoided in advance. Once built, many trunk roads nowadays are provided with meteorological instruments to monitor potential hazards. Visibility sensors trigger illuminated signs when fog occurs, anemometers provide alerts when certain wind speeds are exceeded, and temperature sensors on the road surfaces give warning of impending icy patches.
But in bygone times, the difficulties were much more fundamental. Until the end of the 19th century, many roads on these islands were impassable to carts and wagons after the first autumnal rains set in, and for fully half the year travel could be undertaken only on foot or on horseback. With the coming of spring, the roads were levelled by means of "ploughs" drawn by a team of horses, after which normal traffic could resume ad interim.
One man, almost single-handedly, brought about dramatic change. John Louden McAdam was born in 1756 in Ayr, and having amassed a modest fortune as a young man in the American colonies he purchased an estate in Scotland and became thereby a "road trustee" in his district. At his own expense, and despite much local opposition, he conducted a series of experiments on road making, and in due course arrived at what he believed to be the perfect recipe: he proposed a raised surface of broken stones, each stone angular in shape and weighing no more than six ounces, and the thoroughfare to be provided with drains on either side; the road would then be consolidated into a firm, hard surface by the weight of the traffic passing over it.
McAdam described his methodology in A Practical Essay on the Scientific Repair and Preservation of Roads, published in 1819. It was enough to have McAdam appointed Surveyor-General of British Roads in 1826.
McAdam's methodology was adopted almost universally for trunk routes throughout these islands, and a new word, "macadamised", was coined. With the advent of the motor car it became necessary to devise some means of keeping down the dust, and so a coating of tar and stone chippings was applied to the macadamised road. Tarmacadam had arrived.
John Louden McAdam died 165 years ago today, on November 26th, 1836.