The Colossus of Roads
He returned to Scotland in 1783 and bought an estate in Ayrshire. There he spent some time improving the estate's roads. In so doing, he developed new theories about road-building systems.
Put simply, he argued that bare, dry soil was perfectly strong enough to take the weight of the kind of traffic contemporary roads carried; all you had to do was keep it dry. So he dispensed with the heavy, carefully-laid foundations which had been the basis of all metalled roads since the Romans.
Instead, he ensured that the road-bed was raised, with adequate drainage to carry away rainfall, and concentrated on laying tightly-packed layers of small stone - the stone to be carefully graded for optimum packing.
Further, he reckoned that the best way to compact this surface was to let traffic do the work - with additional aggregate being added to correct imperfections revealed in this way.
"That it is the native soil which really supports the weight of traffic; that while it is preserved in a dray state, it will carry any weight without sinking and that it does in fact carry the road and carriages also; that this native soil must be previously made quite dry and a covering placed over it in that dry state; that the thickness of the road should only be regulated by the quantity of material necessary to form such impervious covering and never by reference to its own power of carrying weight."
Before McAdam, roadbuilding was an expensive business. It needed top quality slabs of stone, skillfully and consistently laid. For the most part, it just didn't happen - it wasn't economically feasible. Where roads had to be built (for military or political reasons) the contractors often cut corners to shave a little extra off the budget. It wasn't uncommon to find that parts of a major road were built on logs, with a covering of small stones; it would pass an initial, cursory inspection, but would start to collapse within a year or so.
And these roads were very expensive to fix.
McAdam's building system was considerably cheaper than previous ones. True, road routes needed to be properly surveyed, and a consistent degree of quality control was required on the stone-work, but the actual work could be carried out by ordinary, unskilled workers. In fact, at most road-building sites, you could find entire families at work - breaking stones to fit through the grading sieves.
Other engineers developed McAdam's work. Richard Edgeworth, particularly, used the stone dust, mixed with water, to fill the gaps between the surface stones, thus providing a much smoother surface for the increasing number of fancy carriages using the roads.
It was this "water-bound macadam" which brought McAdam's name into the English language. In 1825, the philosopher Jeremy Bentham wrote: "MacAdam's system justified the perpetuation of MacAdam's name in popular speech." Which was somewhat ironic, since McAdam condemned this variation on his simple system.
John McAdam was offered a knighthood, but declined it. However his grandson (William Junior) accepted the honour after his (John's) death.
Just one further development was necessary to arrive at the kind of road we would find familiar today - the replacement of the water/dust binding with tar or bitumen, to produce ashphalt. The first bitumenous pavement was laid in 1854, in Paris, but it wasn't until the twentieth century, and the coming of the automobile, that "black top" became common, falsely known as 'Tarmacadam', or 'Tarmac'.
Enclosure to Turnpike
