McAdam's innovation involved using crushed stone of varying sizes, carefully layered and compacted, to create a durable and smooth road surface. This method proved far superior to the previously used methods of gravel and cobblestones, and it quickly became widely adopted.
As McAdam's method gained popularity, the word "macadam" became synonymous with this type of road construction. It eventually evolved to describe the road surface itself, as in "macadam roads" or "macadam pavement."
So, the next time you drive on a smooth, hard-wearing road, remember that it's a testament to the ingenuity of John Loudon McAdam!