A realignment in 1937–38 moved the M-17 designation in Allen Park. Changes made moved the highway to turn east on Southfield Road to US 25 and follow US 25 back to the former routing into Detroit. Another larger change came before 1945 with the completion of the Willow Run Expressway. M-17 was rerouted west of the Wayne–Washtenaw county line along the expressway that replaced Ecorse Road. M-17 was joined by Bypass US 112 (BYP US 112) along the Willow Run Expressway to a southern freeway bypass of Ypsilanti. M-17 continued past the end of BYP US 112 at US 112/Michigan Avenue along a two-lane highway to US 23. It then followed US 23 back to Washtenaw Avenue in Pittsfield Township to rejoin the original alignment of M-17 west to Ann Arbor. The former M-17 through downtown Ypsilanti was redesignated Business M-17 (Bus. M-17).