Recently I paid a visit to CA 111 while on my birthday trip to Joshua Tree National Park in October. I'm not quite done with my CA 111 article but I did find something interesting regarding the history of the southern terminus of the highway:
"At some point between 1935-1938 CA 111 was extended south of Brawley over Legislative Route 201 to US 99 near Heber. According to CAhighways LRN 201 was added to the State Highway System in 1933. CA 111 terminating at US 99 in Heber can be first be seen on the 1938 Division of Highways Map."
The 1938 Division of Highways Map below show CA 111 clearly to US 80 in El Centro. Every topographical map I've been before the 1964 State Highway Renumbering shows CA 111 ending at US 99 just east of Heber:
https://www.davidrumsey.com/luna/servlet/detail/RUMSEY~8~1~239588~5511892:Road-Map-of-the-State-of-California?sort=Pub_List_No_InitialSort%2CPub_Date%2CPub_List_No%2CSeries_No&qvq=q:caltrans%201938;sort:Pub_List_No_InitialSort%2CPub_Date%2CPub_List_No%2CSeries_No;lc:RUMSEY~8~1&mi=0&trs=2
Apparently it wasn't much of a leap for CA 111 to assume US 99 south to the border in Mexicali, it was actually pretty close for several decades!
Finished my blog on CA 111...
This past October I revisited one of my common routes of travel while working in southern California; CA 111 in Coachella Valley. CA 111 was one of the original Sign State Routes which was originally aligned between US 99/60/70 in Whitewater and US 99 in Brawley. CA 111 was extended to US 99 near Heber in the late 1930s before being extended to the Mexican Border in Calexico over former US 99 in 1964. CA 111 for decades has been the main street of Coachella Valley but has begun to be gradually relinquished between CA 86 and I-10. My trip on CA 111 included the odd gap point at CA 86 in Mecca along with the remaining segment of CA 111 from Palm Springs west to I-10.
https://www.gribblenation.org/2019/11/california-state-route-111-in-coachella.html