A bit of history here to make things interesting:
According to Wikipedia:

The McDonald Brothers business began in 1940, with a restaurant opened by
siblings Dick and Mac McDonald in San Bernardino, California. Their
introduction of the "Speedee Service System" in 1948 established the
principles of the modern fast-food restaurant.

According to my good friend and mentor Victor Rubio, most popular, respected
and well known restaurateur in Tijuana during the WWII years through to his
death in 2004:

"I remember the McDonald brothers coming into my restaurant near the
bullring and inviting me to have a beer with them as they shared their love
of bullfighting and mariachi music during their occasional trips to Tijuana.
This was around 1947, I remember because of the brouhaha around Margaret
Truman's first public singing appearance. Tijuana had been and was still a
huge magnet to Hollywood types, sailors and marines before and after the
war. It was not until sometime in 1955 when I put two and two together,
recalling Dick's penchant for taking notes, that I remembered their telling
me of their observing how a small taco stand could serve so many people in
such a short time that they began experimenting with the idea back at their
San Bernardino restaurant. By 1960 you could not go anywhere in the United
States without finding a nearby McDonald's. Truly an amazing story of how
the fast food concept changed the way Americans eat. All from observing a
trio of uneducated taco stand workers."