I took a look at a current map using place names given yesterday to see if such a road exists as I have not been able to find any other mention of a Douglas Highway in Illinois.
It was to enter Illinois at Orangeville near the Wisconsin border which would make it Highway 26 today. It would remain Highway 26 through Freeport and Forrestown.
At Polo, it would become US-52 and remain that through Woosung, Dixon, Lee center, Shaw Center to Mendota. From there it would be US-51 through the rest of the state going south to the Mississippi River.