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John McAdam

McAdam 

John Loudon McAdam was born on September 21, 1756. He invented a new process, "macadamisation", for building roads with a smooth hard surface that was more durable and less muddy than those already in use. The macadam method spread very quickly across the world. The first macadam road in North America, the National Road, was completed in the 1830s and most of the main roads in Europe were macadamized by the end of the nineteenth century.

These roads were adequate for use by horses and carriages or coaches, but they were very dusty and subject to erosion with heavy rain. Later on, they did not hold up to higher speed motor vehicle use. Modern road construction still reflects McAdam's influence. Of subsequent improvements, the most significant was the introduction of tar to bind the road surface's stones together – "tarmac" (for Tar Macadam) – followed later by the use of hot-laid tarred aggregate or tar-sprayed chippings to create better road metalling. More recently, oil-based asphalt laid on reinforced concrete has become a major road surface, but its use of granite or limestone chippings still recalls McAdam's innovation.

As petroleum production increased, the by-product asphalt became available in greater quantities and largely supplanted tar due to its reduced temperature sensitivity. The Macadam construction process also became quickly obsolete due to its high manual labour requirement; however, the somewhat similar tar and chip method, also known as bituminous surface treatment, remains popular.

While the specific Tarmac pavement is not common in some countries today, many people use the word to refer to generic paved areas at airports.

To learn more about the current state of the United States' roads and transportation systems, visit the 2009 Report Card for America's Infrastructure.