Used to be really popular. I remember when my family moved up from L.A. to the Bay Area when I was 9, a highway we used to drive on often went right by a drive-in movie place and we could always see the movies playing on the screens.
Later when I got to be around 13 or 14 my friends started getting their drivers licenses and we would get a carload of people on a Saturday night and go to the drive-in. And half the fun was bringing beer and sneaking people in. We would put a couple of guys in the trunk of the car or someone would lay on the floor in the backseat and we could sneak in people that way.
Or my friend had a pickup truck and one night he drove us to the theater and I hid in the bed of the pickup truck under a blanket. It was funny because we parked and three of us were sitting in the back of the pickup watching the movie and drinking beer and talking loudly and a girl that worked there came up to us because she knew I snuck in. She confronted us but she was cool and let us get away with it.
Later when I got my own car, me and a friend went to the drive in. At the exit to the drive in they used to have these four inch metal spikes across the road that you could safely drive over if you were exiting but if you tried to drive the wrong way they would puncture your tires. I haven’t seen these in decades, I don’t know if they are still used. Me and my friend ingeniously built a wooden contraption that we pulled out and stuck over the spikes so we could drive over and get in the movie free.