 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 carriage tyres that traveled on the road.Macadam roads were being built widely in the United States and Australia in the 1820s and in Europe in the 1830s and 1840s.Macadam roads were adequate for use by horses and carriages or coaches, but they were very dusty and subject to erosion with heavy rain.