Macadam is a type of road construction pioneered by the Scotsman John Loudon McAdam in around 1820. The method simplified what had been considered state-of-the-art at that point. Single sized aggregate layers of stone with a coating of binder as a cementing agent are mixed in an open-structured macadam.
Pierre-Marie-Jérôme Trésaguet is sometimes considered the first person to bring science to road building. A Frenchman from an engineering family, he worked paving roads in Paris from 1757 to 1764. As chief engineer of road construction of Limoges he had opportunity to develop a better and cheaper method of road construction. In 1775 Tresaguet became engineer-general and presented his answer for road improvement in France, which soon became standard practice there. 
Before McAdam, French road director Pierre-Marie-Jérôme Trésaguet had recommended a roadway consisting of three layers of stones laid on a crowned subgrade with side ditches for drainage. The first two layers consisted of angular hand-broken aggregate, maximum size 3 inches (75 mm), to a depth of about 8 inches (200 mm). The third layer was about 2 inches (50 mm) thick with a maximum aggregate size of 1 inch (25 mm).  This top level surface permitted a smoother shape and protected the larger stones in the road structure from iron wheels and horse hooves. To keep the running surface level with the countryside, this road was put in a trench, which created drainage problems. Building the surface as solid as possible, cambering the structure, and building deep side ditches helped to solve drainage problems.”
Thomas Telford was a British surveyor and engineer who applied Tresaguet’s road building theories. In 1801 Telford worked for the British Commission of Highlands Roads and Bridges. He became director of the Holyhead Road Commission between 1815 and 1830. Telford extended Tresaguet’s theories; but Telford emphasized quality stone. He recognized that some of the road problems of the French could be avoided by using cubical stone blocks.
Telford used 300 x 250 x 150 mm partially shaped pitchers, with a slight flat face on the bottom surface. However he turned the other faces more vertically than Tresaguet’s method. The longest edge was arranged crossways to the traffic direction, and the joints were broken in the method of conventional brickwork, but with the smallest faces of the pitcher forming the upper and lower surfaces. 
Broken stone was wedged into the spaces between the tapered perpendicular faces to provide the layer with good lateral control. Telford kept the natural formation level and used masons to camber the upper surface of the blocks. He placed a 150mm layer of stone no bigger than 60mm in size on top of the rock foundation. To finish the road surface he covered the stones with a mixture of gravel and broken stone. This structure came to be known as “Telford pitching.” Telford’s road depended on a resistant structure to prevent water from collecting and corroding the strength of the pavement. Telford raised the pavement structure above ground level whenever possible. Where the structure could not be raised, Telford drained the area surrounding the roadside. Previous road builders in Britain ignored drainage problems and Telford’s rediscovery of these principles was a major contribution to road construction. 
John Loudon McAdam was born in Ayr, Scotland in 1756. In 1787 he became a trustee of the Ayrshire Turnpike in the Scottish Lowlands and during the next seven years this hobby became an obsession. As Surveyor-General of roads for the Bristol Turnpike in 1816, McAdam first put his ideas about road construction into major practice. He also began to actively propagate his ideas in two booklets called Remarks (or Observations) on the Present System of Roadmaking, (which ran nine editions between 1816 and 1827) and A Practical Essay on the Scientific Repair and Preservation of Public Roads, published in 1819. 
McAdam's method was more simple and yet more effective at protecting roadways: he discovered that massive foundations of rock upon rock were unnecessary, and asserted that native soil alone would support the road and traffic upon it, as long as it was covered by a road crust that would protect the soil underneath from water and wear.
Unlike Telford and other road builders of the time, McAdam laid his roads as level as possible. His thirty foot wide road required only a rise of three inches from the center. Cambering and elevation of the road above the water table enabled rain water to run off into ditches on either side. 
Size of stones was central to the McAdam’s road building theory. The lower 200 mm road (7.8 inches) thickness was restricted to stones no larger than 75 mm. (2.9 in) The upper 50 mm (1.9 in) layer of stones was limited to 20 mm size (.787 in ) and stones were checked by supervisors who carried scales. A workman could check the stone size himself by seeing if the stone would fit into his mouth. The importance of the 20 mm stone size was that the stones needed to be much smaller than the 100 mm width of the iron coach tires which traveled on the road.
McAdam believed that the “proper method” of breaking stones for utility and rapidity was accomplished by persons sitting down and using small hammers, breaking the stones so that none of them was larger than six ounces in weight. He also wrote that the quality of the road would depend on how carefully the stones were spread on the surface over a sizeable space, one shovelful at a time.  McAdam directed that no substance which would absorb water and affect the road by frost should be incorporated into the road. Neither was anything to be laid on the clean stone to bind the road. The action of the road traffic would cause the broken stone to combine with its own angles, merging into a level, solid surface which would withstand weather or traffic. 
Through his road building experience McAdam had learned that a layer of broken angular stones would act as a solid mass and would not require the large stone layer previously used to build roads. By keeping the surface stones smaller than the tire width, a good running surface could be created for traffic. The small surface stones also provided low stress on the road, so long as it could be kept reasonably dry. 
McAdam's road building technology was applied to roads by other engineers. One of these engineers was Richard Edgeworth, who filled the gaps between the surface stones with a mixture of stone dust and water, providing a smoother surface for the increased traffic using the roads.  This basic method of construction is sometimes known as water-bound macadam. Although this method required a great deal of manual labor, it resulted in a strong and free-draining pavement. Roads constructed in this manner were described as “macadamised.” 
The greatness of McAdam might be in his effective and economical construction, which was a great improvement over the methods used by his generation. McAdam’s claim to fame might also be in his defeat of the resentment travelers had to the increased traffic on the roads and his advocacy of effective road maintenance and management. He emphasized that roads could be constructed for any kind of traffic. He advocated a central road authority and the trained professional official, who could be paid a salary that would keep him from corruption. This professional could give his entire time to his duties and be held responsible for his actions.
With the advent of motor vehicles, dust became a serious problem on macadam roads. The area of low air pressure created under fast-moving vehicles sucks dust from the road surface, creating dust clouds and a gradual unraveling of the road material. This problem was later rectified by spraying tar on the surface to create tar-bound macadam, more commonly known as tarmac or blacktop. 
A more durable road surface, modern mixed asphalt pavement, was introduced in the 1920s. This pavement method mixed the aggregates into the asphalt with the binding material before they were laid. The macadam surface method laid the stone and sand aggregates on the road and then sprayed it with the binding material. While macadam roads have now been resurfaced in most developed countries, some are preserved along stretches of roads such as the United States' National Road. 
The first macadam road built in the United States was constructed between Hagerstown and Boonsboro, Maryland and was named Boonsboro Turnpike Road. This was the last section of unimproved road between Baltimore on the Chesapeake Bay to Wheeling on the Ohio River. Stagecoaches traveling the Hagerstown to Boonsboro road in the winter took 5 to 7 hours to cover the 10 mile stretch. This road was built using McAdam’s road techniques, except that the finished road was compacted with a cast-iron roller instead of relying on road traffic for compaction. The second American road built using McAdam principles was the Cumberland Road which was 73 miles long and required five years of work. In 1830 the 73-mile Cumberland Road or National Pike Road became the second American road to be built on the "McAdam principle." 
Due to uses of macadam as a road surface in former times, roads in some parts of the United States (as parts of Pennsylvania) are often referred to as macadam, even though they might be made of asphalt or concrete. Similarly, the term "tarmac" is sometimes colloquially misapplied to asphalt roads or aircraft runways.