7 The 100 Most Influential Inventors of All Time 7In 1770 he went to New York City, entering the
counting house of a merchant uncle; he returned to
Scotland with a considerable fortune in 1783. There he
purchased an estate at Sauhrie, Ayrshire. McAdam, who
had become a road trustee in his district, noted that the
local highways were in poor condition. At his own expense
he undertook a series of experiments in road making.
In 1798 he moved to Falmouth, Cornwall, where he
continued his experiments under a government appoint-
ment. In 1815, having been appointed surveyor general of
the Bristol roads, he put his theories into practice.
McAdam had no use for the masonry constructions of his
predecessors and contemporaries. Instead, he showed that
traffic could be supported by a relatively thin layer of
small, single-sized, angular pieces of broken stone placed
and compacted on a well-drained natural formation and
covered by an impermeable surface of smaller stones.
Drainage was essential to the success of McAdam’s method,
and he required the pavement to be elevated above the
surrounding surface. The structural layer of broken stone
was 8 inches (20.32 cm) thick and used stone of 2 to 3 inches
(5 to 8 cm) maximum size laid in layers and compacted by
traffic—a process adequate for the traffic of the time. The
top layer was 2 inches (5 cm) thick, using three-fourths- to
1-inch (2.5 cm) stone to fill surface voids between the large
stones. Continuing maintenance was essential.
Although McAdam drew on the successes and failures
of others, his total structural reliance on broken stone
represented the largest paradigm shift in the history of
road pavements. To document his work, McAdam wrote
Remarks on the Present System of Road-Making (1816) and
Practical Essay on the Scientific Repair and Preservation of
Roads (1819).
As the result of a parliamentary inquiry in 1823 into
the whole question of road making, McAdam’s views