It seems like they were pretty ubiquitous in the 50s/60s, but now they are virtually non existent. What caused the decline?
The biggest single factor for the decline of the drive in theater in the USA is they took up a lot of real estate. The owners of these theaters built them in the 1935 to 1955 time period, but after 1970, the value of the land they occupied grew so much, their owners often sold out to real estate developers, who built shopping malls and housing developments. Between 1985 and 1995, Joe Bob Briggs hosted a late night Saturday show "Drive In Theater" on TMC (The Movie Channel). This show featured B movies that were originally shown in drive in theaters. Every week, Joe Bob Briggs would read an obituary of the drive ins that were closed down during the preceding week. Joe Bob Briggs also wrote two books about the whole Drive in Movie Theater experience.