Milling is the process of removing the top layer of asphalt to a specific depth without disturbing the underlying sub-base. This allows new asphalt to be installed without increasing the height of the road and saves time if the sub-base doesn't need to be repaired. The milling process allows curbs, drainage, and other existing structures to remain undisturbed. Milling is an excellent way to maintain or alter existing grades and reduce project completion times.