The present alignment of Route 66 was cut into the mountain about 1917. It appears the old road was again used as a temporary bypass when Route 66 was paved in this area around 1930.
Oddly enough, numerous maps from around 1915 for the National Old Trails Highway suggest utilization of the alternate route through Yucca to avoid the sharp grades, steep curves, and ironically, the traffic congestion in Oatman and Goldroad. In large part this would become Route 66 after the bypass of Oatman and the Black Mountains portion of the highway in 1952. This is currently the path for I-40.
It should also be noted the primary road to Phoenix from Kingman during the pre war period was Route 66, the National Old Trails Highway before that, west to Yucca, south to Alamo Crossing, a small mill town on the Bill Williams River that is now under the waters of Alamo Lake, and then south to a point near Wenden.
As a final note I have a bit of shameless self promotion to share. My latest book, Ghost Towns of the Southwest, hits stores this weekend. If you would like a signed, first edition copy please drop me a note and I will supply ordering detials.