Conversation so far:
how do you hear the movie at a drive in theater
The movie's sound is provided by broadcasting the soundtrack at a low output power on AM or FM radio to be picked up by a car radio.
How many drive-in movie theaters are left?
There are currently only about 330 drive-in theaters that remain in operation in the United States compared to a peak of about 4,000 in the late 1950's.
Why did drive-in movie theaters decline in number?
Beginning in the late 1960s, drive-in attendance declined as the result of improvements and changes to home entertainment, from color television to cable TV, VCRs and video rental.
What was the first drive-in movie theater?
In 1933, motorists park their automobiles on the grounds of Park-In Theaters, the first-ever drive-in movie theater in Camden, New Jersey.

Question: When was the peak of the popularity of the theater?
Self-contained rewrite: When were drive-in movie theaters most popular?