<https://www.americanroadmagazine.com/forum/topic/1933-building-the-illinois-national-road/#comment-17946>
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"Jim, \n  \nThe 1920 ABB says that the greater part of this route will be under construction in the \"coming season.\"  The road in the 1920 ABB is described as it was described in the 1917 ABB, so I would surmise that between 1917 and 1920 the road was gravel, and dirt, with short stretches of concrete (as described in 1917 ABB).  Then it was significantly upgraded in the summer and fall of 1920. \n  \nDave \n  \nKeep the Show on the Road! \n"
<https://www.americanroadmagazine.com/forum/topic/1933-building-the-illinois-national-road/?tab=comments#comment-17947>