<https://www.americanroadmagazine.com/forum/topic/1933-building-the-illinois-national-road/#comment-17949>
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"Awesome! Thanks Dave! It's great to know when this was done. Illinois hard-surfaced its NR before Indiana, then. Indiana didn't start hard surfacing its NR until no earlier than 1922, according to past research I've done with the road guides. \n  \nLook at all the manual labor to just lay the road surface down, let alone any leveling and grading necessary to prepare the ground. \n  \n\n"
<https://www.americanroadmagazine.com/forum/topic/1933-building-the-illinois-national-road/?tab=comments#comment-17947>